1. EEL 5718 Computer Communications
Medium Access Control (MAC)
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2. Outline Duplexing Channelization MAC protocols Deterministic Random
Hybrid
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3. Multiple Access Control (MAC)
Data link layer = Logical Link Control + MAC
Logical link control (e.g., ARQ) hides the physical connection from higher layers, while MAC is the part interacting with physical characteristics
If link is dedicated, MAC may not be necessary
What if a link is broadcast in nature? E.g., daily conversation
Need coordination---MAC
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4. MAC
WLAN
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5. MAC
MAC: Medium Access Control or Multiaccess Control or Multiple Access Control
MAC: protocols coordinating the use of shared resource (“channel”)
Classification
centralized vs distributed
deterministic vs random
static vs dynamic
reservation-based vs demand-based
QoS-based or fair sharing
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6. Duplexing Control transmissions in both directions
Forward (downlink)
Reverse (uplink)
Time Division Duplexing (TDD)
forward and reserse use the same channel but at different time assignment
time slots in a frame divided into uplink and downlink
Frequency Division Duplexing (FDD)
forward and reverse transmissions use different frequency channels
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7. Channelization (Deterministic MAC)
Resource available in the network will be shared
time
space
frequency
code
Classification
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
Space Division Multiple Access (SDMA)
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8. FDMA Assigns individual channels to individual users
each assigned channel is exclusively used by that user only
Frequency 1
Guard bands 2 …
M-1 M
Time
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9. TDMA
Divide the radio spectrum into time slots, in each slot, only one user is allowed to either transmit or receive (guard times and preambles will be used)
Frequency
Synchronization intervals … W 1 2 3 M 1
Time
1 Frame
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10. TDMA
Transmission will use “frame” (do not be confused with frame in DLC)
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11. CDMA
Instead of slicing time or frequency, different orthogonal codes can be used, e.g., different languages are used in a party
Use all the spectrum: transmitting power is spread over the whole frequency band
Hide the transmitted signal behind the noise floor
Developed in military applications
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12. CDMA
Each user will use a unique code (orthogonal to other users) for communications (pseudo-random sequence is used to generate codes)
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13. Transmitter from one user
Binary
Information
R1 bps
W1 Hz
Radio
Antenna X X
R >> R1bps
W >> W1 Hz
Unique User Binary
Random Sequence
Digital
Modulation
Signal plus residual
interference
Signals
from all
transmitters X X
Binary
Information
Correlate to
User Binary
Random Sequence
Digital
Demodulation

14. CDMA Rough mathematical illustration:
Transmitted signal: si(t) for user i, obtained from PN code ci(t) orthogonal
At receiver (BS) s(t) = s1(t)+s2(t)+…+ sk(t) BS can recovered the signal from user i using correlation: s(t)ci(t) due to  ci(t)sj(t) dt -> 0
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15. CDMA
However, it has near-far problem: a user near a BS may overtake a far user
Power control must be done: transmitting powers are controlled in such a way that all received power at a BS will be the same
Simple interpretation: draw a comparison between the modulated signal by cos(fct) and a spread signal by a PN signal c(t), all signal magnitudes are decreased over a wider band! c(t) can be regarded as a signal with rich components
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16. SDMA
Control the radiated power/energy for each user in space using spot beams (using the same frequency in different space)
Adaptive antennas
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17. Random Access (Distributed)
Coordination of the same shared medium (channel)
Easy to implement and does not need a central controller (distributed in nature)
Major random MACs
Pure Aloha
Slotted Aloha
CSMA, CSMA/CD, CSMA/CA
Tree splitting algorithm
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18. Pure Aloha
In 1970s, Norm Abramson at Univ. of Hawaii devised random access scheme for packet radio: link library among a few islands via radio
Coordination rule: no coordination! Whenever there is data, transmit at will, hope for the best, if collide, delay for random length of time, retransmit
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19. Throughput
Throughput: the percentage of successful transmissions among all transmission attempts
Basic assumptions
Frame size is constant with transmission time t
Merged traffic (new attempts+retransmits) is Poisson with parameter G
G: frame arrivals per frame-time t
traffic load or offered traffic
Collision is assumed as long as there are overlaps between transmitted frames
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20. Aloha Transmissions
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21. Time Diagram t t0-X t0 t0+X t0+X+2tprop t0+X+2tprop
First transmission
Retransmission t
t0-X t0
t0+X
t0+X+2tprop
t0+X+2tprop
Vulnerable
period
Time-out
Backoff
period
Retransmission
if necessary
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22. Vulnerable Period
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23. Formula
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24. Slotted Aloha
Coordination rule: a little bit coordination, “let’s transmit only at certain time instants”
In 1972, Roberts proposed a simple scheme to double the throughput: divide time into fixed time interval (time slot), each user transmits only at the beginning of each slot; if collide, delay fixed number of slots, retransmits
Vulnerable period is reduced to one frame-time: as long as there is no arrival in a frame period, there is no collision
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25. Time Diagram for S-AlohakX
(k+1)X
t0 +X+2tprop
t0 +X+2tprop+B
Time-out
Backoff
period
Vulnerable
period
Retransmission
if necessary
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26. Throughput
Aloha: Smax=18.4%
Slotted Aloha: Smax=36.8%
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27. Aloha vs Slotted Aloha
Maximum throughput for Aloha is 18.4%, while maximum throughput for slotted Aloha is 36.8%, slotted Aloha doubles the throughput of pure aloha
Catch: slotted Aloha needs synchronization of all users, a global clock!
