1. Distributed Medium Access Control in Wireless Networks
Nitin Vaidya
© 2009

2. Medium Access Protocol Performance
Depends on
Channel properties
Physical capabilities
Single interface?
One packet at a time?
One channel at a time?
Antenna diversity?
Assume single interface, single channel, single antenna, one packet at a time, unless specified

3. Assumption Low delay-capacity product Propagation delay small compared
to transmission time

4. Distributed Medium Access Protocols
Coordinated access
Each host is somehow “scheduled” to transmit in certain intervals of time
Schedule chosen to avoid excessive interference between simultaneous transmissions
Random access
Each host “randomly” decides when to transmit

5. “Basic” Medium Access Control (MAC) Protocol
Based on Aloha
Simple rule: Transmit packet immediately (if not transmitting already)

6. Basic Protocol Shortcomings No provision for reliability
No detection of “collisions”

7. Basic Protocol

8. Slotted Access
Synchronized slot boundaries: Window of vulnerability = 1 slot

9. Slotted Access
Slot size = L (packet duration) if propagation delay ignored & slot boundaries synchronized
With propagation delay ≤  ,
slot size L +  , to ensure window of vulnerability of 1 slot
With clock skew ≤  , slot size L +  + , to ensure window of vulnerability of 1 slot

10. Back-of-the-Envelope Analysis
Slotted scheme with synchronized slots
Access probability p = probability that a host will (independently) transmit in a given slot
For optimal throughput, p = 1/n

11. Slotted Aloha
When n   , optimal throughput approaches 1/e = 0.36

12. Each slot overlaps with 1 or 2 slots of other hosts
Unsynchronized Slots
Each slot overlaps with 1 or 2 slots of other hosts

13. Unsynchronized Slots
Assume that each slot overlaps with exactly 2 slots of other hosts
Access probability = p
Throughput =
Optimal p = 1/(2n-1)
Optimal throughput for n   = 1/(2e)

14. Lower curve for unsynchronized slots
Numerical Results
Lower curve for unsynchronized slots

15. Basic Aloha
Slotted access improves performance by reducing the window of vulnerability
Alternative: Reduce collisions using carrier sensing

16. Carrier Sense Multiple Access (CSMA)
Listen-before-you-talk
A host may transmit only if the channel is sensed as idle

17. Energy Detection A potential implementation
sample the signal periodically
sum the square of the sampled values
if sum exceeds a threshold, signal present
Requires non-zero delay to correctly sense the channel status
Another possibility: Detect transition from idle to busy, and vice-versa, instead of presence of a signal

18. Energy Detection – Approximation
Implementation using Carrier Sense (CS) threshold Pcs
If received power < CS threshold  Channel idle
Else channel busy

19. Energy Detection – Approximation
Energy detection accuracy also affected by noise & interference
Energy detection non-deterministic due to presence of noise
The approximation assumes that if received signal exceeds Pcs, the transmission will always be sensed

20. Feature Detection
Detect a “well-known” waveform to know if a transmission is taking place
Preamble
Trade-off between complexity & accuracy of sensing with energy detection and feature detection
Our discussion will assume the approximate characterization of energy detection

21. Carrier Sense Multiple Access (CSMA)
D perceives idle channel although A is transmitting A B C D
distance
power
D’s CS Threshold

22. Carrier Sense Multiple Access (CSMA)
D perceives busy channel when A transmits D B C A
distance
power
D’s CS Threshold

23. Trade-Off Large carrier sense threshold  More transmitters
 Greater spatial reuse & more interference
Trade-off between spatial reuse and interference

24. Impact of CS Threshold on Interference
Suppose C transmits even though A is already transmitting A B C D

25. Impact of CS Threshold on Interference
Suppose C transmits even though A is already transmitting A B C D

26. Hidden Terminals

27. Hidden & Exposed Terminals
Collisions may occur despite carrier sensing
Smaller carrier sensing can help
But increases the incidence of exposed terminals ?

