Medium Access Control for Ad Hoc Wireless Networks: A Survey

1. Medium Access Control for Ad Hoc Wireless Networks: A Survey S. Kumar, V. Raghavan, J. Deng Ad Hoc Networks 4 (2006) 326-358

2. Medium Access Control • Coordinate access from active nodes • Deal with channel contention • Challenges • Wireless communication channel is prone to errors and problems, e.g., hidden/exposed node problems & signal attenuation • This paper provides a comprehensive survey

3. Need for MAC Protocols • Popular CSMA/CD (Carrier Sense Multiple Access/Collision Detection) scheme is not applicable to wireless networks • CSMA suffers hidden node & exposed node problems • Hidden node: A sends to B; C sends to B -> Collision at B • Exposed node: B sends to A; C unnecessarily delays transmission to B • Collision Detection is impossible in wireless communication

4. Classification • Contention-free MAC • TDMA, FDMA, CDMA: Divides channel by time, frequency, or code • More applicable to static networks and/or networks with centralized control • Contention-based MAC • Focus of this survey

5. Classification

6. (Partial) Solutions of Hidden/Exposed Node Problems in CSMA • Use control packets • RTS/CTS (Request-To-Send/Clear-To-Send) • Used by MACA (Multiple Access Control Avoidance) and MACAW (MACA for Wireless LANs) • Use both control packets and carrier sense • CSMA/CA, IEEE 802.11

7. Dynamic Reservation Approaches: Sender- vs. Receiver-initiated • Sender-initiated • A node wanting to send data takes the initiative of setting up the reservation • Most existing schemes • Receiver-initiated • A receiving node polls a potential transmitting node for data • A node can send data after being polled • MACA-By Invitation • A bit more efficient than MACA in terms of transmit & receive turnaround time

8. Single vs. Multiple Channel Protocols • Single channel protocols: Control and packets use the same channel • Multiple channel protocols: Frequency hopping or Separate channels for control & data transmission

9. Frequency Hopping Spread Spectrum (FHSS) • Transmit radio signals by switching a carrier among multiple frequency channels using a pseudo random sequence known to the transmitter and receiver • Spread spectrum signals are resistant to noise & interference • Difficult to intercept • Can share a frequency band with other transmissions • Efficient bandwidth utilization

10. Direct Sequence Spread Spectrum (DSSS) • Phase modulate a sine wave in a pseudo random manner • A pseudo random noise code symbols are called chips • Chip rate is much higher than the information signal bit rate • The sequence of chips is known to the receiver • Resistant to jamming • Multiple users can share a single channel • Relative timing correlation

11. Other criteria for classification • Power-aware • Directional or omnidirectional antennas • QoS-aware • End-to-end (E2E) delay • Packet loss rate (or the probability) • Available bandwidth • Challenges: lack of centralized control, limited bandwidth, node mobility, power/computational constraints, error-prone nature of wireless media

12. I. Non-QoS MAC Protocols • General MAC protocols • MACA (Multiple Access Collision Avoidance) • IEEE 802.11 • MACA-BI • Power aware MAC protocols • PAMAS (Power aware medium access control with signaling) • PCM (Power control medium access control) • PCMA (Power controlled multiple access) • Multiple channel protocols • DBMA (Dual busy tone multiple access), Multichannel CSMA MAC protocol, etc.

13. MACA • If node A wants to transmit to B, it first sends an RTS packet to B, indicating the length of the data transmission to follow • B returns A a CTS packet with the expected length of the transmission • A starts transmission when it receives CTS • RTS, CTS packets are much shorter than data packets • A neighboring node overhearing an RTS defers its own transmission until the corresponding CTS would have been finished • A node hearing the CTS defers for the expected length of the data transmission

14. MACA • MACA can handle hidden node & exposed node problems unsolved by CSMA • Hidden node: A sends to B; C sends to B -> Collision at B ->In MACA, B sends CTS to A; C can hear the CTS & defer its own transmission to B in MACA • Exposed node: B sends to A; C unnecessarily delays transmission to B -> In MACA, C can overhear B’s RTS sent to A but C cannot hear CTS from A; So, C transmits to B

15. MACA • Limitations • MACA does not provide ACK • RTS-CTS approach does not always solve the hidden node problem • Example • A sends RTS to B • B sends CTS to A; At the same time, D sends RTS to C • The CTS & RTS packets collide at C • A transmits data to B; D resends RTS to C; C sends CTS to D • The data & CTS packets collide at B

16. MACAW (MACA for Wireless) • RTS-CTS-DS-DATA-ACK • RTS from A to B • CTS from B to A • Data Sending (DS) from A to B • Data from A to B • ACK from B to A • Random wait after any successful/unsuccessful transmission • Significantly higher throughput than MACA • Does not completely solve hidden & exposed node problems

17. IEEE 802.11 MAC • Very popular wireless MAC protocol • Two modes: DCF (distributed coordination function) & PCF (point coordination function) • DCF is based on CSMA/CA ≈ CSMA + MACA • RTS-CTS-DATA-ACK • Physical carrier sensing + NAV (network allocation vector) containing time value that indicates the duration up to which the medium is expected to be busy due to transmissions by other nodes • Every packet contains the duration info for the remainder of the message • Every node overhearing a packet continuously updates its own NAV • IFS (inter frame spacing) • Short IFS (SIFS), PCF IFS (PIFS), DCF IFS (DIFS), Extended IFS (EIFS)

