1. Wireless Medium Access Control Protocols
CS 851 Seminar
University of Virginia

2. Why Need MAC ?
Wireless medium is an open, shared, and broadcast medium
Multiple nodes may access the medium at the same time
Medium Access Control Protocol:
Define rules to allow distributed nodes to communicate with each other in an orderly and efficiently manner B C A D

3. Ideal MAC Protocol Limited Delay High Throughput Fairness Stability
Scalability
Robustness against channel fading
Low power consumption
Support for multimedia

4. Wireless MAC Issues Half-Duplex Operation Time Varying Channel
Burst Channel Errors
Location Dependent Carrier Sensing
Hidden Terminal
Exposed Terminal
Capture

5. Hidden Terminal Problem
Node B can communicate with A and C both
A and C cannot hear each other
When A transmits to B, C cannot detect the transmission using the carrier sense mechanism
If C transmits to D, collision will occur at B A B C D

6. Exposed Terminal Problem
Node C can communicate with B and D both
Node B can communicate with A and C
Node A cannot hear C
Node D can not hear B
When C transmits to D, B detect the transmission using the carrier sense mechanism and postpone to transmit to A, even though such transmission will not cause collision A X B C D

7. Capture Effect A D B C
Power
Difference
Of A and
D signals
A and D transmit simultaneously to B, the signal strength received by B from D is much higher than that from A, and D’s transmission can be decoded without errors. This will result unfair sharing of bandwidth.

8. Classification of Wireless MAC Protocols
Guaranteed
access
Wireless MAC Protocols
Distributed Mac
Protocols
Centralized MAC
Random

9. Distributed MAC Protocols
Collision avoidance mechanisms
Collision avoidance with out-of-band signaling
Collision avoidance with in-band control messages
Two distributed random access protocols
DFWMAC: Distributed Foundation Wireless MAC (used in IEEE )
EY-NPMA: Elimination Yield-Nonpreemptive Priority Multiple Access (used in HyperLan)

10. Centralized MAC Protocols
Work for centralized wireless networks
Base station has explicit control for who and when to access the medium
All nodes can hear from and talk to base station
All communications go through the base station
The arbitration and complexity are in base station

11. MACA: A New Channel Access Method for Packet Radio Phil Karn 1990

12. Goals , New Ideas, and Main Contributions
Try to overcome hidden & exposed terminal problems
New idea:
Reserve the channel before sending data packet
Minimize the cost of collision (control packet is much smaller than data packet)
Main Contribution:
A three-way handshake MAC protocol : MACA
CSMA/CA  MA/CA  MACA

13. Fundamental Assumptions
Symmetry
A can hear from B  B can hear from A
No capture
No channel fading
Packet error only due to collision
Data packets and control packets are transmitted in the same channel

14. Three-Way Handshake A sends Ready-to-Send (RTS)
B responds with Clear-to-Send (CTS)
A sends DATA PACKET
RTS and CTS announce the duration of the data transfer
Nodes overhearing RTS keep quiet for some time to allow A to receive CTS
Nodes overhearing CTS keep quiet for some time to allow B to receive data packet
CTS (10)
CTS: Clear To Send
RTS (10)
RTS: Request To Send
DATA D
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on. B A C E

15. More Details for MACA Note: no carrier sense
A sends out RTS and set a timer and waits for CTS
If A receives CTS before timer go to zero, OK
Otherwise, A assumes there is a collision at B
Double the backoff counter interval
Randomly pick up a timer from [1,backoff counter]
Send next RTS after timer go to zero
B sends out CTS, then set a timer and waits for data packet
If data packet arrives before timer go to zero, OK
Otherwise, B can do other things
C overhears A’s RTS, set a timer which is long enough to allow A to receive CTS. After the timer goes to zero, C can do other things
D overhears B’s CTS, set a timer which is long enough to allow B to receive data packet.
E overhears A’s RTS and B’s CTS, set a timer which is long enough to allow B to receive data packet.
RTS and CTS can also contain info to allow sender A to adjust power to reduce interference
Note: no carrier sense

16. Hidden Terminal Problem Still Exists (1)
Data packet still might suffer collision
RTS
CTS
RTS
DATA
RTS B C A
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on.

