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) 1.If channel is idle for DIFS, transmit 2.If busy, initiate back-off counter (Randomly choose a back-off value between 0 and CW-1) 3.If channel is idle for DIFS, start decrementing back-off timer; Stop if channel becomes busy 4.Transmit the frame when counter = 0 5.If transmission was successful, set CW = CW min 6.If transmission fails (i.e., no ACK), CS = min{2(CW+1)-1, CW max } 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 1.Use shorter inter-frame spacing & smaller backoff contention window for RT traffic Extension of 802.11 DCF (e.g., 802.11e) 2.Black burst contention RT nodes jam the channel in proportion to waiting time Observe the channel Node with the longest jam transmits 3.Use reserved time slots to provide bounded & required bandwidth for RT traffic; Non-RT traffic is treated like 802.11 4.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], CW min [TC] & CW max [TC] instead of DIFS, CW min & CW max in DCF where AIFS is arbitration inter frame space and TC is traffic class –AIFS[TC] ≥ DIFS can be enlarged for lower priority classes –CW min [TC] & CW max [TC] are set differently according to TC No deterministic guarantee on delay Normal traffic suffers higher delay

29. Questions?