Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel

Hidden Terminals Using A Single Transceiver

Nov 2011Neng Xue

Tianxu Wang

Our Thoughts

Eval & Sum

Protocol Details

Introduction

2

3

4

1

Outlines

Outlines

Introduction

Intro

Multiple Channels available in IEEE 802.11

• 3 channels in 802.11b• 12 channels in 802.11a

1

defer

1

2

Single channel Multiple Channels

A

B

CD

A

B

CD

Intro

Multi-Channel Hidden Terminals

A B CRTS

A sends RTS

Channel 1

Channel 2

D

Intro

Multi-Channel Hidden Terminals

A B CCTS

B sends CTS

Channel 1

Channel 2

C does not hear CTS because C is listening on channel 2

D

Intro

Multi-Channel Hidden Terminals

A B CDATA

C switches to channel 1 and transmits RTS

Channel 1

Channel 2

Collision occurs at B

RTS DD

Intro

Related Work

Previous work on multi-channel MAC

Intro

Muliti-channel CSMA ProtocolAssumes N transceivers per host

Capable of listening to all channels simultaneously

Sender searches for an idle channel and transmits on the channel [Nasipuri99WCNC]

Extensions: channel selection based on channel condition on the receiver side [Nasipuri00VTC]

Disadvantage: High hardware cost

Intro

DCA ProtocolAssumes 2 transceivers per host

One transceiver always listens on control channel

Negotiate channels using RTS/CTS/RES

RTS/CTS/RES packets sent on control channelSender includes preferred channels in RTS Receiver decides a channel and includes in CTSSender transmits RES (Reservation)

Sender sends DATA on the selected data channel

Key Ideas

Protocol Details

Multi-Channel MAC (MMAC) Protocol

Proposed Protocol (MMAC) Assumptions

• Each node is equipped with a single transceiver

• The transceiver is capable of switching channels

• Multi-hop synchronization is achieved by other means

Key Ideas

Idea similar to IEEE 802.11 PSM

Divide time into beacon intervals

At the beginning of each beacon interval, all nodes must listen to a predefined common channel for a fixed duration of time (ATIM window)

Nodes negotiate channels using ATIM messages

Nodes switch to selected channels after ATIM window for the rest of the beacon interval

MMAC

Key Ideas

Each node maintains PCLRecords usage of channels inside the transmission range

High preference (HIGH)Already selected for the current beacon interval

Medium preference (MID)No other vicinity node has selected this channel

Low preference (LOW)This channel has been chosen by vicinity nodesCount number of nodes that selected this channel to break ties

Preferred Channel List (PCL)

Key Ideas

Key Ideas

Channel Negotiation

A

B

C

DTime

ATIM Window

Beacon Interval

Common Channel Selected Channel

Beacon

Key Ideas

Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

Time

ATIM Window

Beacon Interval

Common Channel Selected Channel

Beacon

Key Ideas

Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

ATIM-ACK(2)

ATIM ATIM-RES(2)

Time

ATIM Window

Beacon Interval

Common Channel Selected Channel

Beacon

Key Ideas

Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

ATIM-ACK(2)

ATIM ATIM-RES(2)

Time

ATIM Window

Beacon Interval

Common Channel Selected Channel

Beacon

RTS

CTS

RTS

CTS

DATA

ACK

ACK

DATA

Channel 1

Channel 1

Channel 2

Channel 2

Eval

Performance Evaluation

Simulation ModelSimulation Results

Eval

Simulation Modelns-2 simulator

Transmission rate: 2MbpsTransmission range: 250m

Traffic type: Constant Bit Rate (CBR)Beacon interval: 100ms

Packet size: 512 bytesATIM window size: 20ms

Default number of channels: 3 channels

Compared protocols802.11: IEEE 802.11 single channel protocol

DCA: Wu’s protocolMMAC: Proposed protocol

Eval

Wireless LAN - Throughput

30 nodes 64 nodes

MMAC

DCA

802.11

MMAC shows higher throughput than DCA and 802.11

802.11

DCA

MMAC

Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec)1 10 100 1000 1 10 100 1000

2500

2000

1500

1000

500

Agg

rega

te T

hrou

ghpu

t (K

bps)

2500

2000

1500

1000

500

Eval

Multi-hop Network – Throughput

3 channels 4 channels

MMAC

DCA

802.11802.11

DCA

MMAC

Packet arrival rate per flow (packets/sec)1 10 100 1000

Packet arrival rate per flow (packets/sec)1 10 100 1000

Agg

rega

te T

hrou

ghpu

t (K

bps)

1500

1000

500

0

2000

1500

1000

500

0

Eval

Throughput of DCA and MMAC(Wireless LAN)

DCA MMAC

2 channels

802.11

MMAC shows higher throughput compared to DCA

6 channels

802.11

2 channels

6 channels

Agg

rega

te T

hrou

ghpu

t (K

bps) 4000

3000

2000

1000

0

4000

3000

2000

1000

0

Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec)

Advantages:

• One transceiver

• All the channels

Sum

Summary

Disadvantages:

• Overhead

• Synchronization

Thoughts

Future Works

• A better scheme for clock synchronization

• Switch channels inside the beacon interval

Thoughts

Our thoughts• Should we limit the certain channel assignment when

congestion

• How to decide the ATIM windows length comparing to the data transmission length

• When the RES message was not back, the update to the PCL is a mistake

Thank you