17. Hidden Terminal Problem Still Exists (2)
Data packet still might suffer collision
DATA
RTS
CTS
CTS
RTS
DATA
RTS B C A E
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on.

18. Exposed Terminal Problem Still Exists
Node C can not receive CTS
RTS
DATA
CTS
RTS
CTS B A C
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on.

19. Summary MACA did not solve hidden & exposed terminal problems
MACA did not provide specifications about parameters
What are RTS, CTS packet sizes ?
How to decide timers?
What is initial backoff window size?
A lot things need to do if using MACA

20. MACAW: A Media Access Protocol for Wireless Lan’s V. Bharghavan, A
MACAW: A Media Access Protocol for Wireless Lan’s V. Bharghavan, A. Demers, S. Shenker, and L. Zhang (Sigcomm 1994)

21. Goals, New Ideas, and Main Contributions
This paper refined and extended MACA
New idea: Information sharing
Modified control messages
Four-way handshake (reliable, recover at MAC layer)
Five-way handshake (relieve exposed terminal problem)
RRTS (unfairness)
Modified back-off algorithms
Multiplicative increase and linear decrease (MILD)
Synchronize back-off counter using piggyback message
Multiple stream model (V-MAC)

22. Revisit Hidden Terminal Problem
Data packet still may suffer collision
To recover packet loss at transport layer too slow
Recover at MAC layer is more fast
Need ACK

23. Four-Way Handshake CTS: Clear To Send RTS: Request To Send ACK DATA
Sender sends Ready-to-Send (RTS)
Receiver responds with Clear-to-Send (CTS)
Sender sends DATA PACKET
Receiver acknowledge with ACK
RTS and CTS announce the duration of the transfer
Nodes overhearing RTS/CTS keep quiet for that duration
Sender will retransmit RTS if no ACK is received
If ACK is sent out, but not received by sender, after receiving new RTS, receiver returns ACK instead of CTS for new RTS
CTS(T)
CTS: Clear To Send
ACK
RTS(T)
RTS: Request To Send
DATA
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on.
destination
source

24. Comparison with ACK and without ACK

25. Revisit Exposed Terminal Problem
RTS/CTS/DATA/ACK can not solve exposed terminal problem
When overhearing RTS, the node needs to wait longer enough to allow the data packet being completely transmitted even it does not overhear CTS
To relieve exposed terminal problem,
Let exposed terminal know the DATA packet does be transmitted
Extra message DS (data send)
Five Handshaking to let exposed terminal know how long it should wait

26. Five-Way Handshake CTS: Clear To Send RTS: Request To Send
Sender sends Ready-to-Send (RTS)
Receiver responds with Clear-to-Send (CTS)
Sender sends DATA SENDING (DS)
Sender sends DATA PACKET
Receiver acknowledge with ACK
RTS and CTS announce the duration of the transfer
Nodes overhearing RTS/CTS keep quiet for that duration
CTS
CTS: Clear To Send
ACK
RTS
RTS: Request To Send DS
DS: Data Sending
DATA
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on. B A C

27. Comparison with DS and without DS

28. Comparison with DS and without DS
RTS
CTS
RTS
DATA
CTS
ACK
RTS P1 P2 B2 B1
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on.

