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Wireless MAC Protocols Presenter: George Nychis Xi Liu Outline Carrier Sense MACA MACAW MACA-BI Idle Sense OAR ECHOS Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos Carrier Sense Basic Idea: listen before you send to avoid collisions Why? Avoid wasted transmissions on collision Avoid corruption of other transmission Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos Carrier Sense Design General Components: Automatic Gain Control (AGC) system Baseband processor ρ = Instantaneous signal strength Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos How to Detect Transmissions 1. 2. 3. 4. Preamble Detection AGC unlock indicator Energy Detect De-correlation amplitude Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos Can CS be improved? Exposed Terminal Hidden Terminal Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos Carrier Sense Experiments 60-node wireless sensor network testbed Crossbow Mica2 Radio @ 433MHz & 48.4 Kbps What should we expected? Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos Carrier Sense Experiment Results Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos MACA Addresses hidden terminal and exposed terminal problems How? Throw away carrier sense! Use RTS / CTS Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos MACAW 4 design details 1. 2. 3. 4. Carrier Sense Contention is at the receiver Congestion is location dependent Fairness through learning of congestion levels Propagate synchronization information about contention periods MACA MACAW MACA-BI Idle Sense OAR Echos Fairness in MACAW Channel capture in MACA Backoff doubled every collision Reduce backoff on success Solution: Copy backoffs Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos MACAW: Copying Backoffs This does not always work as wanted Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos MACAW: Additional Design Multiple Stream Model ACK Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos MACAW: Additional Design DS Because carrier sense disabled RTS CTS DS DATA Hears RTS Doesn’t hear CTS Hears DS Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos RRTS Problem: ACK RRTS RTS Backoff Increases Cannot send CTS RRTS prevents P2 from responding CTS X RTS DS DATA RTS CTS RTS lost Carrier Sense MACA MACAW RTS DATA RTS DS MACA-BI Idle Sense OAR Echos MACAW: Conclusions 8% extra overhead for DS and ACK 37% improvement in congestion Future work: Multicast support Copying backoff Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos MACA-BI Design Eliminate RTS Replace CTS with RTR (Ready to Receive) Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos MACA-BI: Data Collision Free is D, and B transmits AA 3. C 2. is C receiving 1.transmitting C transmits an RTRto afrom data Dpacket when B to sends B ananRTR RTRtoto Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos MACA-BI: Results Minimal hidden terminal, CSMA Hidden terminal problems, which has less overhead is contention at the receiver Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos Idle Sense – Basic Idea Make hosts use similar Contention Window (CW) instead of exponential Better fairness backoff 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos Idle Sense - Analysis Result Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos Idle Sense - Principles If we know N, things are much easier estimating N is undesired Pick a fixed target nitarget for all cases Hosts observe ni If ni < nitarget , then CW <- CW + ε ni > nitarget , then CW <- CW / α AIMD has the converging property If Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Time fairness Scaling fairness CW according to transmission rate Fully distributed and Carrier Sense no information exchange MACA MACAW MACA-BI Idle Sense OAR Echos Idle Sense - Throughput Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos Idle Sense - Fairness Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos Idle Sense - Convergence Speed Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos Idle Sense - Time Fairness Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos OAR - Benefits Channel is better utilized, then better throughput No RTS/CTS for subsequent packets Reduce contention time per packet Time fairness Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos ECHOS - Three Ranges in Wireless Obtained from QualNet Rate = 2Mbps CST = -93dBm Power = 15dBm Pr = kPt / d4 transmission range interference range carrier sense range Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos ECHOS - Example Node 5 & 7 can hear only each other Node 3 & 9 can hear only each other Node 1 hear all transmissions Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos ECHOS - AP-CST Basic idea: Stations choose the appropriate CST to be used by each AP and stations Reducing CST may allow several flows to operate together without interference 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos References [E.1] Phil Karn, "MACA - A New Channel Access Method for Packet Radio", 1990. [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. [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. [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. 2005 [E.6] A. Vasan, R. Ramjee, and T. Woo. Echos - Enhanced Capacity 802.11 Hotspots In Proc. of IEEE Infocom'05, Mar 2005 [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, 1997.630970 [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) Q&A Thanks! Questions Why hasn’t MAC changed much?
