Lecture 10: DataLink1 Introduction To Computer Networks מבוא לרשתות תקשורת 89-350 Ethernet and Switches פרופ ' אמיר הרצברג Computer Networking: A Top

Lecture 10: DataLink 1

Introduction To Computer Networks

-89מבוא לרשתות תקשורת 350Ethernet and Switches פרופ' אמיר הרצברג

Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2005. ** Group purchase ?? **

Based on foils by Kurose & Ross ©, see: http://www.aw.com/kurose-ross/

My site: http://AmirHerzberg.com

Course site http://www.cs.biu.ac.il/~89-350/

http://www.aw.com/kurose-ross/

http://amirherzberg.com/


Agenda: link layer technologies: Ethernet Hubs, bridges, switches, etc… A bit about:

wireless (IEEE 802.11 LANs) PPP ATM Frame Relay


Ethernet: “dominant” wired LAN technology, using CSMA/CD: Simple, cheap and familiar (first widely used LAN) IP-friendly

Unreliable, connectionless service

Speed/cost options: 10, 100, 1000 Mbps Max cable (segment) length options: 100m to 5000m

Metcalfe’s EthernetSketch; think of `Ether` as a single cable (segment)


Ethernet Frame Structure

Preamble (8 Bytes): “AA AA AA AA AA AA AA AB” used to synchronize receiver, sender clock rates

MAC Addresses (6 Bytes): Pass to net-layer packets (data) from frames with matching

and broadcast addresses; ignore others

Type/Length (2 Bytes): protocol or length: see next... CRC(4 Bytes): checked at receiver, if error is

detected, the frame is simply dropped Data: 46 to 1500 Bytes (if shorter: add pad)

Why? Maximal: small buffer; Minimal: see soon…

(or length)


Type or Length?

Two (main) flavors of Ethernet formats IEEE 802.3 `Standard Ethernet`: Length

Up to 1500 (max length) Identify higher-layer by LLC header (in data)

Ethernet II (DIX): Type Use these 2 bytes to identify higher-layer

protocol (e.g. IPv4=0800) All identifiers > 1500 Identify end of data by CRC hardware

outputting 00-00-00-00

(or length)


Ethernet CSMA/CD algorithm1. Adaptor creates frame2. If channel idle, transmit

frame. If channel busy, transmit when idle.

3. If transmission completes without detecting collision, done!

4. If adapter detects collision while transmitting, aborts and sends jam signal (48 bits)

5. After abort, enter exponential backoff: after the mth collision, choose K at random from {0,1,2,…,2m-1}. Wait K*512 bit times and return to Step 2 heavy load longer wait Bit time: .1μsec for 10

Mbps Ethernet


Ethernet CSMA/CD algorithm – Subtle issues (See T section 4.3)

Why jam signal? Let source know about collision… No/too short jam signal source may not detect

collision

Why source must know of collision? Isn’t it enough for the `2nd source` to detect collision? No! undetected loss result in expensive re-

transmission Also: source may continue to send… no fairness

Why min-frame-length?? To ensure source knows of collision… before source

completes transmitting the frame! How long till collision detected?


Collision Detection Delay

tprop = max signal propagation time (τ in T4.3.3)

Collision detection may take 2tprop (one slot) A begins transmission at time tA

Worst case: B begins `just before` tA+tprop

B detects collision at tA+tprop (too late…)

A detects collision only at tA+2tprop

A

B

Note: better covered in T, sec. 4.3.3; but note diff. notations

tA

tA+tprop

tA+2tprop


CD Delay & Min Segment Size Collision detection may take 2tprop (one slot) Consider 10Mbps Ethernet (`classical`) Max length 2500m, up to four repeaters

Assuming each repeater takes trepeater<4μsec

tprop=2500/(3*108)+4*trepeater<25μsec With 10Mbps, one bit takes 0.1μsec Minimal frame=2*25/0.1=500b<64 Bytes

A

B

Note: better covered in T, sec. 4.3.3; but note diff. notations

tA

tA+tprop

tA+2tprop


Ethernet Efficiency (Heavy Load)

Simplify: k nodes, all send with probability p during each time slot (2tprop)

Success probability A=kp(1-p)k-1

For max success probability, use p=1/k Success probability A1/e as k∞

Same analysis as we did for slotted Aloha

Probability that j slots until success is: (1-A)j-1∙A Average number of slots till success:

Note: better covered in T, sec. 4.3.5; note diff. notations

1j

1jA1jA )(


Ethernet Efficiency (Heavy Load) Average number of slots till success:

Taylor series of f(x)=1/x2; f(i)(x)=(-1)i mod 2 x-2-i (i+1)!

