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MAC-Layer Capture:A Problem in Wireless Mesh Networks

with Beamforming Antennas

Romit Roy Choudhury Nitin VaidyaECE and CS Department Dept. of ECE Duke University UIUC

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Wireless Everywhere

Internet

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Internet

Omnidirectional Antennas

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IEEE 802.11 with Omni Antenna

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`` Interference management ``A crucial challenge for dense multihop networks

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Managing Interference

Several approaches Dividing network into different channels Power control Rate Control …

Our Approach …Exploiting antenna capabilities to

improve the performance of wireless multihop networks

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From Omni Antennas …

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To Beamforming Antennas

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To Beamforming Antennas

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Outline / Contribution

Antenna Systems A quick overview

Problem of MAC-Layer Capture

Capture-aware MAC protocol: CaDMAC

Capture-aware routing protocol: CaRP

Discussion and Conclusion

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Antenna Systems

Signal Processing and Antenna Design+

Shift to higher frequency communication=

Several existing antenna systems Switched Beam Antennas Steerable Antennas MIMO-Beamforming antennas

Beams can be formed/controlled electronically

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Antenna Beam

Energy radiated toward desired direction Not necessarily line of sight (LoS)

A

Pictorial Model

Main Lobe (High gain)

Sidelobes (low gain)

A

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Directional Reception

Directional reception = Spatial filtering Interference along straight line joining interferer

and receiver

A BC

D

Signal

Interference

No Collision at A

A B

C

D

Signal

Interference

Collision at A

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Expected Spatial Reuse

A B

C

D

Ideally, both links should be concurrent

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Unfortunately,only one of the transmissions proceeds

Fairness in channel access is severely low

Caused byMAC-Layer Capture

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Typically, idle nodes remain in omni mode When signal arrives, nodes get engaged in receiving

the packet Received packet passed to MAC If packet not meant for that node, it is dropped

MAC-Layer Capture

Wastage because the receiver could accomplish useful communication

instead of receiving the unproductive packet

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Capture Example

A B

C

D

Both B and D are omni whensignal arrives from A

A B

C

D

B and D beamform to receivearriving signal

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Outline / Contribution

Antenna Systems A quick overview

Problem of MAC-Layer Capture

Capture-aware MAC protocol: CaDMAC

Capture-aware routing protocol: CaRP

Discussion and Conclusion

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Impact of Capture

Beamforming for transmission and reception onlyis not sufficient

Antenna control necessary during idle state also

A B

C

DA B

C

D

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Capture-Aware MAC (CaDMAC) D monitors all incident traffic Identifies unproductive traffic

Beams that receive onlyunproductive packets areturned off

However, turning beams offcan prevent useful communication in future

MAC Layer Solution

A B

C

D

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CaDMAC Communication

Transmission / Reception usesonly necessary single beam

When node becomes idle, itswitches back to appropriate

beam pattern Depending upon current time window

A B

C

D

A B

C

D

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CaDMAC turns off beams periodically Time divided into cycles Each cycle consists of

1. Monitoring window + 2. Filtering window

1 2

CaDMAC Time Cycles

1 12 2

cycle

time

All beams remain ON,monitors unproductive beams

Node turns OFF unproductivebeams while it is idle.

Can avoid capture

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During Monitoring window, idle nodes are omni

Spatial Reuse in CaDMAC

A B

C

EF

D

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At the end of Monitoring window CaDMACidentifies unproductive links

Spatial Reuse in CaDMAC

A B

C

EF

D

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During Filtering window use spatial filtering

Spatial Reuse in CaDMAC

A B

C

EF

D

Parallel CommunicationsCaDMAC : 3 DMAC & others : ≤ 2Omni 802.11 : 1

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Is MAC-Layer capture eliminated completely?

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CaDMAC cannot eliminate capture completely

Happens because CaDMAC cannot choose routes Avoiding capture-prone links A routing problem

Discussion

YX

AB

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Routing using Beamforming AntennasIncorporating capture-awareness

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Motivating Capture-Aware Routing

YX

AB

YX

AB

D

S

Z DZ

S

Find a route from S to D, given AB exists Options are SXYD, SXZG

Capture No Capture

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Sum capture costs of all beams on the route Capture cost of a Beam j = how much unproductive traffic

incident on Beam j

Route’s hop count Cost of participation

How many intermediate nodes participate in cross traffic

Measuring Route Cost

X

DS

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Uroute = Weighted Combination of1. Capture cost (K)2. Participation cost (P)3. Hop count (H)

Weights chosen based on sensitivity analysis

Unified Routing Metric

ijipij

routeij

kroute HPU ++= !"

#$#

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CaRP Vs DSR Comparison

1

2

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CaRP Vs DSR

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CaRP Vs DSR

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CaRP Vs DSR

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CaRP Vs DSR

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CaRP Vs DSR

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CaRP Vs DSR

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CaRP Vs DSR

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CaRP Vs DSR

DSR CaRP

CaRP prefers a traffic-free direction“Squeezes in” more traffic in given area

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Performance of CaDMAC

CaDMAC

DMAC

802.11

CBR Traffic (Mbps)

Agg

rega

te T

hrou

ghpu

t (M

bps)

CMAC

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Throughput with CaRP CaRP +CaDMAC

DSR +CaDMAC

DSR +802.11

Agg

rega

te T

hrou

ghpu

t (M

bps)

Topology Number

Random Topologies

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Conclusion

Beamforming antenna protocols Extract benefits from transmission and reception

However, antenna control necessary while idle Else MAC-layer Capture can inhibit spatial reuse

Designed Capture-Aware DMAC (CaDMAC) Enables greater reuse, but still room for improvement

Designed Capture-Aware Routing (CaRP) Significant improvements in throughput and latency

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Thank You

Seehttp://www.ee.duke.edu/~romit/for paper and related material