1

LAN & MAC (Medium Access Control) protocols

• Two basic types of networks:– Switched networks: transmission lines, multiplexers,

and switches; routing, hierarchical address for scalability.

– Broadcast networks: a single shared medium, simpler, no routing, messages received by all stations, flat address; however, when users try to transmit messages into the medium, potential conflict, so MAC is needed to orchestrate the transmission from various users.

– LAN is a typical broadcast network.

2

Peer-to-peer protocols VS. MAC• Both are to transfer user information despite transmission impairments• For peer-to-peer:

– Main concern: loss, delay, resequencing– Using control frames to coordinate their actions– Delay-bandwidth is important– Involved only two peer processes

• MAC: – Main concern: interference from users– Using some mechanisms to coordinate the access of channel– Delay-bandwidth is important– Need the coordination from all MAC entities, any one does not

cooperate, the communication will not take place.

3

What are going to be discussed

• Introduction to broadcast networks• Overview of LANs: frame format & placement in

OSI.• Random access: ALOHA & CSMA-CD (Carrier

Sensing Multiple Access with Collision Detection ) i.e., Ethernet.

• Scheduling: token-ring.• LAN standards (brief view)• LAN bridges: used to connect several LANs.

4

12

3

4

5M

Shared MultipleAccess Medium

1. Any transmission from any station can be heard by any other stations2. If two or more stations transmit at the same time, collision occurs

Figure 6.1

Multiple access communications

5

Satellite Channel = fin

= fout

Figure 6.3

Satellite communication involves sharing of uplink and downlink frequency bands

6

AC = authentication centerBSS = base station subsystemEIR = equipment identity registerHLR = home location register

wirelineterminal

MSC

PSTN

BSS BSS

STP SS#7HLRVLR

EIRAC

MSC = mobile switching center PSTN = public switched telephone networkSTP = signal transfer pointVLR = visitor location register

Figure 4.52

Cellular networks: radio shared by mobile users and require MAC

7

Multidrop telephone lines

Inbound line

Outbound line

Figure 6.4

Multi-drop telephone line requires access control

Host

Terminals

8

Ring networks

Multitapped Bus

Figure 6.5

Ring networks and multi-tapped buses require MAC

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Figure 6.6

Wireless LAN: share wireless medium and require MAC

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Medium Sharing Techniques

Static Channelization

Dynamic Medium Access Control

Scheduling Random Access

Figure 6.2

Approaches to sharing transmission medium

Partitioned channelsare dedicated to individual users, sono collision at all.Good for steady trafficand achieve efficient usage of channels

Minimize the incidence of collision to achieve reasonableusage of medium.Good for bursty traffic.

Schedule a orderly accessof medium. Good for heaviertraffic.

Try and error. if no collision,that is good, otherwise wait a random time, try again. Good for light traffic.

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MAC and performance

• Shared medium is the only means for stations to communicate

• Some kind of MAC technique is needed• Like ARQs, which use ACK frame to coordinate

the transmission and consume certain bandwidth, the MAC will need to transfer some coordination information which will consume certain bandwidth of shared medium.

• Delay-bandwidth product plays a key role in the performance of MAC (as in ARQs).

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A transmits at t = 0

Distance d meterstprop = d / seconds

A B

B transmits before t = tprop and detectscollision shortlythereafter

A B

A BA detectscollision at t = 2 tprop

Figure 6.7

Delay-bandwidth product and performance

1. : the speed of light, 3*108 meters/second 2. Before A begins to transmit, A listens to medium, if busy, wait; otherwise, do it (suppose t=0) 3. If B wants to transmit after t=tprop A’s transmission has reached B, so B waits and A captures medium

successfully and transmits its entire message.

(suppose two station A and B want to transmit information)

4. If B wants to transmit before t=tprop, it listens and no transmission is going on, so B begins to transmit, then collision occurs. B detects collision shortly, but A detects collision at t=2tprop

5. Therefore, 2tprop is required to coordinate the access for each packet transmitted.

13

MAC efficiency

And suppose average length of packets is L.

Then efficiency in use of the medium is:

Efficiency = L + 2tpropR

L=

1

1 + 2tpropR

L

=1

1+2a

a=tpropR / L i.e., the ratio of (one-way) delay-bandwidth product to the average packet length.Suppose a = 0.01, then efficiency = 1/1.02 = 0.98

a = 0.1, then efficiency = 1 / 2 = 0.50

•Suppose bit rate of medium is R, then number of bits “wasted” in access coordination is 2tpropR.

