ECE 256, Spring 2009 Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver ________

ECE 256, Spring 2009__________

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

Using A Single Transceiver __________________

Paper by Jungmin So & Nitin VaidyaUniversity of Illinois at Urbana-Champaign

ACM MobiHoc ‘04

Presenter: Sandip Agrawal, Duke University

Acknowledgments

Slides courtesy:

Jungmin So and Nitin Vaidya

http://www.crhc.uiuc.edu/wireless/groupPubs.html

Topics Introduction

o Motivationo Problem Statement

Preliminarieso 802.11 DCF structureo 802.11 PSM mode

Issues in multi-channel environment Other works in multi-channel MAC Proposed MMAC Simulation results Discussions

Motivation Multiple Channels available in IEEE 802.11

802.11b – 14 channels in PHY layer – 3 of them are used (1,6,11)

802.11a – 12 channels – 8 in the lower part of the band for indoor use and rest in higher for outdoor us

‘Exploit multiple channels to improve network throughput’ … why ?

Allow Simultaneous Transmissions

1

defer

1

2

Single channel Multiple Channels

Problem Statement The ideal scenario – use k channels to improve

throughput by a factor of k• Reality is different…Nodes listening on different

channels cannot talk to each other

Constraint : Single Transceiver - Can listen to only one channel at a time

Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance (at least possible cost and complexity)

1 2

Topics Introduction

Motivation Problem Statement

Preliminaries 802.11 DCF structure 802.11 PSM mode


802.11 DCF (Distributed Coordinate Function)

Designed for sharing a single channel between the hosts

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 DCF

A B C D

A

B

C

D

Time

802.11 DCF

A B C DRTS

A

B

C

D

RTS

Time

802.11 DCF

A B C DCTS

A

B

C

D

RTS

CTS

SIFS

NAV Time

802.11 DCF

A

B

C

D

A B C D

RTS

CTS

DATA

SIFS

NAV

NAV

Time

DATA

802.11 DCF

A

B

C

D

A B C D

RTS

CTS

DATA

SIFS

ACK

NAV

NAV

Time

ACK

802.11 DCF

A

B

C

D

A B C D

RTS

CTS

DATA

SIFS

ACK

NAV

NAV

Time

Contention Window

802.11 PSM (Power Saving Mechanism)

A

B

C

Time

Beacon

ATIM Window

Beacon Interval

Doze mode – less energy consumption but no communication• ATIM – Ad hoc Traffic Indication Message


A

B

C

Time

Beacon

ATIM

ATIM Window

Beacon Interval


A

B

C

Time

Beacon

ATIM

ATIM-ACK

ATIM Window

Beacon Interval


A

B

C

Time

Beacon

ATIM

ATIM-ACK

ATIM-RES

ATIM Window

Beacon Interval


A

B

C

Time

Beacon

ATIM

ATIM-ACK

DATAATIM-RES

Doze Mode

ATIM Window

Beacon Interval


A

B

C

Time

Beacon

ATIM

ATIM-ACK

DATA

ACK

ATIM-RES

Doze Mode

ATIM Window

Beacon Interval

In essence … All nodes wake up at the beginning of a beacon interval

for a fixed duration of time (ATIM window)

Exchange ATIM 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

Topics Introduction




Multi-channel Hidden Terminals

Multi-channel Hidden Terminals

Observations

1. Nodes may listen to different channels2. Virtual Carrier Sensing becomes difficult3. The problem was absent for single channel

Possible approaches

1. Use multiple transcievers2. Exploit synchronization technique available from IEEE

802.11 PSM

Topics Introduction




Nasipuri’s Protocol

N transceivers per host- Capable of listening all channels

simultaneously

• Find an idle channel and transmit – sender’s policy

• Channel selection should be based on channel condition on receiver side

• High hardware cost

Wu’s Protocol

2 transceivers per host One transceiver always listens on control channel

• Sender includes preferred channel list in RTS, receiver picks one and tells in CTS

• Sender sends DATA on the selected data channel

No synchronization required

Control channel’s BW becomes 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

Topics Introduction




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

- Multi-hop synch is achieved by other means

MMAC

Idea similar to IEEE 802.11 PSM

- 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 agreed upon channel after the ATIM window duration

MMAC Preferred Channel List (PCL)

- For a node, PCL records usage of channels inside Transmission 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

- In ATM window, sender transmits ATIM …. Includes its PCL

- Receiver selects a channel based on sender’s PCL and its own PCL• Order of preference: HIGH > MID > LOW• Tie breaker: Receiver’s PCL has higher priority• For “LOW” channels: channels with smaller count have higher priority

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

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

MMAC

A

B

C

DTime

ATIM WindowBeacon Interval

Common Channel Selected Channel

Beacon

MMAC

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

TimeATIM Window


Beacon

MMAC

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

ATIM-ACK(2)

ATIM ATIM-RES(2)

Time

ATIM Window


Beacon

MMAC

ATIM

ATIM-ACK(1)

ATIM-RES(1)

ATIM-ACK(2)

ATIM ATIM-RES(2)

Time

ATIM Window

Beacon Interval


Beacon

RTS

CTS

RTS

CTS

DATA

ACK

ACK

DATA

Channel 1

Channel 1

Channel 2

Channel 2

Topics Introduction




Parameters

Transmission rate: 2MbpsTransmission range: 250mTraffic type: Constant Bit Rate (CBR)Beacon interval: 100ms

Packet size: 512 bytesATIM window size: 20msDefault number of channels: 3 channels

Compared protocols802.11: IEEE 802.11 single channel protocolDCA: Wu’s protocolMMAC: Proposed protocol

WLAN - Throughput

MMAC

DCA

802.11

MMAC

DCA

802.11

30 nodes64 nodes

MMAC shows higher throughput than DCA and 802.11

Multihop 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

Throughput of DCA and MMAC

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)

Analysis of Results 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

Topics Introduction




Discussions 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

Instead of counting source-destination pair for calculating channel usage, counting the number of pending packets may be a better idea

Starvation can occur with common source and multiple destinations

Questions??? While criticizing Wu’s protocol – control channel

‘prevents the data channel from being fully utilized’ … why ?

Source and Destinations may not be in one hop distance and may not be communicated within a beacon interval

Thank You!

Documents

ECE 256, Spring 2009 __________ Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver __________________

ECE 256, Spring 2009 Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver ________