1
The Data-Link Layer
Ethernet ARP and LANs
Based on slides from the Computer Networking A Top Down Approach Featuring the Internet by Kurose and Ross
The Data Link Layer
Our goals
Understand principles behind data link layer services
Sharing a broadcast channel multiple access
Link layer addressing
Interconnection of different LAN segments
Instantiation and implementation of various link layer
technologies
2
2
Link Layer Introduction
Some terminology
hosts and routers are nodes
communication channels that
connect adjacent nodes along
communication path are links
wired links
wireless links
LANs
layer-2 packet is a frame
encapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
3
Link layer context
Datagram transferred by
different link protocols over
different links
eg Ethernet on first link
frame relay on intermediate
links 80211 on last link
Each link protocol provides
different services
eg may or may not provide
rdt over link
transportation analogy
trip from Princeton to Lausanne
limo Princeton to JFK
plane JFK to Geneva
train Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
4
3
Physical Media
physical link
Transmitted data bit propagates across link
guided media
signals propagate in solid media (eg copper fiber)
unguided media
signals propagate freely (eg radio bands)
The physical Layer defines the ldquorepresentationrdquo of bits
It‟s also provides protection by both detecting and correcting
corrupted bits (See how it works)
5
Physical Media (Examples)
Twisted Pair (TP)
Two insulated copper wires May be shielded or not
Category 3
Traditional phone wires Supports 10-Mbps Ethernet
Category 5
Supports 100Mbps Ethernet (ldquoFast-Ethernetrdquo)
6
4
Physical Media (Examples)
Coaxial cable
Two concentric shielded wires
Baseband
single channel on cable
broadband
multiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber
Glass fiber carrying light pulses
Single mode or multi mode
High point-to-point speed
Very low error rate (caused by low fiber‟s attenuation)
Secured
Common used for 100-Mbps Ethernet and 1000-Mbps
Ethernet (ldquoGigabit Ethernetrdquo)
8
5
Physical Media (Examples)
Many more
Radio bands (WiFi high bit error rate)
Microwave (Requires hosts in light of sight)
Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE
802 model
Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol)
Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts
(actually NICs) who are part of the network
Different for IP addresses
Channel Access
Defines the set of rules which allows the hosts to use the (possibly
shared) medium
10
6
Link Layer Services
Flow Control
pacing between adjacent sending and receiving nodes
Error Detection
errors caused by signal attenuation noise
receiver detects presence of errors
signals sender for retransmission or drops frame
Error Correction
receiver identifies and corrects bit error(s) without resorting to
retransmission
Half-duplex and full-duplex
with half duplex nodes at both ends of link can transmit but not at same
time
RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in a
frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc
extracts datagram passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
7
Link Types
Three types of ldquolinksrdquo
point-to-point (P2P)
PPP for dial-up access
Point-to-point link between
Ethernet switch and host
Switched Networks
ATM (used for WAN)
Broadcast
Traditional Ethernet and its
predecessors
Upstream HFC
80211 wireless LAN
We‟ll focus on Broadcast
media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same
time
Multiple ACcess (MAC) Protocol
distributed algorithm that determines how nodes share channel
ie determine when node can transmit
communication about channel sharing must use channel itself
no out-of-band channel for coordination
14
8
Human Analogy
15
Rules for bdquoparty conversation‟
Give everyone a chance to speak
Dont speak until you are spoken to
Dont monopolize the conversation
Raise your hand if you have a question
Dont interrupt when someone is speaking
Dont fall asleep when someone else is talking
MAC Protocols measures
Assume a shared medium with a channel rate of R [bpsec]
Efficient
When one node wants to transmit it ca send at rate R
Fair
When N users want to transmit each can send at average rate RN
Decentralized
No special node uses to coordinate transmission (no ldquoleaderrdquo)
No synchronization of clocks or slots
Fault tolerant
Simple
Should be very fast and implemented in NICs firmware
16
9
MAC Protocol Types
Three broad classes
Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands
or by code)
allocate piece to a node for exclusive its use
Random Access
Channel not divided allow collisions
ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo
Nodes take turns but nodes with more to send can take longer
turns
Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMA
TDMA (Time Division Multiple Access)
Access the channel in roundsldquo
Each station gets fixed length slot (length = packets trans time) in each
round Each slot called a Time-Slot
Unused slots go idle
Example 6-station LAN 134 have packtes slots 256 idle
18
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
2
Link Layer Introduction
Some terminology
hosts and routers are nodes
communication channels that
connect adjacent nodes along
communication path are links
wired links
wireless links
LANs
layer-2 packet is a frame
encapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
3
Link layer context
Datagram transferred by
different link protocols over
different links
eg Ethernet on first link
frame relay on intermediate
links 80211 on last link
Each link protocol provides
different services
eg may or may not provide
rdt over link
transportation analogy
