Wireless MAC Protocols Presenter: George Nychis Xi Liu

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 collisionAvoid corruption of other transmission


Carrier Sense Design

General Components:Automatic Gain Control (AGC) systemBaseband processor

ρ = Instantaneous signal strength


How to Detect Transmissions

1. Preamble Detection

2. AGC unlock indicator

3. Energy Detect

4. De-correlation amplitude


Can CS be improved?

Exposed Terminal Hidden Terminal


Carrier Sense Experiments

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

What should we expected?


Carrier Sense Experiment Results


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


MACA

Addresses hidden terminal and exposed terminal problems

How?Throw away carrier sense!Use RTS / CTS


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


MACAW

4 design details1. Contention is at the receiver

2. Congestion is location dependent

3. Fairness through learning of congestion levels

4. Propagate synchronization information about contention periods


Fairness in MACAW

Channel capture in MACA Backoff doubled every collision Reduce 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


RTSDSDATARTS

RRTS Problem:

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 ACK 37% improvement in congestion

Future work:Multicast supportCopying backoff


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


MACA-BI Design

Eliminate RTS Replace CTS with RTR (Ready to

Receive)


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


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 unfairness

colliding hosts will be penalized No ACK != collision Slow hosts limits the throughput of fast

hosts capture leads to long-term unfairness


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


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 - 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


Idle Sense - Throughput


Idle Sense - Fairness


Idle Sense - Convergence Speed


Idle Sense - Time Fairness


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


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 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 throughput

No RTS/CTS for subsequent packets Reduce contention time per packet Time fairness


ECHOS (Enhance Capacity 802.11 Hotspots) AP-CST: Access Point (modifies) Carrier

Sense Thresholdmultiple flows can co-exist by dynamically

modifying the carrier sense threshold RNC-SC: Radio Network Controller (uses)

Secondary Channelscentralized algorithm that assigns hosts to

cells/channels and adjust transmission power


ECHOS - Three Ranges in Wireless Obtained from QualNet

Rate = 2Mbps CST = -93dBm Power = 15dBm Pr = kPt / d4

transmission range interference range carrier sense range


ECHOS - Example

Node 5 & 7 can hear only each other

Node 3 & 9 can hear only each other

Node 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


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 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


ECHOS - RNC-SC

Measure and determine if a cell is overloadedMAC 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)

Q & A

Thanks!

Questions

Why hasn’t MAC changed much?

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Wireless MAC Protocols Presenter: George Nychis Xi Liu