28. Hidden & Exposed Terminals
Cannot eliminate all collisions using carrier sensing
Trade-off between hidden and exposed terminals
Optimal carrier sense threshold function of network “topology” and traffic characteristics

29. Collision Detection
Ethernet uses carrier sensing & collision detection (CSMA/CD)
Transmitter also listens to the channel
Mismatch between transmitted & received signal indicates mismatch
Stop transmitting immediately once collision is detected
 Reduces time lost on a collision

30. Collision Detection in Wireless Networks
Receiving while transmitting: Received signal dominated by transmitted signal
Collision occurs at receiver, not the transmitter
 Collision detection difficult at the transmitter without feedback from the receiver

31. Collision Avoidance Use mechanisms to reduce occurrence of collisions
For collisions to occur, transmissions must overlap in time
Two types of overlapping transmissions
Simultaneous
Concurrent

32. Parameter  Worst-case propagation delay 
Worst-case carrier sensing delay 
 =  +  A
Signal sensed C   

33. Simultaneous Transmissions
Transmission by C in this interval cannot
be prevented by carrier sensing
= Simultaneous transmissions

34. Concurrent Transmissions
Overlapping transmissions, which are not simultaneous, are said to be concurrent
Simultaneous and concurrent transmissions desirable if they do not cause too much interference to ongoing transmissions
Trade-off between spatial reuse & interference

35. Reliability Forward error correction Retransmissions
Stop-and-wait protocol
Send a packet
Start timeout interval, and
Wait for Ack
If no Ack within timeout interval, retransmit

36. Retransmission Protocol

37. Overhead of Collisions
Function of
Cost of collisions
Frequency of collisions

38. A Mechanism to Reduce Collision Cost
“Reserve” the wireless channel before transmitting data
Send short control packets for reservation
Collision may occur for control packets, but they are short  lower collision cost
Once channel reserved, data transmission (hopefully) reliable

39. RTS-CTS Control Packets
RTS = Request-to-Send
CTS = Clear-to-Send

40. RTS-CTS Exchange A sends RTS to B
Duration of proposed transmission specified in RTS
B responds with CTS to acknowledge receipt of RTS
Host A sends data to B on receipt of CTS
Other hosts overhearing RTS keep quiet for duration of proposed transmission

41. RTS-CTS Better to use RTS-CTS if …
… data packets large , collisions frequent

42. RTS-CTS
RTS-CTS reduce collision cost due to simultaneous transmissions
If data packets too small, sending RTS-CTS not beneficial
A possible implementation:
Send RTS-CTS only for data packets with size > RTS-threshold

43. Solutions for Hidden Terminals
Busy-tone
Virtual carrier sensing
Carrier sensing mechanism discussed earlier will be referred to as physical carrier sensing, to differentiate with virtual carrier sensing

44. Busy-Tone Mechanism Host A transmits data to B
Host B transmits busy-tone while receiving the data
Host C defers transmission if received busy-tone power exceeds threshold Pcs

45. Busy-Tone Mechanism
If C transmits even though B is sending a busy-tone, then

46. Busy-Tone Mechanism
Interference bound = Pcs , independent of path gain between A and C
Unlike with physical carrier sensing
Sensing threshold for busy-tone Pcs is a good bound on the interference
 Interference can be controlled by controlling Pcs

47. Multiple Interferers
To allow k interferes, Pcs should be smaller by a factor of k

48. Issues with Busy-Tone Mechanism
Overhead of spectrum used for busy-tone
Differences in channel gain of data and busy-tone channel

49. Issues with Busy-Tone Mechanism
Reliability of Ack packets
Busy-tone from host B will protect reception of data at host B. To protect Ack sent to host A:
Host A may also send a busy-tone during the reception of Ack
What about the short interval between completion of data transmission and start of Ack transmission?
Other hosts should sense data and busy-tone channel, and stay idle long enough after the channels go idle to allow for the Ack

50. Issues with Busy-Tone Mechanism

51. Virtual Carrier Sensing
RTS specifies duration of transmission
CTS also includes the duration
Any host hearing RTS or CTS stay silent as shown
RTS
CTS

52. Virtual Carrier Sensing
Host C may not receive RTS from A and still cause collision with Ack reception at host A
Assume SINR-threshold model
 SINR  necessary for reliable delivery

53. SINR for RTS reception at C is upper bounded as
If C transmits while A is receiving an Ack from B, SINR for Ack reception at A is upper bounded as
RTS
CTS

54. It is possible to find path gains for which we have
and 

55. Virtual Carrier Sensing
C’s silent interval below is not adequate to ensure reliable Ack reception at A
Similarly, D’s silent interval not adequate to ensure reliable data reception at B
RTS
CTS

56. Virtual Carrier Sensing - Modification
Greater protection from interference
Reduce book-keeping with multiple nearby transmitters

