Download - TANENBAUMComputer Networks 11 Chapter 4 – 1 The Medium Access Sublayer Multiple Access Protocols
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Chapter 4 – 1The Medium Access Sublayer
Multiple Access Protocols
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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
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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
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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
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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
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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
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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
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Frames in Pure ALOHA
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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
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ALOHA cont’dP0 = probability that a frame does not
suffer a collisionS = Probability of a transmission
succeedingS = G P0
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ALOHA Frame Collision Period
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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
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ALOHA Throughput
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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
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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
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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
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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
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Channel Utilization for Random Access Protocols
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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
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CSMA/CD States
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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?
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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
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The basic bit-map protocol
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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%.
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Binary Countdown Example
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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)
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WDMA 2
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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
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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