trip from Princeton to Lausanne
limo Princeton to JFK
plane JFK to Geneva
train Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
4
3
Physical Media
physical link
Transmitted data bit propagates across link
guided media
signals propagate in solid media (eg copper fiber)
unguided media
signals propagate freely (eg radio bands)
The physical Layer defines the ldquorepresentationrdquo of bits
It‟s also provides protection by both detecting and correcting
corrupted bits (See how it works)
5
Physical Media (Examples)
Twisted Pair (TP)
Two insulated copper wires May be shielded or not
Category 3
Traditional phone wires Supports 10-Mbps Ethernet
Category 5
Supports 100Mbps Ethernet (ldquoFast-Ethernetrdquo)
6
4
Physical Media (Examples)
Coaxial cable
Two concentric shielded wires
Baseband
single channel on cable
broadband
multiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber
Glass fiber carrying light pulses
Single mode or multi mode
High point-to-point speed
Very low error rate (caused by low fiber‟s attenuation)
Secured
Common used for 100-Mbps Ethernet and 1000-Mbps
Ethernet (ldquoGigabit Ethernetrdquo)
8
5
Physical Media (Examples)
Many more
Radio bands (WiFi high bit error rate)
Microwave (Requires hosts in light of sight)
Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE
802 model
Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol)
Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts
(actually NICs) who are part of the network
Different for IP addresses
Channel Access
Defines the set of rules which allows the hosts to use the (possibly
shared) medium
10
6
Link Layer Services
Flow Control
pacing between adjacent sending and receiving nodes
Error Detection
errors caused by signal attenuation noise
receiver detects presence of errors
signals sender for retransmission or drops frame
Error Correction
receiver identifies and corrects bit error(s) without resorting to
retransmission
Half-duplex and full-duplex
with half duplex nodes at both ends of link can transmit but not at same
time
RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in a
frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc
extracts datagram passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
7
Link Types
Three types of ldquolinksrdquo
point-to-point (P2P)
PPP for dial-up access
Point-to-point link between
Ethernet switch and host
Switched Networks
ATM (used for WAN)
Broadcast
Traditional Ethernet and its
predecessors
Upstream HFC
80211 wireless LAN
We‟ll focus on Broadcast
media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same
time
Multiple ACcess (MAC) Protocol
distributed algorithm that determines how nodes share channel
ie determine when node can transmit
communication about channel sharing must use channel itself
no out-of-band channel for coordination
14
8
Human Analogy
15
Rules for bdquoparty conversation‟
Give everyone a chance to speak
Dont speak until you are spoken to
Dont monopolize the conversation
Raise your hand if you have a question
Dont interrupt when someone is speaking
Dont fall asleep when someone else is talking
MAC Protocols measures
Assume a shared medium with a channel rate of R [bpsec]
Efficient
When one node wants to transmit it ca send at rate R
Fair
When N users want to transmit each can send at average rate RN
Decentralized
No special node uses to coordinate transmission (no ldquoleaderrdquo)
No synchronization of clocks or slots
Fault tolerant
Simple
Should be very fast and implemented in NICs firmware
16
9
MAC Protocol Types
Three broad classes
Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands
or by code)
allocate piece to a node for exclusive its use
Random Access
Channel not divided allow collisions
ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo
Nodes take turns but nodes with more to send can take longer
turns
Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMA
TDMA (Time Division Multiple Access)
Access the channel in roundsldquo
Each station gets fixed length slot (length = packets trans time) in each
round Each slot called a Time-Slot
Unused slots go idle
Example 6-station LAN 134 have packtes slots 256 idle
18
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
3
Physical Media
physical link
Transmitted data bit propagates across link
guided media
signals propagate in solid media (eg copper fiber)
unguided media
signals propagate freely (eg radio bands)
The physical Layer defines the ldquorepresentationrdquo of bits
It‟s also provides protection by both detecting and correcting
corrupted bits (See how it works)
5
Physical Media (Examples)
Twisted Pair (TP)
Two insulated copper wires May be shielded or not
Category 3
Traditional phone wires Supports 10-Mbps Ethernet
Category 5
Supports 100Mbps Ethernet (ldquoFast-Ethernetrdquo)
6
4
Physical Media (Examples)
Coaxial cable
Two concentric shielded wires
Baseband
single channel on cable
broadband
multiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber
Glass fiber carrying light pulses
Single mode or multi mode
High point-to-point speed
Very low error rate (caused by low fiber‟s attenuation)
Secured
Common used for 100-Mbps Ethernet and 1000-Mbps
Ethernet (ldquoGigabit Ethernetrdquo)
8
5
Physical Media (Examples)
Many more
Radio bands (WiFi high bit error rate)
Microwave (Requires hosts in light of sight)
Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE
802 model
Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol)
Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts
(actually NICs) who are part of the network
Different for IP addresses
Channel Access
Defines the set of rules which allows the hosts to use the (possibly
shared) medium
10
6
Link Layer Services
Flow Control