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Examples of efficiency

• Ethernet (CSMA-CD): – Efficiency = 1/(1+6.44a) where a = tpropR/L.

• Token-ring networks:– Efficiency = 1/(1+a’ ) where a’ = ring-latency in

bits/L where ring-latency contains:• The sum of bit delays introduced at each ring adapter.

• Delay-bandwidth product where delay is the time required for a bit to circulate around the ring.

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(a)

RAM

RAMROM

Ethernet Processor

(b)

Figure 6.10

Typical LAN structure and network interface card

1. NIC is parallel with memory but serial with network2. ROM stores the implementation of MAC3. Unique physical address burn into ROM4. A hardware in NIC recognizes physical, broadcast & multicast addresses.5. NIC can be Set to “promiscuous” mode to catch all transmissions.

A LAN connects servers, workstations,Printers, etc., together to achieve sharing

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Data LinkLayer

802.3CSMA-CD

802.5Token Ring

802.2 Logical Link Control

PhysicalLayer

MAC

LLC

802.11Wireless

LAN

Network Layer Network Layer

PhysicalLayer

OSIIEEE 802

Various Physical Layers

OtherLANs

Figure 6.11

IEEE 802 LAN standards

One LLC and several MACs, each MAC has an associated set of physical layers.MAC provides connectionless transfer. Generally no error control because of relatively error free.MAC protocol is to direct when they should transmit frames into shared medium.

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PHY

MAC

PHY

MAC

PHY

MAC

Unreliable Datagram Service

Figure 6.12

The MAC sublayer provides unreliable datagram service

Important: all three MAC entities must cooperate to provide datagram service, I.e., the interactionbetween MAC entities is not between pairs of peers, but rather all entities must monitor all frames.

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PHY

MAC

PHY

MAC

PHY

MAC

Reliable Packet Service

LLCLLC LLC

A C

A C

Figure 6.13LLC can provide reliable packet transfer service

LLC provides three HDLC services: 1. Unacknowledged connectionless service, recall HDLC hasunnumbered frames; 2. Reliable connection-oriented service in the form of HDLC ABM mode;3. Acknowledged connectionless service, need to add two unnumbered frames to HDLC frame set.

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DestinationSAP Address

Source SAP Address Information

1 byte 1

Control

1 or 2

Destination SAP Address Source SAP Address

I/G

7 bits1

C/R

7 bits1

I/G = Individual or group address C/R = Command or response frame

Figure 6.14

LLC PDU structure and its support for several SAPs

•LLC provides additional addressing, i.e., SAP (Service Access Point). Like PPP, LLC can support several different network connections with different protocols at the same time. •Typical SAPs: IP: 06, IPX: E0, OSI packets: FE etc. •In practice, LLC SAP specifies in which buffer the NIC places the frame, thus allowing the appropriate network protocol to retrieve the data.

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Figure 6.15LLC PDU and MAC frame

Header overhead: TCP, IP: >=20 LLC: 3 or 4

MAC: 26

LLC Header

IP

Data

MAC Header FCS

LLC PDU

IP Packet

MAC frame

IP Header

TCPsegments

Chaos

Orderly

Unreliable

Reliable

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Random Access• Why random access?

– Reaction time (i.e. 2 times of propagation delay) is very important for performance, e.g. in Stop-and-Wait, when reaction time is small (i.e. the ACK will arrive soon) the performance is very good, however, if reaction time is large, then performance is very bad.

– Therefore, proceed the transmission without waiting for ACK and deal with collision/error after the fact, i.e. random access.

• Three types of random accesses:– ALOHA, slotted ALOHA, and CSMA-CD

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ALOHA• Basic idea:

– let users transmit whenever they have data to be sent.

– When collision occurs, wait a random time ( why? ) and retransmit again.

• Differences between regular errors &collision– Regular errors only affect a single station

– Collision affects more than one

– The retransmission may collide again

– Even the first bit of a frame overlaps with the last bit of a frame almost finished, then two frames are totally destroyed.

