Understanding the Real-World Performance of Carrier Sense

MIT Computer Science and Artificial Intelligence LaboratoryNetworks and Mobile Systems

http://nms.csail.mit.edu

Kyle Jamieson, Bret Hull, Allen Miu, Hari Balakrishnan

Introduction

• Carrier sense is a crucial building block for many radio networks– Wireless sensor networks– Wireless local area

networks• Performance depends

on carrier sense

MAC layer

Physical layer

Application layer

Carrier sense

A research direction

• Let’s quantify how well carrier sense performs in real-world radio networks

• Let’s study diverse radio networks and draw high-level conclusions– Modulation type– Network size (number of nodes)– Data rates

Experimental setup

Experimental testbedSensor network 802.11b/g LAN

Nodes 60 3

Radio Chipcon CC1000 Atheros 5212

Data rate 38.4 Kbps 1 to 54 Mbps

Modulation FM narrowband OFDM/DSSS

MAC B-MAC (software) 802.11 (hardware)

Sensor network testbed• 60-node Mica2

sensor network• Six radio hops in

diameter• Ethernet

backchannel to log packet receptions

100 ft.

16,076 sq. ft.

http://mistlab.csail.mit.edu

Outline

• IntroductionImplementing carrier sense• Benefits of carrier sense• Drawbacks of carrier sense• Conclusion

How carrier sense works:energy detectionS

igna

l stre

ngth

(dB

m)

Time

Squelch (“noise floor”)Instantaneous signal strength

Energy detect clearEnergy detect busy

How carrier sense works: other mechanisms

• Preamble detection• Decorrelation amplitude

– Unique to spread-spectrum radios

• AGC unlock– True when AGC adjusts rapidly

Spreading code

×× Received data

Spreading code

Transmit data

PacketPreamble

Outline

• Introduction• Implementing carrier senseBenefits of carrier sense• Drawbacks of carrier sense• Conclusion

Aggregate load lowers link delivery rate

WSN experiment with all nodes sending, carrier sense on

~360 links > 70% at 4 pps

Carrier sense improves link delivery rates

Carrier sense avoids collisions under high load

Only 80 links in the network

are > 70% without CS

Carrier sense improves throughput

Large-scale experiment with an offered load of 1 pps/node

Outline

• Introduction• Implementing carrier sense• Benefits of carrier senseDrawbacks of carrier sense• Conclusion

Sender-side decision;receiver-side collision

R

S

Will any transmissions

collide with mine?

Carrier sense is at best a heuristic for predicting transmissions’ success

Exposed terminals fool carrier sense

R S S΄ R΄

Carrier sense indicates busy, yet the transmission would have succeeded (S, S’ are exposed terminals)

Missed transmission opportunity

Carrier sense misses transmit opportunities

Large-scale experiment with CS energy detect, 0.25 pps per node

Carrier sense misses transmit opportunities

Large-scale experiment with carrier sense off, 0.25 pps per node

Capture fools carrier sense

R captures B’s transmission despite A’s concurrent transmission

R

A

B

Missed transmission opportunity

Capture prevalent at low bit rates

At some low 802.11 bit rates, node B should disable carrier sense

Collision

Capture

Hidden terminals fool carrier sense

R S’S

Carrier sense is

free!

Carrier sense indicates free, yet both transmissions fail (S, S’ are hidden terminals)

Related work

• Capture-aware MAC– Whitehouse et al., Em-Nets ’05– Priyantha, PhD thesis ‘05

• Channel sampling to infer congestion– CODA, Wan et al., SenSys ’04

• Models to pick carrier sense sensitivity– Yang and Vaidya, INFOCOM ’05

Conclusion and future research

• An experimental evaluation of the benefits and drawbacks of carrier sense

• Algorithm to track correlation between signal strengths and packet reception

• Use a congestion control algorithm: CODA or Fusion [SenSys] and turn off or reduce carrier sense