Avg. number of slots till success = A∙A-2=e Time till success: e∙2tprop

1i

i

1j

1j A11i1AA1jA ))(()(

1i

i

1i

ii

2A11i1

i

1A1f1fAf

A

1))((

!

))(()()(

)(

proptrans

trans

et2t

t

efficiency

A מתקבל j=1עבור i=j-1 ולכן עברתי אל

(f(A)-ל)טור טיילור (וצמצום f(x)=1/x2)הצבת

A=1/eכי


Ethernet CSMA/CD efficiency tprop = max prop between 2 nodes in LAN

ttrans = time to transmit max-size frame

Much more efficient at high loads (cf. ALOHA), while still decentralized, simple, and cheap Efficiency goes to 1 as tprop goes to 0

Smaller physical network diameter (distance)

Goes to 1 as ttrans goes to infinity Slower transmission, longer (minimal) frames

Different tradeoffs several Ethernet variants…

transprop tt51

1

/efficiency

e25


Bus Ethernet: 10Base2, 10Base5,…

10: 10Mbps coaxial cable in `bus` topology 10Base2: `thin` cable, 200 meters max segment length 10Base5: `regular` cable, 500 m max segment length Use (up to 4) repeaters to connect segments

repeater amplifies bits from one interface to its other interface(s): physical layer device only! Total length up to 2500 m (efficiency constraints…)

Faster versions also available, e.g. using Fiber


Repeaters: fixing the signal Amplify inversely to the link

Restore original signal Except for harmonics over cut-off frequency…

Decode to restore digital bit stream If noise and distortion are not too bad… Problem: synchronization, latency (delay)

Transmit (hopefully original signal) again…

g(t)

g(t)|7Before Repeater

Recovered signal


10BaseT, 100BaseT and beyond… 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub (why shorter?)

Hubs are physical-layer devices (like repeaters): bits coming in one link go out all other links no frame buffering no CSMA/CD at hub: adapters detect collisions provides net management functionality

Also Gb, 10Gb Ethernets…

hub

nodes


Outline

Ethernet Hubs, bridges, switches, etc… A bit about:



Interconnecting segments Physical layer (same collision domain):

Repeater: amplify for longer distance Hubs: connect multiple segments / hosts

Data link layer (separate collision domains) Bridges: connect and forward frames btw two or

more segments (usually only to dest segment) Switches: connect multiple segments/hosts; allow

efficient simultaneous forwarding (AB, CD,…)• Some switches allow `cut-thru` forwarding (later)• Otherwise, much like multi-port bridges

Network layer: Routers (compare later)


Bridges/Switches Link layer device

stores and forwards Ethernet frames• Exception: cut-thru switches (later)

examines frame header and selectively forwards frame based on MAC dest address

when frame is to be forwarded on segment, uses CSMA/CD to access segment

transparent hosts are unaware of presence of bridges

plug-and-play, self-learning bridges do not need to be configured


Bridges/Switch: traffic isolation

Bridge installation breaks LAN into LAN segments Bridges/Switches filter packets:

same-LAN-segment frames not usually forwarded onto other LAN segments

segments become separate collision domains

bridge collision domain

collision domain

= hub

= host

LAN (IP network)

LAN segment LAN segment


Self learning A bridge/switch has a bridge/switching table entry in table:

(Node LAN Address, Interface, TTL) Bridges/switches learn which hosts can be reached

through which interfaces when frame received, bridge “learns” location of

sender: incoming LAN segment records sender/location pair in bridge table Forward:

• To segment of destination (if known) • Otherwise: to all segments (but of sender)


Learning, Filtering and ForwardingWhen switch receives frame from X to Y on intf I:Add X to segment(I) // learningindex switch table using MAC dest addressif entry found for destination

then{ if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated } else flood

forward on all but the interface on which the frame arrived


Interconnection without backbone

Would work by self learning, but… Not recommended, for two reasons:

- single point of failure at Computer Science hub- all traffic between EE and SE must path over CS segment


Backbone configuration

Recommended !


Disabled

Bridges Spanning Tree for increased reliability, connect segments by multiple bridges multiple

paths to dest with multiple paths, cycles may result - bridges may multiply and forward

frame forever solution: organize bridges in a spanning tree by disabling subset of

interfaces Exercise: design spanning-tree protocol for bridges

Hint: PIF


Bridges vs. Routers Both store-and-forward devices

Routers: network layer devices Bridges: link layer devices

Routers use IP address to route (req. separate net addresses); provide (only) IP service; implement routing

Bridges implement learning & spanning tree; provide LAN services (e.g. QOS) – with some degradation