57. “Space Reserved” by Virtual CS
RTS
RTS
Not circular in reality

58. Physical & Virtual CS
Physical carrier sensing (PCS) & virtual carrier sensing (VCS) may be used simultaneously
Channel assumed idle only if both PCS and VCS indicate that the channel is idle

59. p-Persistence A dialog may consist of Data Data-Ack RTS-CTS-Data-Ack
Dialog begins at a valid transmission opportunity

60. p-Persistence Recall: Slotted access used slot size = packet size
Exploiting physical carrier sensing to reduce slot size
Goal: Choose slot size to minimize number of opportunities for another host to interfere

61. p-Persistence Slot size Back-of-the-envelope analysis:
Consider n nodes that can sense each other
p = access probability for each slot

62. Duration required for a transmission or collision

63. Efficiency of channel access

65. Unsynchronized Slots
A sub-optimal approach

66. Backoff Intervals
Bounded number of valid opportunities skipped before transmitting a packet
Example:
Choose backoff interval uniformly in range [Bmin, Bmax]
Initialize a counter by this value
Decrement counter after each slot in which channel detected idle
On a valid opportunity, if counter 0, transmit

67. Responding to Packet Loss
Packet loss may occur due to
Collision from a simultaneous transmission
Other causes (e.g., concurrent transmission)
To reduce collisions with simultaneous transmissions, access probability should be reduced
May be achieved by increasing the window over which backoff interval is chosen
Exponential backoff : [0,cw-1]  [0,2cw-1]

68. IEEE 802.11 Distributed Coordination Function (DCF)
Physical & virtual carrier sensing (RTS-CTS)
Contention window (cw) : Backoff chosen uniformly in [0,cw-1]
Exponential backoff after a packet loss
Contention window reset to CWmin on a success

69. IEEE DCF

70. IEEE 802.11 Wireless MAC Distributed and centralized MAC components
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
DCF suitable for multi-hop ad hoc networking
DCF is a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol
Inter-frame spacing: SIFS & DIFS

71. IEEE 802.11 DCF Uses RTS-CTS exchange to avoid hidden terminal problem
Any node overhearing a RTS or CTS cannot transmit for the duration of the transfer
Uses ACK to achieve reliability
Any node receiving the RTS cannot transmit for the duration of the transfer
To prevent collision with ACK when it arrives at the sender
When B is sending data to C, node A will keep quite A B C

72. IEEE 802.11 Pretending a circular range RTS = Request-to-Send RTS A BF
Pretending a circular range

73. IEEE 802.11 NAV = remaining duration to keep quiet
RTS = Request-to-Send
RTS A B C D E F
NAV = 10
NAV = remaining duration to keep quiet

74. IEEE
CTS = Clear-to-Send
CTS A B C D E F

75. IEEE
CTS = Clear-to-Send
CTS A B C D E F
NAV = 8

76. IEEE
DATA packet follows CTS. Successful data reception acknowledged using ACK.
DATA A B C D E F

77. IEEE
ACK A B C D E F

78. IEEE
Reserved area
ACK A B C D E F

79. Backoff Interval
When transmitting a packet, choose a backoff interval in the range [0,cw-1]
cw is contention window
Count down the backoff interval when medium is idle
Count-down is suspended if medium becomes busy
When backoff interval reaches 0, (optionally) transmit RTS

80. DCF Example B1 = 25 B2 = 20 B1 = 5 B2 = 15 data wait data wait B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
IFS ignored in figure.
RTS-CTS-Ack not shown.
cw = 32

81. Backoff Interval
The time spent counting down backoff intervals is a part of MAC overhead
Choosing a large cw leads to large backoff intervals and can result in larger overhead
Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously)

82. Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed
IEEE DCF: contention window cw is chosen dynamically depending on collision occurrence

83. Binary Exponential Backoff in DCF
When a node fails to receive CTS in response to its RTS, it increases the contention window
cw is approximately doubled (up to an upper bound)
When a node successfully completes a data transfer, it restores cw to Cwmin
cw follows a sawtooth curve

84. Contention Resolution Overhead
Channel contention resolved using backoff
Nodes choose random backoff interval from [0, CW]
Count down for this interval before transmission
Overhead (time not used for reliable data): Backoff, collisions, RTS/CTS, Ack
Random
backoff
RTS/CTS
Data Transmission/ACK