Chapter 4 – 1 The Medium Access Sublayer

TANENBAUM Computer Networks 1 1

Chapter 4 – 1The Medium Access Sublayer

Multiple Access Protocols


Broadcast ChannelsAlso called

Multiaccess Channels Random Access Channels

Determining who will use the channel next is a problem

Medium Access Control (MAC) sublayer solves this problem

MAC is a sublayer (bottom part) of data link layer


Static Channel Allocation Usually done by FDM or TDM Not an efficient method for data traffic. E.g.

Let The capacity of a channel be C bps The mean time delay of the channel be T (seconds) Frame arrival rate is a random variable from Poisson

distribution with mean frames/second Frame length is a random variable from exponential

probability density function with mean 1/ bits/frame Then

T = 1 / (C - ) (result from queuing theory) Now, let the channel be divided into N subchannels with

capacity C/N and mean input rate /N TFDM= 1 / ((C/N) – (/N) = N / (C - ) = NT The mean delay is N times worse


Dynamic Channel Allocation Assumptions 1Station Model

Generates frames at a rate of frames/unit time (Frame generation is Poisson Distribution)

Once a frame is generated, the station is blocked until the frame is successfully transmitted

Single Channel Assumption All stations transmit and receive with equal

priority over a unique channel


Dynamic Channel Allocation Assumptions 2Collision Assumption

Overlapping transmission by two or more stations at the same time garbles the frames (collision)

All stations detect collisions There are no errors other than those

generated by collisionsContinuous Time

Frame transmission can begin at any instantSlotted Time

Time is divided into slots Frame transmission always begins with a slot


Dynamic Channel Allocation Assumptions 3Carrier Sense

Stations can tell if the channel is in use LANs generally have carrier sense

No Carrier Sense Stations can not sense the channel before

trying to use it Satellite networks do not have carrier sense


Pure ALOHAUsers transmit any time If there is collision

sender knows about it after a certain time, waits random amount of time, sends the frame again

Contention systems Systems in which multiple users share a

common channel in a way that can lead to conflicts

To maximize throughput, frames have uniform size


Frames in Pure ALOHA


ALOHA AssumptionsFrame time=time to transmit one frameNumber of frames generated in a frame

time is a Poisson Distribution with mean N.If N>1, every frame will suffer a collision0<N<1 is reasonable

Probability of k transmission attempts in a frame time is Poisson with parameter G.

Pr[k]=Gk e-G/k! • For small N, G N• For large N, G>N


ALOHA cont’dP0 = probability that a frame does not

suffer a collisionS = Probability of a transmission

succeedingS = G P0


ALOHA Frame Collision Period


Efficiency of ALOHA

Referring to Fig 4-2, the vulnerable period is two frame times

The probability that no frame is transmitted during this period is e-2G

Pr[0]=e-G in one frame period so P0=e-2G in two frame periods

Therefore S = G e-2G

The maximum of S occurs at G=0.5, S=1/2e


ALOHA Throughput


Slotted ALOHA 1Can only transmit at the beginning of a slotVulnerable period is halvedHence S = G e-G

S peaks at G = 1Probability that a frame avoids a collision is

e-G

The probability of a collision is 1-e-G

Probability of a transmission requiring exactly k attempts is Pk=e-G(1-e-G)k-1


Slotted ALOHA 2Expected number of transmissions, E, per

each created frame is

E = k Pk = ke-G(1-e-G)k-1= d/dG(1-e-G)k =

k=1 k=1 k=1

d/dG (1-e-G)k = d/dG eG = eG

k=1

Conclusion: Performance exponentially degrades by the load


Carrier Sense Multiple Access (CSMA) Protocols ALOHA does not listen to the channel

before it transmits, ending up with poor performance

Carrier Sense Protocols Stations listen the channel if there is any

transmission going on before they transmit

Computer Networks 1 17

Persistent and Nonpersistent CSMA 1-persistent CSMA

Stations transmit with probability 1 whenever they find the channel idle

Nonpersistent CSMA If the channel is idle before the first attempt, transmit If the channel is already in use, wait for a random amount of

time, and then listen to the channel for transmission P-persistent CSMA

Applies to slotted channels If the channel is idle,

transmit with probability p Defer transmission until the next slot with probability q = 1 – p If, in the mean time, someone else transmits, wait a random time

If channel busy Wait for the next slot


Channel Utilization for Random Access Protocols


CSMA with Collision Detection (CSMA/CD)collision Detection

Abort transmission as soon as detect collision If is the time the signal propagates between

two farthest stations, the station has to wait 2 to make sure that no collision has occurred

CSMA/CD model has contention, transmission and idle periods

Contention period is modeled as a slotted ALOHA with slot size 2


CSMA/CD States


Collision-Free ProtocolsAssumptions

There are N stations Each station has a unique address (0 to N-1)

hardwired to it

Question Which station gets the channel after a

successful transmission?


A Bit-Map Protocol (Reservation Protocol)Two rounds of transmission cycle

First Round (Contention Period)Consists of N slots each reserved for a particular

stationIn this period, each station transmits

• 1 if it has a frame to transmit• 0 if it has no frame to transmit

At the completion of the first round everybody knows who wants to transmit

Second Round (Transmission Period)Stations transmit according to the order formed in

the first roundThere will not be any collisions


The basic bit-map protocol


Reservation Protocol Performance :Binary CountdownEach station has a binary station addressA station wanting to transmit broadcasts its

address starting with the high-order bit The bits from each station are boolean

Or’edArbitration Rule

As soon as a station sees that a high-order bit position that is 0 in its address is overwritten by 1, it gives up

Channel Efficiency is d/(d+log2N) If station address is the first field in the frame

then efficiency is 100%.


Binary Countdown Example


Wavelength Division Multiple Access (WDMA) ProtocolsAll optical LANs divide the spectrum into

wavelength bandsEach station is assigned two channels

Narrow channel: Control channel to signal the station

Wide channel: Station outputs data frames

Narrow channel is divided into m time slotsWide channel is divided into n+1 slots

n for data output 1 for status (to indicate which slots on both

channels are free)


WDMA 2


WDMA 3Both connection-oriented and

connectionless traffic is supportedEach station has:

A fixed-wavelength receiver for listening to its own control channel

A tunable transmitter for sending on other station’s control channel

A fixed-wavelength transmitter for outputting data frames

A tunable receiver for selecting a data transmitter to listen to


WDMA Connection Setup Procedure A tunes its data receiver to B’s data channel and

waits for the status slot to learn about a free B control slot (on 4 of A)

A chooses a free control slot and sends a CONNECTION REQUEST (on 2 of A)

B assigns a data slot to A by announcing it in the status slot (on 3 of B, B also tunes 4 to A’s 3)

A reads this announcement and a unidirectional connection from A to B is established (A transmits on 4 in the slot assigned by B)

If the request was for two way communications, B would repeat the same procedure

Chapter 4 – 1 The Medium Access Sublayer

Documents

Transcript of Chapter 4 – 1 The Medium Access Sublayer