CMAP: Harnessing Exposed

Terminalsin Wireless Networks

Mythili Vutukuru

Joint work with Kyle Jamieson and Hari Balakrishnan

2

The Problemu

v

x

y

Which transmissions concurrently? Increase throughput by maximizing

concurrency.

Xz

3

u

v yz

Exposed terminal problem.

Today’s Solution: CSMA

x

Energy > carrier sense threshold

4

Key Insight Existing solutions: rules to predict which

concurrent transmissions increase throughput.

Instead, watch and discover which concurrent transmissions increase throughput.

5

CMAP: Conflict Map Map of conflicting transmissions based on

empirical evidence. Built in distributed, online manner.

Exposed terminals – 2x gain on CSMA.

u

v yz

x u v & x y NO!

6

Roadmap CMAP Design

• Conflict maps

• ACK & Backoff Policy

Implementation

Evaluation

7

What is a “conflict”?u

v y

x

Conflict – throughput lower when concurrent. Loss rate threshold to decide conflicts.

No Conflict.Conflict.

50% loss

8

Discovering Conflictsu

v y

x

Loss rate of uv when x is concurrent

>50% then infer conflict at v. Conflict entries timed out periodically.

When u transmits to me, x causes interference.

pq

9

Populating the Conflict Map

u

v y

x

When u transmits to me, x causes interference.

Do not transmit to v when x anyone.

Do not transmit to anyone when u v.

Conflict map

z

10

Channel Access Decisions Nodes always overhear channel.

Consult conflict map before transmission.

Carrier sense always disabled!

11

Roadmap CMAP Design

• Conflict maps

• ACK & Backoff Policy

Implementation

Evaluation

12

Windowed ACKs

Sliding window of packets at sender.

u

yz

xX

13

Backoff Policy

Cannot defer when hidden terminals. Exponential backoff. When loss rate in ACKs > threshold.

u

v y

xDo not transmit to v when x anyone.

u must hear x.

14

Roadmap CMAP Design

• Conflict maps

• ACK & Backoff Policy

Implementation

Evaluation

15

Implementation Challenges1. At receiver: Identify colliding senders.

2. At sender: Identify ongoing transmissions.

PHYMAC

Trailer

16

Implementation Options Software radios – Partial Packet Recovery.

[Jamieson and Balakrishnan, SIGCOMM 2007]

Commodity hardware – separate header and trailer packets.

Header pkt Trailer pkt

17

Prototype Implementation

MadWifi DriverAtheros 802.11 card

Conflict Maps ACKs & Backoff

CSMA, ACKs & Backoff disabled.

Click Kernel Module

PHYMAC

18

Roadmap CMAP Design

• Conflict maps

• ACK & Backoff Policy

Implementation

Evaluation

19

Evaluation

50-node 802.11a indoor testbed. Does CMAP improve throughput by

increasing concurrency?

20

Two Senders In Range Senders in range. 1400-byte UDP @ 6 Mbps. 50 unique sets of four nodes. CMAP, CSMA, no CS no acks.

Exposed terminals. Interfering transmissions.

21

Two Senders In Range

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10

Aggregate Throughput (Mbits/s)

CD

F

CSMA No CS, No Acks

CSMA better.

No CS better.

22

Two Senders In Range

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10

Aggregate Throughput (Mbits/s)

CD

F

CSMA No CS, No Acks Ideal

Ideal is max of CSMA & No CS

23

Two Senders In Range

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10

Aggregate Throughput (Mbits/s)

CD

F

Ideal CMAP

CMAP traces ideal curve.

24

Multiple Concurrent Senders

AP-client networks. Tree-based mesh networks.

25

Multiple Concurrent Senders

0

2

4

6

8

10

12

14

16

18

3 4 5 6

Number of concurrent senders

Agg

rega

te th

roug

hput

(Mbi

ts/s

)

CSMACMAP

AP-client – 20-47% better over CSMA.

Mesh – 52% better over CSMA.

26

Hidden terminals – backoff ensures CMAP similar to CSMA.

CMAP without windowed ACKs gets only half the gains → windowed ACKs useful.

CMAP’s gains hold across multiple bit-rates.

More results in the paper

27

Related Work

RTS but no CTS → exposed. [Karn, Shukla et al.]

Offline training to identify exposed terminals. [Mittal and Belding]

u

yz

xRTS

CTS

X

28

Limitations Losses until conflict map entries populated.

Unequal pkt sizes → longer to detect

conflicts.

Cannot detect conflicts when interfering

node’s headers cannot be decoded.

29

Contributions MAC to improve throughput by increasing

concurrency. Key idea: watch and discover conflicts. Experiments show increased throughput.

• 2x improvement over CSMA with exposed terminals.

• ~50% improvement in AP and mesh networks.