Wireless MAC MACA Protocols MACAW Idle Sense OAR...

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Wireless MAC Protocols

Presenter: George NychisXi Liu

Outline

Carrier SenseMACAMACAWMACA-BIIdle SenseOARECHOS

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 collisionAvoid corruption of other transmission


Carrier Sense Design

General Components:Automatic Gain Control (AGC) systemBaseband processor

ρ = Instantaneous signal strength


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How to Detect Transmissions

1. Preamble Detection2. AGC unlock indicator3. Energy Detect4. De-correlation amplitude


Can CS be improved?

Exposed TerminalHidden Terminal


Carrier Sense Experiments

60-node wireless sensor network testbedCrossbow Mica2Radio @ 433MHz & 48.4 Kbps

What should we expected?


Carrier Sense Experiment Results


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Do we always need it?

Oblivious to capture effect

Performs poorly with high load

Nodes that sent above the diagonal ρ = σachieved higher throughput


MACA

Addresses hidden terminal and exposed terminal problems

How?Throw away carrier sense!Use RTS / CTS


Collisions in MACA

Preferably, small RTS packet collidesStill chance of data collisionCan we reduce this?

Automatic Power ControlInclude “S-meter” in CTSReduce RTS powerDon’t want to reduce CTS powerNeed extra hardware


MACAW

4 design details1. Contention is at the receiver2. Congestion is location dependent3. Fairness through learning of congestion levels4. Propagate synchronization information about

contention periods


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Fairness in MACAW

Channel capture in MACABackoff doubled every collisionReduce backoff on success

Solution: Copy backoffs


MACAW: Copying Backoffs

This does not always work as wanted


MACAW: Additional Design

Multiple Stream Model

ACK


MACAW: Additional Design

DSBecause carrier sense disabled

RTS

CTS

Doesn’t hear CTS

Hears RTSDS

Hears DS

DATA


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RTSDSDATARTS

RRTSProblem:

CTS

RTS

Cannot send CTSBackoff Increases

ACKRRTS

RRTS prevents P2 from responding

RTS

CTS

DSDATARTS

RTS lost

X


MACAW: Conclusions

8% extra overhead for DS and ACK37% improvement in congestion

Future work:Multicast supportCopying backoff


MACA-BI

Simplified version of MACAMACAW: 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


MACA-BI Design

Eliminate RTSReplace CTS with RTR (Ready to Receive)


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MACA-BI: Benefits

Reduced turnaround timeKeeps MACA functionalityData collision free just like MACALess vulnerable to control packet corruption (half as many control packets)Receiver driven allows traffic regulation


MACA-BI: Data Collision Free

1. C transmits a data packet to B2. C is transmitting to D, and B transmits an RTR to A3. C is receiving an RTR from D when B sends an RTR to A


MACA-BI: Results

Minimal hidden terminal, CSMAhas less overheadHidden terminal problems, whichis contention at the receiver


Idle Sense - Problems of 802.11 DCF

Short-term unfairnesscolliding hosts will be penalized

No ACK != collisionSlow hosts limits the throughput of fast hostscapture leads to long-term unfairness


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Idle Sense – Basic Idea

Make hosts use similar Contention Window (CW)instead of exponential backoffBetter fairness

Adapting CW to varying traffic conditionsHosts observe the number of idle slots between two transmission attemptsIntuitively, this is an indicator of how many hosts are currently contendingAdjust CW to an optimal value accordinglyAll hosts converge to a similar CW


Idle Sense - Analysis Result


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

If ni > nitarget , then CW <- CW / α

AIMD has the converging property


Idle Sense - PropertiesDecouple contention control with frame loss

Solves the capture effect problemNo exponential backoff due to bad channel qualityRate adaptation protocol

Similar CWshort-term fairness

Time fairnessScaling CW according to transmission rate

Fully distributedand no information exchange


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Idle Sense - Throughput


Idle Sense - Fairness


Idle Sense - Convergence Speed


Idle Sense - Time Fairness


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Rate Adaptation Protocols

Auto Rate Fallback (ARF):senders attempt to use higher transmission rate after consecutive successesrevert to lower rate after failures

Receiver Based Auto Rate (RBAR):receiver measures channel quality piggyback in CTSsender decide transmission rate according to this information

Idle Sense


Opportunistic Auto Rate (OAR)

Observation:Coherence time (duration where hosts have better channel quality) is at least several packet time

IdeaIf the channel is of high quality, user can transmit multiple packetsTemporal fairness vs. throughput fairness


OAR - Implementation Issues

How to estimate channel conditionUse ARF, RBAR, Idle Sense

How to transmit several packetUtilize 802.11 fragmentationset more fragments bitclear fragment number subfield


OAR - Benefits

Channel is better utilized, then better throughputNo RTS/CTS for subsequent packetsReduce contention time per packetTime fairness


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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


ECHOS - Three Ranges in Wireless

Obtained from QualNetRate = 2MbpsCST = -93dBmPower = 15dBmPr = kPt / d4

transmission rangeinterference rangecarrier sense range


ECHOS - Example

Node 5 & 7 can hear only each otherNode 3 & 9 can hear only each otherNode 1 hear all transmissions


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


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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


ECHOS - AP-CST

Basic idea: choose the appropriate CST to be used by each AP and stationsReducing CST may allow several flows to operate together without interference

StationsGiven 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

APMinimum SNR of the AP at its clients decides the CST of AP


ECHOS - RNC-SCMeasure and determine if a cell is overloaded

MAC service time: time between frame is submitted and ACK is acknowledgedaverage over all users

Choose client and secondary channelOnly when secondary AP and client can operate at low transmit power


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)

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Q & A

Thanks!

Questions

Why hasn’t MAC changed much?

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