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

Using A Single Transceiver

Jungmin So and Nitin Vaidya

University of Illinois at Urbana-Champaign

• Multiple Channels available in IEEE 802.11– 3 channels in 802.11b– 12 channels in 802.11a

• Utilizing multiple channels can improve throughput– Allow simultaneous transmissions

Motivation

1

defer

1

2

Single channel Multiple Channels

Problem Statement• Using k channels does not translate into throughput

improvement by a factor of k– Nodes listening on different channels cannot talk to each other– Requires modification of coordination schemes among the nodes

• Constraint: Each node has only a single transceiver– Capable of listening to one channel at a time

• Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance– Modify 802.11 DCF to work in multi-channel environment

1 2

802.11 Distributed Coordination Function

• Virtual carrier sensing

– Sender sends Ready-To-Send (RTS)

– Receiver sends Clear-To-Send (CTS)

– RTS and CTS reserves the area around sender and receiver for the duration of dialogue

– Nodes that overhear RTS and CTS defer transmissions by setting Network Allocation Vector (NAV)

802.11 Distributed Coordination Function

A

B

C

D

A B C D

Time

802.11 Distributed Coordination Function

A

B

C

D

A B C D

RTS

Time

RTS

802.11 Distributed Coordination Function

A

B

C

D

A B C D

RTS

CTS

SIFS

NAV Time

CTS

802.11 Distributed Coordination Function

A

B

C

D

A B C D

RTS

CTS

DATA

SIFS

NAV

NAV

Time

DATA

802.11 Distributed Coordination Function

A

B

C

D

A B C D

RTS

CTS

DATA

SIFS

ACK

NAV

NAV

Time

ACK

802.11 Distributed Coordination Function

A

B

C

D

A B C D

RTS

CTS

DATA

SIFS

ACK

NAV

NAV

DIFS

Time

Contention Window

802.11 Power Saving Mechanism• Time is divided into beacon intervals

• All nodes wake up at the beginning of a beacon interval for a fixed duration of time (ATIM window)

• Exchange ATIM (Ad-hoc Traffic Indication Message) during ATIM window

• Nodes that receive ATIM message stay up during for the whole beacon interval

• Nodes that do not receive ATIM message may go into doze mode after ATIM window

802.11 Power Saving Mechanism

A

B

C

Time

Beacon

ATIM Window

Beacon Interval

Issues in Multi-Channel Environment

Multi-Channel Hidden Terminal Problem

Multi-Channel Hidden Terminals

A B CRTS

A sends RTS

Channel 1

Channel 2

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

Multi-Channel Hidden Terminals

A B CDATA

C switches to channel 1 and transmits RTS

Channel 1

Channel 2

Collision occurs at B

RTS

Related Work

Previous work on multi-channel MAC

Nasipuri’s Protocol

• Assumes N transceivers per host– Capable of listening to all channels simultaneously– Always have information for all channels

• Disadvantage: High hardware cost

Wu’s Protocol [Wu00ISPAN]Dynamic Channel Assignment

• Assumes 2 transceivers per host– One transceiver always listens on control channel

• Negotiate channels using RTS/CTS/RES

– RTS/CTS/RES packets sent on control channel– Sender includes preferred channels in RTS – Receiver decides a channel and includes in CTS– Sender sends DATA on the selected data channel

Wu’s Protocol (cont.)

• Advantage– No synchronization required

• Disadvantage– Each host must have 2 transceivers– Control channel bandwidth is an issue

• Too small: control channel becomes a bottleneck• Too large: waste of bandwidth• Optimal control channel bandwidth depends on traffic load,

but difficult to dynamically adapt

MMAC

Assumptions

- All channels have same BW and none of them are overlapping channels

- Nodes have only one transceiver

- Transceivers are capable of switching channels but they are half-duplex

- Channel switching delay is approx 250 us, avoid per packet switching

- Nodes synchronized: Begin their beacon intervals same time

MMAC

Steps –

- Divide time into beacon intervals

- At the beginning, nodes listen to a pre-defined channel for ATIM window duration

- Channel negotiation starts using ATIM messages

- Nodes switch to the selected channel after the ATIM window duration

MMAC

Preferred Channel List (PCL)

- For a node, PCL records usage of channels inside Tx range

- HIGH preference – always selected

- MID preference – others in the vicinity did not select the channel

- LOW preference – others in the vicinity selected the channel

MMAC

Channel Negotiation

- Sender transmits ATIM to the receiver and includes its PCL in the ATIM packet

- Receiver selects a channel based on sender’s PCL and its own PCL

- Receiver sends ATIM-ACK to sender including the selected channel

- Sender sends ATIM-RES to notify its neighbors of the selected channel

Channel Negotiation

A

B

C

DTime

ATIM Window

Beacon Interval

Common Channel Selected Channel

Beacon

Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

Time

ATIM Window

Beacon Interval

Common Channel Selected Channel

Beacon

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

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

Performance Evaluation

Simulation Model

Simulation Results

Simulation Model• ns-2 simulator• Transmission rate: 2Mbps• Transmission range: 250m• Traffic type: Constant Bit Rate (CBR)• Beacon interval: 100ms

• Packet size: 512 bytes• ATIM window size: 20ms• Default number of channels: 3 channels

• Compared protocols– 802.11: IEEE 802.11 single channel protocol– DCA: Wu’s protocol– MMAC: Proposed protocol

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

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

Analysis

For DCA: BW of control channel significantly affects the performance and it’s difficult to adapt control channel BW

- For MMAC:

1. ATIM window size significantly affects performance

2. ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon interval – reduced overhead

3. ATIM window size can be adapted to traffic load

Conclusion

• MMAC requires a single transceiver per host to work in multi-channel ad hoc networks

• MMAC achieves throughput performance comparable to a protocol that requires multiple transceivers per host

Future Work

• Dynamic adaptation of ATIM window size based on traffic load for MMAC

• Efficient multi-hop clock synchronization

• Routing protocols for multi-channel environment

Thank you!

Sanhita Ganguly