29. Unfairness CTS RTS DATA ACK RTS
Using RTS/CTS/DATA/ACK or RTS/CTS/DS/DATA/ACK might cause unfairness
A sends data to B; D sends data to C
A and D have enough data to send
C can hears from B and D, but not A
B can hear from A and C, but not D
A is in luck and gets the channel
D sends RTS and times out
Backoff window for D repeatedly doubles
For the next transmission:
A picks a random number from a smaller window
Unequal probability of channel access
Throughput for flow A B > 90 %
Throughput for flow D  C ~ 0%
RTS
CTS
RTS
DATA
ACK
RTS
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on. B A C D

30. Request for RTS (RRTS)
Try to solve unfairness by having C do the contending for D
RRTS
RRTS: Request for RTS
ACK
CTS
RTS
RTS
DATA
RTS B A
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on. C D

31. Comparison with RRTS and without RRTS (1)

32. Comparison with RRTS and without RRTS (2)

33. Multiple Stream Model (V-MAC)
Node
Single Stream MAC
MAC
Node
Multiple Stream MAC
Single stream model merge traffic from different flows into a mixed stream and uses a single MAC
Multiple stream model uses multiple MAC (one flow one MAC) to achieve fairness
This idea was used by Intersil Company to proposed a new MAC for IEEE e in 2001

34. Comparison V-MAC and MAC

35. Backoff Algorithms
When collision occurs, node A pick up a random number T from [1,Bo], then retransmits RTS after T time unit
How to determine Bo
After each collision Bo_new = Fun_inc(Bo_old)
After each successful transmission Bo_new = Fun_dec(Bo_old)
Binary exponential backoff (BEB) algorithm
Fun_inc(Bo_old)=min{2*Bo_old, Bo_max}
Fun_dec(B_old)=Bo_min
Multiplicative increase linear decease (MILD)
Fun_inc(Bo_old)=min{1.5*Bo_old, Bo_max}
Fun_dec(B_old)=max{Bo_old -1, Bo_min}

36. Information Sharing in Backoff Algorithms
When a node sends a packet, it embeds its current backoff counter in the packet header. Other nodes which overhears the packet copy the value as itself backoff counter
Key idea: all nodes have the same backoff counter to achieve fairness

37. Comparison BEB and BEB-Copy

38. Comparison BEP-COPY and MILD-Copy

39. Evaluation of MACAW

40. Evaluation of MACAW

41. Open Problems How to design a good backoff algorithm?
Adaptive MAC to achieve fairness in ad-hoc networks
Do upper layer operations need to tightly relate to MAC?
Reliable multicast MAC in ad-hoc networks

42. Evaluation of MACAW

43. Evaluation of MACAW

44. Per-Destination Backoff

45. What Are We Going to Learn?
Understand and appreciate fundamental principles in Wireless Medium Access Control Protocols
Basic theory
A real MAC protocol: IEEE b (next week)
Open problems

46. Hidden Terminal Problem Still Exists
Data packet still might suffer collision
RTS
CTS
RTS
DATA
RTS B C A
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on.
Four-way handshake (add ACK message)
Sender will redo RTS/CTS/DATA/ACK if no ACK is received
If ACK is sent out, but not received by sender, after receiving new RTS, receiver returns ACK instead of CTS for new RTS

47. Request for RTS (RRTS) CSMA/CA with binary exponential backoff
IEEE data transmission is accomplished via a four-way handshake
CTS
CTS: Clear To Send
RTS
RTS: Request To Send
DATA
A BR is a region in which all nodes are in radio range of one another.
And try to access the channel through a carrier sense medium access
protocol.
In IEEE the nodes communicate through a 4 way handshake.
The source sends a request to send, which is responded by a clear to
Send message from the destination. Then the source transmits the data
And finally the destination sends the acknowledgment. All other nodes
Defer while this transmissions are going on.
destination
source

48. Some Background
CSMA/CD (carrier sense multiple access/ collision detection)
Every node senses the carrier before transmitting
If the carrier is not clear, the node defers transmission for a specified period. Otherwise, transmits
While transmitting, the sender is listening to carrier and sender stops transmitting if collision has been detected
Due to hidden & exposed terminal problem
Contention/collision will occur at receiver side
Carrier sense (send side) approach is inappropriate for wireless networks