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28. Carrier Sense Multiple Access
CSMA
Coordination rule: sense before transmit, or listen before you speak
How to sense: detect whether there is any transmission in the channel
Many operating modes
non-persistent
1-persistent
p-persistent
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29. Non-persistent CSMA A station senses the channel
If the channel is busy, it will not continue sensing the channel, instead, it delays a random period of time, repeats the same procedure
If channel is idle, it transmits
If a collision occurs, it will delay a random period of time, starts all over again
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30. 1-persistent CSMA A station (a user, a transmitter) senses the channel
If the channel is busy, it will wait until the channel is idle (sensing all the time)
Whenever it senses the channel is idle, it will transmit
If a collision occurs, it will delay a random period of time, start it all over again
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31. p-persistent CSMA
Observation: if you sense idle, others too, collision is surely occur
p-persist CSMA applies to slotted channels
A station senses the channel
If it senses idle channel, it transmits with probability p, with probability q=1-p, it defers to next slot, repeats the same procedure
If it senses busy channel, it delays a random number of slots, starts all over
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32. Comparison
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33. CSMA with Collision Detection
CSMA/CD, widely used in LAN (IEEE 802.3)
If we know there is a collision, why continue? New improvement: stop transmission if collision is detected
Efficient when frame is long
How collision detection is made? Detect power increase at the receiver
How soon (late) a collision is detected? 2t - double of the propagation time between two farthest stations
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34. CSMA/CD Time Diagram
Time consists of idle period, contention period and transmission period
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35. Performance Throughput frame contention Pmax
Psuccess is maximized at p=1/n:
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36. Performance
Compute contention interval: the average number of minislots that elapsed until a station capture the channel: J is the random number
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37. Binary Exponential Backoff
Algorithm for delayed transmission: random period of time
Binary exponential backoff algorithm:
first time collision, delay either 0 or 1 slot with probability 1/2
i-th time collision, delay either 0, 1, 2,…, or 2^i-1 slots with equal probability
Usually, when i=16, i.e., after 16 consecutive collisions, the controller will throw the towel or wave the white flag
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38. CSMA/CA CSMA/CD does not work in wireless channel
difficult to detect collisions in a radio environment
difficult to control the wireless channel
hidden and exposed terminal problems
CSMA/CA is used in wireless environments
sense before transmit
gain the access right via contention (slotted Aloha)
wait for channel grant of usage
busy-tone may be used
IEEE standard
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39. Tree Splitting Algorithm
Capetanakis (1979) developed a tree splitting algorithm, which can improve throughput to 43%
Idea: when a collision occurs, say in the k-th slot, all nodes not involved in the collision go into a waiting mode, and all those involved in the collision split into two groups. In the next slot, only one group is allowed to transmit, if collision continues, further splitting is done until no collision occurs, then the second group will allowed to transmit in the similar manner, etc.
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40. Tree Splitting Algorithm
Splitting methods
flip a coin, or 50/50 at each node
splitting time of arrivals into half
How to handle new arrivals
delay all new arrivals until collision is resolved
allow new arrivals at end of each collision resolution period
Refer to Bertsekas and Gallager’s Data Networks
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41. Centralized MAC
Scheduling approach, a type of central coordinator has to make a decision on channel access
Classification
deterministic
TDMA, TDMA, CDMA
reservation systems
polling
token-passing protocols
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42. Reservation-based MAC
Any transmission must make reservation
A reservation interval is divided into M minislots, stations use their corresponding minislots to announce their intention to transmit and make reservation accordingly
By listening the reservation interval, stations can determine the order of transmissions
Variable-length packets can be handled if the reservation message packet-length information
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43. Reservation-based MAC
Operations
Reservation
interval
Data
Transmissions r d d d r d d d
time
1 frame
1 frame
Each station has own
minislot for making
reservations r = 1 2 3 M
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44. Variations
A station can reserve more than one slot per packet transmission per minislot
Random access-reservation based protocol
Instead of assigning minislots to stations, using random access such as slotted Aloha to contend for access right on each minislots, then whoever wins at the minislot will transmit accordingly
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45. Polling Used in wired fixed networks Nodes take turns for transmission
A central node is the coordinator
Operations
the central node sends a poll message to node asking whether there is anything to transmit
if yes, transmit in response to the poll
if no, the central node polls the next node
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46. (a)
Shared inbound line
Outbound line
Central
Controller
(b)
(c)
Central
Controller

47. Token Passing Protocols (Distributed Polling)
Used in ring architecture
Tokens (a short packet) is circling around the ring
Operations
whoever seizes the token has the sole right to transmit
when a node finishes its transmission, it will regenerate the token and reinsert it on the ring
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48. Token-Passing Ring listen mode transmit mode input from ring output to
delay
delay
to device
from device
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49. Hybrid MAC
Some kinds of combinations of random access and reservation MACs can create new classed of MAC protocols
contention + contention-free
random access + reservation
MAC + channel condition
MAC + QoS
more
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50. Further Reading Textbook: Chapter 6 Tanenbaum’s book: Chapter 4
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