pacing between adjacent sending and receiving nodes
Error Detection
errors caused by signal attenuation noise
receiver detects presence of errors
signals sender for retransmission or drops frame
Error Correction
receiver identifies and corrects bit error(s) without resorting to
retransmission
Half-duplex and full-duplex
with half duplex nodes at both ends of link can transmit but not at same
time
RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in a
frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc
extracts datagram passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
7
Link Types
Three types of ldquolinksrdquo
point-to-point (P2P)
PPP for dial-up access
Point-to-point link between
Ethernet switch and host
Switched Networks
ATM (used for WAN)
Broadcast
Traditional Ethernet and its
predecessors
Upstream HFC
80211 wireless LAN
We‟ll focus on Broadcast
media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same
time
Multiple ACcess (MAC) Protocol
distributed algorithm that determines how nodes share channel
ie determine when node can transmit
communication about channel sharing must use channel itself
no out-of-band channel for coordination
14
8
Human Analogy
15
Rules for bdquoparty conversation‟
Give everyone a chance to speak
Dont speak until you are spoken to
Dont monopolize the conversation
Raise your hand if you have a question
Dont interrupt when someone is speaking
Dont fall asleep when someone else is talking
MAC Protocols measures
Assume a shared medium with a channel rate of R [bpsec]
Efficient
When one node wants to transmit it ca send at rate R
Fair
When N users want to transmit each can send at average rate RN
Decentralized
No special node uses to coordinate transmission (no ldquoleaderrdquo)
No synchronization of clocks or slots
Fault tolerant
Simple
Should be very fast and implemented in NICs firmware
16
9
MAC Protocol Types
Three broad classes
Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands
or by code)
allocate piece to a node for exclusive its use
Random Access
Channel not divided allow collisions
ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo
Nodes take turns but nodes with more to send can take longer
turns
Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMA
TDMA (Time Division Multiple Access)
Access the channel in roundsldquo
Each station gets fixed length slot (length = packets trans time) in each
round Each slot called a Time-Slot
Unused slots go idle
Example 6-station LAN 134 have packtes slots 256 idle
18
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
4
Physical Media (Examples)
Coaxial cable
Two concentric shielded wires
Baseband
single channel on cable
broadband
multiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber
Glass fiber carrying light pulses
Single mode or multi mode
High point-to-point speed
Very low error rate (caused by low fiber‟s attenuation)
Secured
Common used for 100-Mbps Ethernet and 1000-Mbps
Ethernet (ldquoGigabit Ethernetrdquo)
8
5
Physical Media (Examples)
Many more
Radio bands (WiFi high bit error rate)
Microwave (Requires hosts in light of sight)
Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE
802 model
Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol)
Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts
(actually NICs) who are part of the network
Different for IP addresses
Channel Access
Defines the set of rules which allows the hosts to use the (possibly
shared) medium
10
6
Link Layer Services
Flow Control
pacing between adjacent sending and receiving nodes
Error Detection
errors caused by signal attenuation noise
receiver detects presence of errors
signals sender for retransmission or drops frame
Error Correction
receiver identifies and corrects bit error(s) without resorting to
retransmission
Half-duplex and full-duplex
with half duplex nodes at both ends of link can transmit but not at same
time
RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in a
frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc
extracts datagram passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
7
Link Types
Three types of ldquolinksrdquo
point-to-point (P2P)
PPP for dial-up access
Point-to-point link between
Ethernet switch and host
Switched Networks
ATM (used for WAN)
Broadcast
Traditional Ethernet and its
predecessors
Upstream HFC
80211 wireless LAN
We‟ll focus on Broadcast
media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same
time
Multiple ACcess (MAC) Protocol
distributed algorithm that determines how nodes share channel
ie determine when node can transmit
communication about channel sharing must use channel itself
no out-of-band channel for coordination
14
8
Human Analogy
15
Rules for bdquoparty conversation‟
Give everyone a chance to speak
Dont speak until you are spoken to
Dont monopolize the conversation
Raise your hand if you have a question
Dont interrupt when someone is speaking
Dont fall asleep when someone else is talking
MAC Protocols measures
Assume a shared medium with a channel rate of R [bpsec]
Efficient
When one node wants to transmit it ca send at rate R
Fair
When N users want to transmit each can send at average rate RN
Decentralized
No special node uses to coordinate transmission (no ldquoleaderrdquo)
No synchronization of clocks or slots
Fault tolerant
Simple
Should be very fast and implemented in NICs firmware
16
9
MAC Protocol Types
Three broad classes
Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands
or by code)
allocate piece to a node for exclusive its use
Random Access
Channel not divided allow collisions
ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo
Nodes take turns but nodes with more to send can take longer
turns
Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMA
TDMA (Time Division Multiple Access)
Access the channel in roundsldquo
Each station gets fixed length slot (length = packets trans time) in each
round Each slot called a Time-Slot
Unused slots go idle
Example 6-station LAN 134 have packtes slots 256 idle
18
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
5
Physical Media (Examples)
Many more
Radio bands (WiFi high bit error rate)
Microwave (Requires hosts in light of sight)
Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE
802 model
Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol)
Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts
(actually NICs) who are part of the network
Different for IP addresses
Channel Access
Defines the set of rules which allows the hosts to use the (possibly
shared) medium
10
6
Link Layer Services
Flow Control
pacing between adjacent sending and receiving nodes
Error Detection
errors caused by signal attenuation noise
receiver detects presence of errors
signals sender for retransmission or drops frame
Error Correction
receiver identifies and corrects bit error(s) without resorting to
retransmission
Half-duplex and full-duplex
with half duplex nodes at both ends of link can transmit but not at same
time
RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in a
frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc
extracts datagram passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
7
Link Types
Three types of ldquolinksrdquo
point-to-point (P2P)
PPP for dial-up access
Point-to-point link between
Ethernet switch and host
Switched Networks
ATM (used for WAN)
Broadcast
Traditional Ethernet and its
predecessors
Upstream HFC
80211 wireless LAN
We‟ll focus on Broadcast
media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same
time
Multiple ACcess (MAC) Protocol
distributed algorithm that determines how nodes share channel
ie determine when node can transmit
communication about channel sharing must use channel itself
no out-of-band channel for coordination
14
8
Human Analogy
15
Rules for bdquoparty conversation‟
Give everyone a chance to speak
Dont speak until you are spoken to
Dont monopolize the conversation
Raise your hand if you have a question
Dont interrupt when someone is speaking
Dont fall asleep when someone else is talking
MAC Protocols measures
Assume a shared medium with a channel rate of R [bpsec]
Efficient
When one node wants to transmit it ca send at rate R
Fair
When N users want to transmit each can send at average rate RN
Decentralized
No special node uses to coordinate transmission (no ldquoleaderrdquo)
No synchronization of clocks or slots
Fault tolerant
Simple
Should be very fast and implemented in NICs firmware
16
9
MAC Protocol Types
Three broad classes
Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands
or by code)
allocate piece to a node for exclusive its use
Random Access
Channel not divided allow collisions
ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo
Nodes take turns but nodes with more to send can take longer
turns
Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMA
TDMA (Time Division Multiple Access)
Access the channel in roundsldquo
Each station gets fixed length slot (length = packets trans time) in each
round Each slot called a Time-Slot
Unused slots go idle
Example 6-station LAN 134 have packtes slots 256 idle
18
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
6
Link Layer Services
Flow Control
pacing between adjacent sending and receiving nodes
Error Detection
errors caused by signal attenuation noise
receiver detects presence of errors
signals sender for retransmission or drops frame
Error Correction
receiver identifies and corrects bit error(s) without resorting to
retransmission
Half-duplex and full-duplex
with half duplex nodes at both ends of link can transmit but not at same
time
RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in a
frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc
extracts datagram passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
7
Link Types
Three types of ldquolinksrdquo
point-to-point (P2P)
PPP for dial-up access
Point-to-point link between
Ethernet switch and host
Switched Networks
ATM (used for WAN)
Broadcast
Traditional Ethernet and its
predecessors
Upstream HFC
80211 wireless LAN
We‟ll focus on Broadcast
media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same
time
Multiple ACcess (MAC) Protocol
distributed algorithm that determines how nodes share channel
ie determine when node can transmit
communication about channel sharing must use channel itself
no out-of-band channel for coordination
14
8
Human Analogy
15
Rules for bdquoparty conversation‟
Give everyone a chance to speak
Dont speak until you are spoken to
Dont monopolize the conversation
Raise your hand if you have a question
Dont interrupt when someone is speaking
Dont fall asleep when someone else is talking
MAC Protocols measures
Assume a shared medium with a channel rate of R [bpsec]
Efficient
When one node wants to transmit it ca send at rate R
Fair
When N users want to transmit each can send at average rate RN
Decentralized
No special node uses to coordinate transmission (no ldquoleaderrdquo)
No synchronization of clocks or slots
Fault tolerant
Simple
Should be very fast and implemented in NICs firmware
16
9
MAC Protocol Types
Three broad classes
Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands
or by code)
allocate piece to a node for exclusive its use
Random Access
Channel not divided allow collisions
ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo
Nodes take turns but nodes with more to send can take longer
turns
Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMA
TDMA (Time Division Multiple Access)
Access the channel in roundsldquo
Each station gets fixed length slot (length = packets trans time) in each
round Each slot called a Time-Slot
Unused slots go idle
Example 6-station LAN 134 have packtes slots 256 idle
18
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
7
Link Types
Three types of ldquolinksrdquo
point-to-point (P2P)
PPP for dial-up access
Point-to-point link between
Ethernet switch and host
Switched Networks
ATM (used for WAN)
Broadcast
Traditional Ethernet and its
predecessors
Upstream HFC
80211 wireless LAN
We‟ll focus on Broadcast
media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same
time
Multiple ACcess (MAC) Protocol
distributed algorithm that determines how nodes share channel
ie determine when node can transmit
communication about channel sharing must use channel itself
no out-of-band channel for coordination
14
8
Human Analogy
15
Rules for bdquoparty conversation‟
Give everyone a chance to speak
Dont speak until you are spoken to
Dont monopolize the conversation
Raise your hand if you have a question
Dont interrupt when someone is speaking
Dont fall asleep when someone else is talking
MAC Protocols measures
Assume a shared medium with a channel rate of R [bpsec]
Efficient
When one node wants to transmit it ca send at rate R
Fair
When N users want to transmit each can send at average rate RN
Decentralized
No special node uses to coordinate transmission (no ldquoleaderrdquo)
No synchronization of clocks or slots
Fault tolerant
Simple
Should be very fast and implemented in NICs firmware
16
9
MAC Protocol Types
Three broad classes
Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands
or by code)
allocate piece to a node for exclusive its use
Random Access
Channel not divided allow collisions
ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo
Nodes take turns but nodes with more to send can take longer
turns
Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMA
TDMA (Time Division Multiple Access)
Access the channel in roundsldquo
Each station gets fixed length slot (length = packets trans time) in each
round Each slot called a Time-Slot
Unused slots go idle
Example 6-station LAN 134 have packtes slots 256 idle
18
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
8
Human Analogy
15
Rules for bdquoparty conversation‟
Give everyone a chance to speak
Dont speak until you are spoken to
Dont monopolize the conversation
Raise your hand if you have a question
Dont interrupt when someone is speaking
Dont fall asleep when someone else is talking
MAC Protocols measures
Assume a shared medium with a channel rate of R [bpsec]
Efficient
When one node wants to transmit it ca send at rate R
Fair
When N users want to transmit each can send at average rate RN
Decentralized
No special node uses to coordinate transmission (no ldquoleaderrdquo)
No synchronization of clocks or slots
Fault tolerant
Simple
Should be very fast and implemented in NICs firmware
16
9
MAC Protocol Types
Three broad classes
Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands
or by code)
allocate piece to a node for exclusive its use
Random Access
Channel not divided allow collisions
ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo
Nodes take turns but nodes with more to send can take longer
turns
Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMA
TDMA (Time Division Multiple Access)
Access the channel in roundsldquo
Each station gets fixed length slot (length = packets trans time) in each
round Each slot called a Time-Slot
Unused slots go idle
Example 6-station LAN 134 have packtes slots 256 idle
18
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
9
MAC Protocol Types
Three broad classes
Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands
or by code)
allocate piece to a node for exclusive its use
Random Access
Channel not divided allow collisions
ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo
Nodes take turns but nodes with more to send can take longer
turns
Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMA
TDMA (Time Division Multiple Access)
Access the channel in roundsldquo
Each station gets fixed length slot (length = packets trans time) in each
round Each slot called a Time-Slot
Unused slots go idle
Example 6-station LAN 134 have packtes slots 256 idle
18
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
10
Channel Partitioning MAC protocols FDMA
FDMA (Frequency Division Multiple Access)
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
Unused transmission time in frequency bands go idle
example 6-station LAN 134 have packets frequency bands 256 idle
frequ
ency
ban
ds
19
TDM FDM summary
FDM Enables the division of a channel with capacity C
bits per seconds to N sub-channels each gets a different
frequency range and capacity of CN
TDM The division of channel to N sub-channels each
gets CN capacity by giving the entire channel to each of
the N stations for 1N of the time
The division makes each sub channel less busy but the
overall waiting time is bigger by a factor of N compared
to having one channel (Little theorem)
20
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
11
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular satellite etc) standards
unique ldquocoderdquo assigned to each user ie code set partitioning
all users share same frequency but each user has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)21
Random Access MAC Protocols
From here on we‟ll focus on Random Access MAC protocols
When node has packet to send
Transmit at full channel data rate R
No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo
Random access MAC protocol specifies
How to detect collisions
How to recover from collisions