18. 802.11 (DCF mode) • If channel is idle for DIFS, transmit • If busy, initiate back-off counter (Randomly choose a back-off value between 0 and CW-1) • If channel is idle for DIFS, start decrementing back-off timer; Stop if channel becomes busy • Transmit the frame when counter = 0 • If transmission was successful, set CW = CWmin • If transmission fails (i.e., no ACK), CS = min{2(CW+1)-1, CWmax} • Control packets, i.e., RTS, CTS, and ACK packets, are sent after the medium has been free for SIFS.

19. MACA-BI • Receiver initiated • Reduce number of control packets • RTR (Ready To Receive) & DATA rather than RTS-CTS-DATA • Receiver needs a traffic prediction algorithm • Works well given predictable traffic patterns

20. Power aware MAC protocols • Minimize expensive retransmissions due to collisions • Transceivers should be kept in standby mode as much as possible • Switch to low power mode sufficient for the destination to receive the packet • Two categories • Alternate between sleep and awake cycles • Vary transmission power

21. PAMAS (Power aware medium access control with signaling) • RTS-CTS exchanges over a signaling channeling • Data transmission over a separate data channel • Receiver sends out a busy tone, while receiving a data packet over the signaling channel • Nodes listen to the signaling channel to determine when it is optimal to power down transceivers • A node powers itself off if it has nothing to transmit and its neighbor is transmitting • A node powers off if at least one neighbor is transmitting and another is receiving • Use of ACK and transmission of multiple packets can enhance performance • Radio transceiver turnaround time was not considered

22. PCM: Power Control Medium access control • Send RTS & CTS packets using max available power • Send DATA & ACK with the min power required to communicate between the sender and receiver • Based on the received signal strength of the RTS/CTS packet, adjust the power level for DATA transmission • Drawbacks • Requires rather accurate estimation of the received signal strength, which is hard in wireless communication • Difficult to implement frequent changes in the transmission power level

23. PCMA: Power controlled multiple access • Control transmit power of the sender • The receiver is just able to receive the packet • Avoid interfering other neighboring nodes not involved in the packet exchange • Two channels: one for busy tone & another for data • Request Power To Send (RTPS) & Accept Power To Send (APTS) on the data channel • Every receiver periodically sends out a busy tone • Sender does carrier sensing

24. II. QoS-Aware MAC protocols • Prioritized QoS • Prioritize network flows • Parameterized QoS • Reserve resources for E2E path • A new stream is not admitted if there’s not enough resources -> Already admitted streams are not affected • Soft-QoS: Brief disruptions are acceptable • Dynamic-QoS: Range of QoS • Different applications, different QoS requirements • Audio/video streaming requires reserved share of channel capacity; Soft-QoS with some transient violations is acceptable • A lot to do to support audio/video streaming over a wireless channel • Inter-vehicle communication requires guaranteed delivery of short bursts of data within a delay bound • Very little prior work has been done!

25. QoS-aware MAC protocols • For real-time (RT) applications, MAC protocols should support resource reservation for RT traffic in addition to addressing hidden/exposed terminal problems • Synchronous schemes: TDM variations requiring time synchronization • Asynchronous approaches: No need for global time synchronization

26. Categories of QoS-aware MAC protocols • Use shorter inter-frame spacing & smaller backoff contention window for RT traffic • Extension of 802.11 DCF (e.g., 802.11e) • Black burst contention • RT nodes jam the channel in proportion to waiting time • Observe the channel • Node with the longest jam transmits • Use reserved time slots to provide bounded & required bandwidth for RT traffic; Non-RT traffic is treated like 802.11 • Provide fair channel allocation to different flows unlike 1-3

27. RT-MAC • Drop tardy packets • Check before sending a packet, when its backoff timer expires, and when a transmission is unacknowledged • Eliminate possibility of collision • When a packet is actually sent, include the backoff value in the packet • A node hearing the transmission chooses a different backoff value • Advantage: Significantly reduced the mean packet delay, missed deadlines, and collisions compared to 802.11 • Drawbacks • Contention window may become very large in a network with many nodes • No guarantee on E2E delay (or deadline miss ratio)

28. DCF with priority classes • Use a shorter IFS and backoff time for higher priority data (Deng et al) • Normal node waits for DIFS, but high priority node only waits for PIFS • Small contention window for a high priority flow • EDCF (Enhanced DCF) in 802.11e takes a similar approach to supporting QoS • Use AIFS[TC], CWmin[TC] & CWmax[TC] instead of DIFS, CWmin & CWmax in DCF where AIFS is arbitration inter frame space and TC is traffic class • AIFS[TC] ≥ DIFS can be enlarged for lower priority classes • CWmin[TC] & CWmax[TC] are set differently according to TC • No deterministic guarantee on delay • Normal traffic suffers higher delay

29. Questions?