1. Week3 The Medium Access Sublayer
Multiple Access Protocols
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2. Overal Internet Architecture

3. MAC Sublayer

4. MAC Architecture

5. Broadcast through a Single Channel
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
Broadcast Channels, also called
Multiaccess Channels
Random Access Channels
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6. Static Channel Allocation
Usually done by FDM or TDM
Not an efficient method for data traffic:
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
Tis means that he average delay is N times worse
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7. Dynamic Channel Allocation Assumptions 1
Station Model: block and wait
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: equal rights
All stations transmit and receive with equal priority over a unique channel
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8. Dynamic Channel Allocation Assumptions 2
Collision 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 collisions
Continuous Time
Frame transmission can begin at any instant of time
Slotted Time
Time is divided into very narrow time slots
Frame transmission always begins with a slot
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9. Dynamic Channel Allocation Assumptions 3
Carrier Sense
Stations can detect 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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10. Pure ALOHA Users transmit any time If there is a 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 must have uniform sizes
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11. Frames in Pure ALOHA
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12. ALOHA Assumptions Frame time=time to transmit one frame
Number of frames generated in a frame time is a Poisson Distribution with mean N.
If N>1, every frame will suffer a collision
0<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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13. ALOHA cont’d P0 = probability that a frame does not suffer a collision
S = Probability of a transmission succeeding
S = G P0
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14. ALOHA Frame Collision Period
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15. Efficiency of ALOHA
Referring to the figure on prev. page, 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 troughput S = G e-2G
The maximum of S occurs at G=0.5, S=1/2e
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16. ALOHA Throughput
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17. Slotted ALOHA 1 Can only transmit at the beginning of a slot
Vulnerable period is halved
Hence S = G e-G
S peaks at G = 1
Probability 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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18. Slotted ALOHA 2
Expected 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= k= 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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19. 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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20. Persistent and Nonpersistent 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

21. Channel Utilization for Random Access Protocols
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22. CSMA with Collision Detection (CSMA/CD)
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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23. CSMA/CD States
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24. Collision-Free Protocols
Assumptions
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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25. A Bit-Map Protocol (Reservation Protocol)
Two rounds of transmission cycle
First Round (Contention Period)
Consists of N slots each reserved for a particular station
In 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 round
There will not be any collisions
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26. The basic bit-map protocol
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27. Reservation Protocol Performance :Binary Countdown
Each station has a binary station address
A station wanting to transmit broadcasts its address starting with the high-order bit
The bits from each station are boolean Or’ed
Arbitration 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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28. Binary Countdown Example
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29. Wavelength Division Multiple Access (WDMA) Protocols
All optical LANs divide the spectrum into wavelength bands
Each 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 slots
Wide 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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30. WDMA 2
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31. WDMA 3
Both connection-oriented and connectionless traffic is supported
Each 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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32. 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
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33. The Medium Access Sublayer
IEEE Standard and Ethernet
Ethernet Frame Structure (Ethernet Encapsulation)
preamble SFD DA SA type
Data
CRC
60 to 1514 bytes
synchronize
the receiver
type
Cyclic Redundancy Check
0800: IPv4 datagram
0806: ARP request/reply
8035: RARP request/reply
86DD: IPv6
start frame
delimiter

34. Ethernet Frame (cont’d)
2 byte type that indicates what kind of data follows, e.g., 0800 for an IP packet
Then the data, maximum 1500 bytes, minimum 46 bytes
Data field must be padded with extra bytes if fewer than 46 bytes are supplied