Examples of random access MAC protocols
ALOHA
slotted ALOHA
CSMACD (Ethernet)
CSMACA (Wireless)
22
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
12
Aloha Protocol
Invented at the ‟70s in Hawaii
Intended for Radio networks but suitable for every
network where the station can listen to the channel while
broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If
collision is detected between frames back off and try
again later
If two frames are broadcast at the same time on the channel a
collision occurs and the both need to retransmit
23
Aloha Protocol
All hosts
Transmit on one frequency (fT)
Receive on other frequency (fR)
There is a central node which repeats whatever it receives
from fT frequency on the other fR frequency
The central node used as a repeater
Collisions are detected by the hosts
Receiving corrupted data (host knows what should be
received)
24
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
13
Aloha Protocol
1 Accept a new frame arrives
2 Transmit immediately and listen
If a collision occurred wait a random time and
repeat to stage 2
Otherwise go back to stage 1 to handle a new frame
25
Aloha Protocol
Simple
Robust against failure of a host
Distributed (excluding the central node which uses as a
repeater)
High load implicates low utilization of the channel and high
delays
26
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
14
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a
station starts transmitting in a time unit is p
Then The probability that exactly one node transmits
in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation
yields maximum point at with utilization
of
Trivial improvement
Why be vulnerable for 2 time units
Synchronize and use slotted time May transmit only
at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
15
Slotted Aloha
Assumptions
all frames same size
time is divided into equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation
when node obtains fresh frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros
single active node can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
30
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
16
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary
Very popular at the beginning of time (ie 70s to 80s)
Very simple to handle
Lots and lots of basic probabilities calculations for
students
Major problem Nodes don‟t check what‟s going on in the
channel each acting on its own No manners
32
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
17
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
Protocol
1 Listen to the channel
2 If channel sensed idle transmit entire frame
3 If channel sensed busy defer transmission by
1-Persistent CSMA
Wait until channel is quiet and transmit immediately If collision
occurs wait a random time and listen again (go to 1)
Non-Persistent CSMA
Wait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission
CSMA human analogy don‟t interrupt others
33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA
When transmitting try to sense if there is a collision
collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection
easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
18
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots
adapter doesn‟t transmit if it
senses that some other
adapter is transmitting that is
carrier sense
transmitting adapter aborts
when it senses that another
adapter is transmitting that is
collision detection
Before attempting a
retransmission adapter
waits a random time that
is random access
36
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
19
Unreliable connectionless service
Connectionless No handshaking between sending and
receiving adapter
Unreliable receiving adapter doesn‟t send acks or nacks to
sending adapter
stream of datagrams passed to network layer can have gaps
gaps will be filled if app is using TCP
otherwise app will see the gaps
37
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other
transmitters are aware of
collision 48 bits
Bit time 1 microsec for 10 Mbps
Ethernet
for K=1023 wait time is about
50 msec
Exponential Backoff
Goal adapt retransmission
attempts to estimated current
load
heavy load random wait will be
longer
first collision choose K from
01 delay is K 512 bit
transmission times
after second collision choose K
from 0123hellip
after ten collisions choose K
from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
20
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it
starts to transmit frame If it
senses channel busy waits until
channel idle and then transmits
3 If adapter transmits entire
frame without detecting
another transmission the
adapter is done with frame
4 If adapter detects another
transmission while transmitting
aborts and sends jam signal
5 After aborting adapter enters
exponential backoff after
the mth collision adapter
chooses a K at random from
012hellip2m-1 Adapter waits
K512 bit times and returns to
Step 2
40
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
21
Ethernet Minimum Packet Size
41
Summary of MAC protocols
What do you do with a shared media
Channel Partitioning by time frequency or code
Time Division Frequency Division Code Division
Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in
others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211 (Wireless)
42
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
22
LAN technologies
Data link layer so far
MAC protocols The random protocol approach
Next LAN technologies
Addressing
Ethernet
Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address used to get datagram from one interface to another physically-
connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM but can be modified
44