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tt0t0-X t0+X t0+X+2tprop

t0+X+2tprop

Vulnerableperiod

Time-out Backoff period: B Retransmission if necessary

First transmission Retransmission

Figure 6.16

Suppose L: the average frame length, R: rate, X=L/R: frame time1. Transmit a frame at t=t0 (and finish transmission of the frame at t0+X )2. If ACK does not come after t0+X+2tprop or hear collision, wait for random time: B3. Retransmit the frame at t0+X+2tprop+BTwo modes: collide only from time to time and snowball effect collision

ALOHA random access scheme

Vulnerable period: t0-X to t0+X, (2X seconds) if any other frames are transmitted during the period, the collision will occur.

Therefore the probability of a successful transmission is the probability that there is no additional transmissions in the vulnerable period.

When collision occurs?

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The performance of ALOHA•Let S be the arrival rate of new frames in units of frames/X seconds,

S is also the throughput of the system.

•Let G be the total arrival rate in units of frames/X seconds, G

contains the new and retransmissions and is the total load.

•Assume that aggregate arrival process resulting from new and

retransmitted frames has a Poisson distribution with an average

number of arrivals of 2G frames/2X seconds, i.e.,

P[k transmissions in 2X seconds] = (2G)k

k!e-2G , k=0,1,2,…

S=G*P[no collision] =G*P[0 transmission in 2X seconds]

=G*(2G)0

0!e-2G =G e-2G

Therefore, the throughput of the system is:

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Figure 6.17

Throughput S versus load G for ALOHA

What results can be obtained from the graph?1.peak value at G=0.5 with S=0.1842.for any given S, there are two values of G, corresponding to the two modes: occasional collision mode with S G and frequent collision mode with G >> S

00.020.040.060.080.1

0.120.140.160.180.2

G

S

0.184

Ge-2G

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Slotted ALOHA Synchronize the transmissions of stations

–All stations keep track of transmission time slots and are allowed to initiate transmissions only at the beginning of a time slot.

S=GP[no collision] =GP[0 transmission in X seconds]

=G(G)0

0!e-G =G e-G

Therefore, the throughput of the system is:

Suppose a packet occupies one time slot–Vulnerable period is from t0-X to t0, i.e., X seconds long.

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t(k+1)XkX t0+X+2tprop

t0+X+2tprop

Time-out Backoff period: B Retransmission if necessary

First transmission Retransmission

Figure 6.16

Slotted ALOHA random access scheme

Vulnerable period: t0-X to t0 , i.e., X seconds long

t0=(k+1)X

=nX

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0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.01563

0.03125

0.0625

0.125

0.25 0.5 1 2 4 8

Ge-G

Ge-2G

G

S0.184

0.368

Figure 6.17

Throughput S versus load G for ALOHA and slotted ALOHA

Peak value at G=1 with S=0.368 for slotted ALOHA, double compared with ALOHA.In LAN, propagation delay may be negligible and uncoordinated access of shared medium is possible but at the expense of significant wastage due to collisions and at very low throughput. Throughput of ALOHAs is not sensitive to the reaction time because stations act independently.

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CSMA (Carrier sensing multiple access)

• Problem with ALOHAs: low throughput because the collision wastes transmission bandwidth.

• Solution: avoid transmission that are certain to cause collision, that is CSMA. Any station listens to the medium, if there is some transmission going on the medium, it will postpone its transmission.

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A

Station A begins transmission at t=0

A

Station A captureschannelat t=tprop

Figure 6.19

CSMA random access scheme

Suppose tprop is propagation delay from one extreme end to the other extreme end of the medium.When transmission is going on, a station can listen to the medium and detect it.

After tprop, A’s transmission will arrive the other end; every station will hear it and refrain from the transmission, so A captures the medium and can finish its transmission.

Vulnerable period = tpropBut in ALOHAs, it is X or 2X In LAN,generally, tprop < X

sense

sense

sense

sense

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Three different CSMA schemes

• Based on how to do when medium is busy:– 1-persistent CSMA– Non-persistent CSMA– p-persistent CSMA

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sense channel when want to transmit a packet, if channel is busy, thensense continuously, until the channel is idle, at this time, transmit the frame immediately.

1-persistent CSMA

If more than one station are sensing, then they will begin transmissionthe same time when channel becomes idle, so collision. At this time, each station executes a backoff algorithm to wait for a random time, andthen re-senses the channel again.

