1. Wireless MAC Protocols Presenter: George Nychis Xi Liu

2. Outline Carrier Sense MACA MACAW MACA-BI Idle Sense OAR ECHOS Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

3. Carrier Sense Basic Idea: listen before you send to avoid collisions Why?  Avoid wasted transmissions on collision  Avoid corruption of other transmission Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

4. Carrier Sense Design General Components:  Automatic Gain Control (AGC) system  Baseband processor ρ = Instantaneous signal strength Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

5. How to Detect Transmissions 1. Preamble Detection 2. AGC unlock indicator 3. Energy Detect 4. De-correlation amplitude Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

6. Can CS be improved? Exposed Terminal Hidden Terminal Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

7. Carrier Sense Experiments 60-node wireless sensor network testbed  Crossbow Mica2  Radio @ 433MHz & 48.4 Kbps What should we expected? Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

8. Carrier Sense Experiment Results Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

9. Do we always need it? Oblivious to capture effect Performs poorly with high load Nodes that sent above the diagonal ρ = σ achieved higher throughput Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

10. MACA Addresses hidden terminal and exposed terminal problems How?  Throw away carrier sense!  Use RTS / CTS Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

11. Collisions in MACA Preferably, small RTS packet collides Still chance of data collision Can we reduce this?  Automatic Power Control Include “S-meter” in CTS Reduce RTS power Don’t want to reduce CTS power Need extra hardware Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

12. MACAW 4 design details 1. Contention is at the receiver 2. Congestion is location dependent 3. Fairness through learning of congestion levels 4. Propagate synchronization information about contention periods Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

13. Fairness in MACAW Channel capture in MACA  Backoff doubled every collision  Reduce backoff on success Solution: Copy backoffs Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

14. MACAW: Copying Backoffs This does not always work as wanted Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

15. MACAW: Additional Design Multiple Stream Model ACK Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

16. MACAW: Additional Design DS  Because carrier sense disabled RTS CTS Doesn’t hear CTS Hears RTS DS Hears DS DATA Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

17. RTSDSDATA RTS RRTS Problem: CTS RTS Cannot send CTS Backoff Increases ACK RRTS RRTS prevents P2 from responding RTS CTS DS DATA RTS RTS lost X Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

18. MACAW: Conclusions 8% extra overhead for DS and ACK 37% improvement in congestion Future work:  Multicast support  Copying backoff Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

19. MACA-BI Simplified version of MACA MACAW: Increased MACA’s overhead  Increased 3-way to 5-way handshake +Turnaround time +Preamble Bits +Control Bits  Turnaround time has more effect with higher channel speeds Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

20. MACA-BI Design Eliminate RTS Replace CTS with RTR (Ready to Receive) Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

21. MACA-BI: Benefits Reduced turnaround time Keeps MACA functionality Data collision free just like MACA Less vulnerable to control packet corruption (half as many control packets) Receiver driven allows traffic regulation Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

22. MACA-BI: Data Collision Free 1. C transmits a data packet to B 2. C is transmitting to D, and B transmits an RTR to A 3. C is receiving an RTR from D when B sends an RTR to A Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

23. MACA-BI: Results Minimal hidden terminal, CSMA has less overhead Hidden terminal problems, which is contention at the receiver Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

24. Idle Sense - Problems of 802.11 DCF Short-term unfairness  colliding hosts will be penalized No ACK != collision Slow hosts limits the throughput of fast hosts capture leads to long-term unfairness Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

25. Idle Sense – Basic Idea Make hosts use similar Contention Window (CW)  instead of exponential backoff  Better fairness Adapting CW to varying traffic conditions  Hosts observe the number of idle slots between two transmission attempts  Intuitively, this is an indicator of how many hosts are currently contending  Adjust CW to an optimal value accordingly  All hosts converge to a similar CW Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

26. Idle Sense - Analysis Result Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

27. Idle Sense - Principles If we know N, things are much easier  estimating N is undesired Pick a fixed target n i target for all cases  Hosts observe n i  If n i < n i target, then CW <- CW + ε  If n i > n i target, then CW <- CW / α  AIMD has the converging property Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

28. Idle Sense - Properties Decouple contention control with frame loss  Solves the capture effect problem  No exponential backoff due to bad channel quality  Rate adaptation protocol Similar CW  short-term fairness Time fairness  Scaling CW according to transmission rate Fully distributed  and no information exchange Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

29. Idle Sense - Throughput Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

30. Idle Sense - Fairness Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

31. Idle Sense - Convergence Speed Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

32. Idle Sense - Time Fairness Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

33. Rate Adaptation Protocols Auto Rate Fallback (ARF):  senders attempt to use higher transmission rate after consecutive successes  revert to lower rate after failures Receiver Based Auto Rate (RBAR):  receiver measures channel quality  piggyback in CTS  sender decide transmission rate according to this information Idle Sense Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

34. Opportunistic Auto Rate (OAR) Observation:  Coherence time (duration where hosts have better channel quality) is at least several packet time Idea  If the channel is of high quality, user can transmit multiple packets  Temporal fairness vs. throughput fairness Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

35. OAR - Implementation Issues How to estimate channel condition  Use ARF, RBAR, Idle Sense How to transmit several packet  Utilize 802.11 fragmentation  set more fragments bit  clear fragment number subfield Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

36. OAR - Benefits Channel is better utilized, then better throughput No RTS/CTS for subsequent packets Reduce contention time per packet Time fairness Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

37. ECHOS (Enhance Capacity 802.11 Hotspots) AP-CST: Access Point (modifies) Carrier Sense Threshold  multiple flows can co-exist by dynamically modifying the carrier sense threshold RNC-SC: Radio Network Controller (uses) Secondary Channels  centralized algorithm that assigns hosts to cells/channels and adjust transmission power Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

38. ECHOS - Three Ranges in Wireless Obtained from QualNet  Rate = 2Mbps  CST = -93dBm  Power = 15dBm  P r = kP t / d 4 transmission range interference range carrier sense range Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

39. ECHOS - Example Node 5 & 7 can hear only each other Node 3 & 9 can hear only each other Node 1 hear all transmissions Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

40. ECHOS - Observations 1 The use of physical carrier sensing with default fixed carrier sensing threshold (CST) can unnecessarily couple together several flows reducing per-flow and aggregate throughput Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

41. ECHOS - Observation 2 The optimum value of CST is that which the carrier sense range of the transmitter just covers the interference range of the receiver Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

42. ECHOS - AP-CST Basic idea:  choose the appropriate CST to be used by each AP and stations  Reducing CST may allow several flows to operate together without interference Stations  Given the signal strength of a station at AP, CST of a station is set to maximum signal strength that can be ignored while attempting to transmit,  Then it is adjusted so that it can hear other stations from inside its own cell – avoid hidden terminal AP  Minimum SNR of the AP at its clients decides the CST of AP Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

43. ECHOS - RNC-SC Measure and determine if a cell is overloaded  MAC service time: time between frame is submitted and ACK is acknowledged  average over all users Choose client and secondary channel  Only when secondary AP and client can operate at low transmit power Carrier SenseMACAMACAWMACA-BIIdle SenseOAREchos

44. References [E.1] Phil Karn, "MACA - A New Channel Access Method for Packet Radio", 1990.MACA - A New Channel Access Method for Packet Radio [E.2] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang. MACAW: A Media Access Protocol for Wireless LANs, In Proc. ACM SIGCOMM, London, U.K., September 1994, pp. 212-225.MACAW: A Media Access Protocol for Wireless LANs [E.3] B. Sadeghi, V. Kanodia, A. Sabharwal, and E. Knightly, Opportunistic Media Access for Multirate Ad Hoc Networks, in Proceedings of ACM MOBICOM 2002.Opportunistic Media Access for Multirate Ad Hoc Networks [E.5] Martin Heusse, Franck Rousseau, Romaric Guillier, and Andrzej Duda. Idle Sense: An Optimal Access Method for High Throughput and Fairness in Rate Diverse Wireless LANs In Proc. of ACM SIGCOMM, Aug. 2005Idle Sense: An Optimal Access Method for High Throughput and Fairness in Rate Diverse Wireless LANs [E.6] A. Vasan, R. Ramjee, and T. Woo. Echos - Enhanced Capacity 802.11 Hotspots In Proc. of IEEE Infocom'05, Mar 2005Echos - Enhanced Capacity 802.11 Hotspots [E.7] F. Talucci, M. Gerla, and L. Fratta. MACA-BI (MACA by invitation) a receiver oriented access protocol for wireless multihop networks. In Proceedings of IEEE PIMRC, 1997.630970MACA-BI (MACA by invitation) a receiver oriented access protocol for wireless multihop networks [E.8] Kyle Jamieson, Bret Hull, Allen K. Miu, Hari Balakrishnan. Understanding the Real-World Performance of Carrier Sense. ACM SIGCOMM Workshop on Experimental Approaches to Wireless Network Design and Analysis (E-WIND)Understanding the Real-World Performance of Carrier Sense

45. Q & A Thanks!

46. Questions Why hasn’t MAC changed much?