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
23
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure
uniqueness)
Analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
24
ARP Address Resolution Protocol
Each IP node (Host Router)
on LAN has ARP table
ARP Table IPMAC address
mappings for some LAN
nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after
which address mapping will be
forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network)
A wants to send datagram to B
and B‟s MAC address not in A‟s
ARP table
A broadcasts ARP query packet
containing Bs IP address
Dest MAC address = FF-FF-
FF-FF-FF-FF
all machines on LAN receive
ARP query
B receives ARP packet replies to
A with its (Bs) MAC address
frame sent to A‟s MAC address
(unicast)
A caches (saves) IP-to-MAC
address pair in its ARP table until
information becomes old (times
out)
soft state information that
times out (goes away) unless
refreshed
ARP is ldquoplug-and-playrdquo
nodes create their ARP tables
without intervention from net
administrator
48
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
25
Routing to another LANwalkthrough send datagram from A to B via R
assume A know‟s B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B
A uses ARP to get R‟s MAC address for 111111111110
A creates link-layer frame with Rs MAC address as dest frame
contains A-to-B IP datagram
A‟s adapter sends frame
R‟s adapter receives frame
R removes IP datagram from Ethernet frame sees its destined to B
R uses ARP to get B‟s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
50
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
26
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs
first widely used LAN technology
Simple cheap
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology prevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs
Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
27
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network
layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
used to synchronize receiver sender clock rates what is
the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes
if adapter receives frame with matching destination address or with
broadcast address (eg ARP packet) it passes data in frame to net-layer
protocol
otherwise adapter discards frame
MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is
simply dropped Before CRC there is a padding field for the
CRC to pad to 64 bytes
54
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
28
55
Ethernet Technology 10Base2
10 10Mbps 2 under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces physical layer device only
Ethernet technology 100BaseT
10100 Mbps rate latter called ldquofast ethernetrdquo
T stands for Twisted Pair
Nodes connect to a hub ldquostar topologyrdquo 100 m max
distance between nodes and hub
twisted pair
hub
56
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
29
Ethernet technology 100BaseT
Problem must keep minimal packet size when bandwidth
increases
With fixed cable length and propagation speed must
increase minimal size proportionally to bandwidth
increase
Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits
Solutions
Cable length limited to 100m
Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode CSMACD is used short distances between
nodes to be efficiency
uses hubs called here ldquoBuffered Distributorsrdquo
Full-Duplex at 1 Gbps for point-to-point links
10 Gbpsec now
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
30
Hubs
QWhy not just one big LAN
Limited amount of supportable traffic on single LAN
all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum
cable length
large ldquocollision domainrdquo (can collide with many
stations)
limited number of stations 8025 (token ring) have
token passing delays at each station
59
Lecture 360
Hubs (Multiport repeaters Bus in a box)
Physical Layer devices essentially repeaters operating at bit
levels repeat received bits on one interface to all other
interfaces
Can‟t interconnect 10BaseT amp 100BaseT (because
segments don‟t share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design)
with backbone hub at its top
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
31
61
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains node may collide with any
node residing at any segment in LAN
Extends max distance between nodes but all the segments become
one large collision domain
Hub Advantages
simple inexpensive device
Multi-tier provides graceful degradation portions of the LAN
continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
62
Bridges
Link Layer devices operate on Ethernet frames examining
frame header and selectively forwarding frame based on its
destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit
Store and forward element So different types of Ethernet
types can be connected
Transparent no need for any change to hosts LAN adapters
Forwarding is selective do not always flood All connected
segments can work independently in parallel
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
32
63
Bridge Filtering
bridges learn which hosts can be reached through which
interfaces maintain filtering tables
when frame received bridge ldquolearnsrdquo location of sender
incoming LAN segment
records sender location in filtering table
filtering table entry
(Node LAN Address Bridge Interface Time Stamp)
stale entries in Filtering Table dropped (TTL can be 60 minutes)
64
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_port
if (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
33
65
Bridge Learning example
Suppose C sends frame to D and D replies back with frame
to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
66
