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 MACA MACAW MACA-BI Idle Sense OAR Echos
Carrier Sense Design
General Components:Automatic Gain Control (AGC) systemBaseband processor
ρ = Instantaneous signal strength
Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos
How to Detect Transmissions
1. Preamble Detection
2. AGC unlock indicator
3. Energy Detect
4. 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 testbedCrossbow Mica2Radio @ 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 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
Carrier Sense 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
DSBecause carrier sense disabled
RTS
CTS
Doesn’t hear CTS
Hears RTSDS
Hears DS
DATA
Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos
RTSDSDATARTS
RRTS Problem:
CTS
RTS
Cannot send CTSBackoff Increases
ACKRRTS
RRTS prevents P2 from responding
RTS
CTS
DSDATARTS
RTS lost
X
Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos
MACAW: Conclusions
8% extra overhead for DS and ACK 37% improvement in congestion
Future work:Multicast supportCopying 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
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
Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos
MACA-BI: Results
Minimal hidden terminal, CSMAhas less overheadHidden terminal problems, whichis 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 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 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
If ni > nitarget , then CW <- CW / α
AIMD has the converging property
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 fairness
Time fairness Scaling CW according to transmission rate
Fully distributed and no information exchange
Carrier Sense 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 packetsTemporal fairness vs. throughput fairness
Carrier Sense MACA MACAW MACA-BI Idle Sense OAR Echos
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
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 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
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: 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 Sense MACA MACAW MACA-BI Idle Sense OAR Echos
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
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?
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