Problem with 1-persistent CSMA is “high collision rate”.

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sense channel when want to transmit a packet, if channel is idle, then transmit the packet immediately. If busy, run backoff algorithm immediately to wait a random time and then re-sense the channel again.

Non-persistent CSMA

Problem with non-persistent CSMA is that when the channel becomes idle from busy, there may be no one of waiting stations beginning the transmission, thus waste channel bandwidth,

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sense channel when want to transmit a packet, if channel is busy, then persist sensing the channel until the channel becomes idle. If the channel is idle, transmit the packet with probability of p, and wait, with probability of 1-p, additional propagation delay tprop and then re-sense again

p-persistent CSMA

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.02

0.03

0.06

0.13

0.25

0.5 1 2 4 8 16

32

64

1-PersistentCSMA

0.53

0.45

0.16

S

G

0.01

0.1

1

Figure 6.21 - Part 2

Throughput versus load G for 1-persistent (three different a=tprop/X )

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.02

0.03

0.06

0.13

0.25

0.5 1 2 4 8 16

32

64

Non-PersistentCSMA

0.81

0.51

0.14

S

G

0.01

0.1

1

Figure 6.21 - Part 1

Throughput versus load G for non-persistent (three different a=tprop/X )

1-persistent is sharper than non-persistent.a=tprop/X has import impact on the throughput.When a approaches 1, both 1-persistent and non-persistent isworse than ALOHAs.

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CSMA-CD• When the transmitting station detects a collision, it stops its

transmission immediately, Not transmit the entire frame which is already in collision.

• The time for transmitting station to detect a collision is 2tprop.• In detail: when a station wants to transmit a packet, it senses channel, if it is busy, use one of above three algorithms (i.e., 1-

persistent, non-persistent, and p-persistent schemes). The transmitter senses the channel during transmission. If a collision occurred and was sensed, transmitter stops its left transmission of the current frame; moreover, a short jamming signal is transmitted to ensure other stations that a collision has occurred and backoff algorithm is used to schedule a future re-sensing time.

• The implication: frame time X >= 2tprop, , since X=L/R, which means that there is a minimum limitation for frame length.

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A begins to transmit at t=0

A BB begins to transmit at t= tprop-B detectscollision at t= tprop

A B

A B

A detectscollision at t= 2 tprop-

It takes 2 tprop to find out if channel has been captured

Figure 6.22

The reaction time in CSMA-CD is 2tprop

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0

0.2

0.4

0.6

0.8

1

0.01 0.1 1

Aloha

Slotted Aloha

1-P CSMANon-P CSMA

CSMA/CD

a

max

Figure 6.24

Maximum achievable throughput of random access schemes

1. When a is small, i.e, tprop << X, the CSMA-CD is best and all CSMAs are better than ALOHAs.

2. When a is approaching 1, CSMAs become worse than ALOHA.3. ALOHAs are not sensitive to a because they do not depend on reaction time.

= tprop /X

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Summary of random access schemes

(Continuous) ALOHA: try to send a frame anytime, if collision, wait random time, resend. Vulnerable period: 2X , maximum throughput: 0.184.

Slotted ALOHA: send a frame at the beginning of a time slot. If collision, wait a random time to a new time slot, and resend again.Vulnerable period: X , maximum throughput: 0.368.

1-persistent CSMA: listen before transmission, if busy, continuously listen until channel become idle, then transmit immediately. If collision, wait a random time, re-listen. Vulnerable period: tprop , throughput: 0.53 for a=tprop/X=0.01

Non-persistent CSMA: listen before transmission, if busy, wait a random time, re-listen. if idle, transmit. If collision, wait a random time, re-listen. Vulnerable period: tprop , throughput: 0.81 for a=tprop/X=0.01

p-persistent CSMA: persist listening to the channel until idle. At this time, with probability p, transmit the packet, and with probability of 1-p, do not transmit but wait additional tprop and then re-listen. If collision, wait random time, re-listen. Vulnerable period: tprop throughput dependent on p.CSMA-CD: using any one of above three. Listen during transmission. If collision,

stop its transmission immediately. Vulnerable period: tprop. throughput: >0.90 for a=tprop/X=0.01

Important: a=tprop/X affects performance. Requirement: frame length can not below certain value for given tprop, (the distance of LAN).