Bridge Learning example
D generates reply to C sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1 so selectivelyforwards frame out via interface 1
C 1
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
34
67
What will happen with loops
Incorrect learning
A
B
1 1
22
A 1 A 122
68
What will happen with loops
Frame looping
A
C
1 1
22
C C
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
35
69
What will happen with loops
Frame looping
A
B
1 1
22
B2 B1
70
Introducing Spanning Tree
Allow a path between every LAN without causing loops
(loop-free environment)
Bridges communicate with special configuration
messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause
loops)
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
36
How to Construct a Spanning Tree
Bridges run a distributed spanning tree
Algorithm
Select what ports (and bridges) should actively forward
frames
Finding the root flooding
Building a tree Bellman-Ford Algorithm
Can combine efficiently
Standardized in IEEE 8021 specification
71
72
Spanning Tree Requirements
Each bridge is assigned a unique identifier
A broadcast address for bridges on a LAN
A unique port identifier for all ports on all bridges
MAC address
Bridge id + port number
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
37
73
Spanning Tree Concepts
Root Bridge The bridge with the lowest bridge ID value is elected the
root bridge
One root bridge chosen among all bridges
Every other bridge calculates a path to the root bridge
74
Spanning Tree Concepts
Path Cost A cost associated with each port on each bridge
default is 1
The cost associated with transmission onto the LAN
connected to the port
Can be manually or automatically assigned
Can be used to alter the path to the root bridge
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
38
75
Spanning Tree Concepts
Root Port The port on each bridge that is on the path towards the
root bridge
The root port is part of the lowest cost path towards
the root bridge
If port costs are equal on a bridge the port with the
lowest ID becomes root port
76
Spanning Tree Concepts
Root Path Cost The minimum cost path to the root bridge
The cost starts at the root bridge
Each bridge computes root path cost independently
based on their view of the network
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
39
77
Spanning Tree Concepts Designated
Bridge Only one bridge on a LAN at one time is chosen the
designated bridge
This bridge provides the minimum cost path to the root
bridge for the LAN
Only the designated bridge passes frames towards the
root bridge
78
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designatedbridge
B8
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
40
79
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
80
Spanning Tree Algorithm
An Overview 1 Determine the root bridge among all bridges
2 Each bridge determines its root port
The port in the direction of the root bridge
3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
41
81
Spanning Tree Algorithm
Selecting Root Bridge Initially each bridge considers itself to be the root bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
Best one wins
(lowest root IDcostpriority)
82
Spanning Tree Algorithm
Selecting Root Ports Each bridge selects one of its ports which has the
minimal cost to the root bridge
In case of a tie the lowest uplink (transmitter) bridge ID is
used
In case of another tie the lowest port ID is used
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
42
83
Spanning Tree Algorithm
Select Designated Bridges
Initially each bridge considers itself to be the designated
bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge
The bridge and port ID of the bridge the sending bridge considers root
The root path cost for the sending bridge
3 Best one wins
(lowest IDcostpriority)
84
ForwardingBlocking State
Root and designated bridges will forward frames to and
from their attached LANs
All other ports are in the blocking state
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
43
85
Ethernet Switches
layer 2 (frame) forwarding
filtering using LAN addresses
Switching A-to-B and A‟-to-B‟
simultaneously no collisions
large number of interfaces
often individual hosts star-
connected into switch
Ethernet but no collisions
Confused with Ethernet
bridgeshellip
86
Ethernet Switches
cut-through switching frame forwarded from input to
output port without awaiting for assembly of entire frame
slight reduction in latency
combinations of shareddedicated 101001000 Mbps
interfaces
Offers VLANS (Virtual LANs)
Nowadays routers are actually combined with
Ethernet switches
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
44
87
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffic
isolation
no yes yes
plug amp play yes no yes
optimal
routing
no yes no
cut
through
yes no yes
88
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
45
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC
Physical Media
Coax Twisted Pairs Fibers
Link Types
Point-to-point Broadcast Switched
Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo
Portioning MAC protocols
TDMA FDMA CDMA
Random Access MAC protocols
Aloha Slotted Aloha
LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP
LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are divided to other
layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91
46
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN functionality
There are various media which offer different methods for communication
(OSI so called Physical layer)
Each LAN technology uses different MAC (Medium Access Control)
method to use its corresponding medias
What kind of medias do we have
What kind of corresponding MAC protocols do we have
91