Routers vs. Bridges

Bridges + and - + Bridge operation is simpler requiring less

packet processing+ Bridge tables are self learning, no loops - All traffic confined to spanning tree, even

when alternative bandwidth is available- Spanning tree is rarely optimized for traffic

- Exercise: demonstrate net where traffic-aware choice of spanning tree is important

- Bridges do not offer protection from broadcast storms


Routers vs. Bridges

Routers + and -+ arbitrary topologies can be supported, cycling is

limited by TTL counters (and good routing protocols)

+ provide protection against broadcast storms- require IP address configuration (not plug and play)- require higher packet processing

bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)


Ethernet Switches Essentially a multi-interface

bridge layer 2 (frame) forwarding,

filtering using LAN addresses Switching: A-to-A’ and B-to-

B’ simultaneously, no collisions

large number of interfaces often: individual hosts, star-

connected into switch Ethernet, but no

collisions!


Ethernet Switches

cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frameslight reduction in latency

combinations of shared/dedicated, 10/100/1000 Mbps interfaces


Typical Organization LAN(without backup switch… add one!)

Dedicated

Shared


Summary comparison

hubs bridges routers switches

traffi cisolation

no yes yes yes

plug & play yes yes no yes

optimalrouting

no no yes no

cutthrough

yes no no yes


Outline

Ethernet Hubs, bridges, switches, etc… A bit about:



Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access):

AB

C

Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each

othermeans A, C unaware of their

interference at B

A B C

A’s signalstrength

space

C’s signalstrength

Signal fading: B, A hear each other B, C hear each other A, C can not hear each other interfering at B Reality is worse than figure!


IEEE 802.11: multiple access Like Ethernet, uses CSMA:

random access carrier sense: don’t collide with ongoing transmission

Unlike Ethernet: Ack, no Collision Detection no collision detection – transmit all frames to completion ACK: to detect loss without collision detection

Why no collision detection? difficult to receive (sense collisions) when transmitting due

to weak received signals (fading) can’t sense all collisions in any case: hidden terminal,

fading And… loss may be due to (higher) error rate of wireless

Goal: avoid collisions: CSMA/C(ollision)A(voidance)


IEEE 802.11 MAC Protocol: CSMA/CA[simplified]

802.11 sender1 if sense channel idle then

- transmit entire frame (no CD)2 if sense channel busy then

- start random backoff timer- timer counts down while channel idle- transmit when timer expires- if no ACK within DIFS, increase

random backoff interval, repeat 2- DIFS>SIFS+2tprop

802.11 receiverif frame received OK then return ACK else ignore (no NACK!)SIFS: max time till Ack sent (SIFS=Short

Inter-Frame Space)

sender receiver

data

ACK

SIFS(e.g.10s)


IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender (when trying to send)1 if sense channel idle for DIFS then

- transmit entire frame (no CD)

[DIFS>SIFS+2Tprop for priority to ACK]

2 if sense channel busy then

- start random backoff timer

- timer counts down while channel idle

- transmit when timer expires

- if no ACK, increase random backoff interval, repeat 2

802.11 receiverif frame received OK

- return ACK

sender receiver

DIFS

data

ACK

SIFS(e.g.10s)


Ack

Data

Next MPDU

Src

Dest

Other

Contention Window

Defer Access Backoff after Defer

DIFS

SIFS

DIFS

Acknowledgment should arrive within SIFS Senders wait for DIFS no-carrier time, then

exponential backoff delay [slot=Tprop]

802.11 MAC OperationData Frames and their ACK


Problems with 802.11 MAC as above Technical problems:

`code` not precise, esp. re backoff, count-down Missing elements…

Spec also allows PCF (Point Coordination Function): polling to coordinate senders to ensure QoS

SIFS < PIFS < DIFS (priorities!) Can’t detect collision while sending…

Wasteful – esp. for long packets Idea for long packets: reserve channel to avoid

collisions – RTS/CTS [optional] mechanism…


RTS/CTS [optional in 802.11 MAC] Sender sends small request-to-send (RTS) to AP

RTSs may collide with each other (but are short) Include indication of length of packet

transmission Receiver broadcasts clear-to-send CTS in response

to RTS CTS heard by all nodes

sender transmits data frame other stations defer transmissions for time

specified in CTS

Avoid data frame collisions completely using small reservation packets!


Collision Avoidance: RTS-CTS exchange

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer


Data Link Layer: Summary

principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing, ARP

link layer technologies: Ethernet, hubs, bridges, switches, IEEE 802.11 LANs, PPP, ATM, Frame Relay

End of introduction to networking… More: advanced networking & security

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Lecture 10: DataLink1 Introduction To Computer Networks מבוא לרשתות תקשורת 89-350 Ethernet and Switches פרופ ' אמיר הרצברג Computer Networking: A Top