ECE 256, Spring 2008

ECE 256, Spring 2008

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

Using A Single Transceiver

Jungmin So & Nitin VaidyaUniversity of Illinois at Urbana-Champaign

(Paper presented at ACM MobiHoc ‘04)

Presenter: Rahul Ghosh, ECE Dept., Duke University

Acknowledgments

ECE 256 / CS 215, Spring 2008

Slides courtesy:

Jungmin So and Nitin Vaidya

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

Topics

ECE 256 / CS 215, Spring 2008

IntroductionMotivationProblem Statement

Preliminaries802.11 DCF structure802.11 PSM mode

Issues in multi-channel environmentOther works in multi-channel MACProposed MMACSimulation resultsDiscussions

Motivation

ECE 256 / CS 215, Spring 2008

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

More number of parallel communications possibleStandard supports …

802.11b – 14 channels in PHY layer – 3 of them are used802.11a – 12 channels – 8 in the lower part of the spectra

and rest in higher

1

defer1

2

Problem Statement

ECE 256 / CS 215, Spring 2008

The ideal scenario – use k channels to improve throughput by a factor of kReality is different…

Nodes on listening to different channels can not talk to each otherListen one channel at a time – constraint with single transcieverGoal: Exploit multiple channels using a single transcieverRequires modification of coordination schemes among the nodes

1 2

Topics

ECE 256 / CS 215, Spring 2008




Preliminaries

ECE 256 / CS 215, Spring 2008

802.11 DCF (Distributed Coordinate Function)Designed for sharing a single channel between the hostsVirtual 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

ECE 256 / CS 215, Spring 2008

A B C D

A

B

C

D

Time

802.11 DCF

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A B C DRTS

A

B

C

D

RTS

Time

802.11 DCF

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A B C DCTS

A

B

C

D

RTS

CTS

SIFS

NAV Time

802.11 DCF

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A

B

C

D

A B C D

RTS

CTS

DATA

SIFS

NAV

NAV

Time

DATA

802.11 DCF

ECE 256 / CS 215, Spring 2008

A

B

C

D

A B C D

RTS

CTS

DATA

SIFS

ACK

NAV

NAV

Time

ACK

Preliminaries

ECE 256 / CS 215, Spring 2008

802.11 PSM (Power Saving Mode)Doze mode – less energy consumption but no communicationATIM – Ad hoc Traffic Indication Message

A

B

C

Time

Beacon

ATIM Window

Beacon Interval

Preliminaries

ECE 256 / CS 215, Spring 2008

A

B

C

Time

Beacon

ATIM

ATIM Window

Beacon Interval

Preliminaries

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A

B

C

Time

Beacon

ATIM

ATIM-ACK

ATIM Window

Beacon Interval

Preliminaries

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A

B

C

Time

Beacon

ATIM

ATIM-ACK

ATIM-RES

ATIM Window

Beacon Interval

Preliminaries

ECE 256 / CS 215, Spring 2008

A

B

C

Time

Beacon

ATIM

ATIM-ACK

DATAATIM-RES

Doze Mode

ATIM Window

Beacon Interval

Preliminaries

ECE 256 / CS 215, Spring 2008

A

B

C

Time

Beacon

ATIM

ATIM-ACK

DATA

ACK

ATIM-RES

Doze Mode

ATIM Window

Beacon Interval

In essence …

ECE 256 / CS 215, Spring 2008

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

ECE 256 / CS 215, Spring 2008




Multi-channel Hidden Terminals

ECE 256 / CS 215, Spring 2008

Multi-channel Hidden Terminals

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Observations

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

Possible approaches

1. Exploit synchronization technique available from IEEE 802.11 PSM

2. Use multiple transcievers

Topics

ECE 256 / CS 215, Spring 2008




Related Works

ECE 256 / CS 215, Spring 2008

Nasipuri et. al proposed for a scheme with 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

Cost becomes higher

Related Works

ECE 256 / CS 215, Spring 2008

Wu et. al talks about a scheme with 2 transceivers per host

1. RTS/CTS/RES packets sent on control channel2. Sender includes PCL list in RTS, receiver picks one and

tells in CTS3. Sender transmits RES and sends data on agreed channel

No synch is requiredPer packet channel switching can be expensiveControl channel’s BW becomes an issue

Topics

ECE 256 / CS 215, Spring 2008




MMAC

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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

ECE 256 / CS 215, Spring 2008

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 agreed upon channel after the ATIM window duration

MMAC

ECE 256 / CS 215, Spring 2008

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

ECE 256 / CS 215, Spring 2008

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

MMAC

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A

B

C

DTime

ATIM WindowBeacon Interval

Common Channel Selected Channel

Beacon

MMAC

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A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

TimeATIM Window


Beacon

MMAC

ECE 256 / CS 215, Spring 2008

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

ATIM-ACK(2)

ATIM ATIM-RES(2)

Time

ATIM Window


Beacon

MMAC

ECE 256 / CS 215, Spring 2008

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

ECE 256 / CS 215, Spring 2008




Parameters

ECE 256 / CS 215, Spring 2008

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

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Multihop Network - Throughput

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Analysis

ECE 256 / CS 215, Spring 2008

- 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

ECE 256 / CS 215, Spring 2008




Discussions

ECE 256 / CS 215, Spring 2008

- 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

- Beaconing mechanism may fail to synchronize in a multi-hop network – probabilistic beaconing may help

- 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

Two Questions

ECE 256 / CS 215, Spring 2008

- 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

Documents

ECE 256, Spring 2008