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Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver. Jungmin So & Nitin Vaidya University of Illinois at Urbana-Champaign. ECE 256 - Spring 2010. Acknowledgments. Slides from: Jungmin So and Nitin Vaidya

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ECE 256 - Spring 2010


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slide1

Multi-Channel MAC for Ad Hoc Networks:

Handling Multi-Channel Hidden Terminals

Using A Single Transceiver

Jungmin So & Nitin Vaidya

University of Illinois at Urbana-Champaign

ECE 256 - Spring 2010

acknowledgments
Acknowledgments

Slides from:

Jungmin So and Nitin Vaidya

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

Modified and Presented by:

Federico Gonzalez

ECE 256 - Spring 2010

motivation

1

1

2

defer

Motivation
  • ‘Exploit multiple channels to improve network throughput’ … why ?
  • Greater parallel communication is possible

ECE 256 - Spring 2010

problem statement

1

2

Problem Statement
  • The ideal scenario – use k channels to improve throughput by a factor of k
  • Reality is different…
  • Nodes on listening to different channels can not talk to each other
  • Listen one channel at a time – constraint with single transceiver
  • Goal: Exploit multiple channels using a single transceiver
  • Requires modification of coordination schemes among the nodes

ECE 256 - Spring 2010

preliminaries
Preliminaries
  • 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)

ECE 256 - Spring 2010

802 11 dcf

A

B

C

D

Time

A

B

C

D

802.11 DCF

ECE 256 - Spring 2010

802 11 dcf1

Time

A

RTS

B

C

D

802.11 DCF

RTS

A

B

C

D

ECE 256 - Spring 2010

802 11 dcf2

CTS

A

B

C

D

Time

A

NAV

RTS

B

CTS

C

SIFS

D

802.11 DCF

ECE 256 - Spring 2010

802 11 dcf3

DATA

A

B

C

D

Time

A

NAV

NAV

RTS

B

CTS

DATA

C

SIFS

D

802.11 DCF

ECE 256 - Spring 2010

802 11 dcf4

ACK

A

B

C

D

Time

A

NAV

NAV

RTS

B

CTS

DATA

ACK

C

SIFS

D

802.11 DCF

ECE 256 - Spring 2010

802 11 psm power saving mode

Beacon

Time

A

B

C

ATIM Window

Beacon Interval

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

ECE 256 - Spring 2010

802 11 psm power saving mode1

Beacon

Time

ATIM

A

B

C

ATIM Window

Beacon Interval

802.11 PSM (Power Saving Mode)

ECE 256 - Spring 2010

802 11 psm power saving mode2

Beacon

Time

ATIM

A

B

ATIM-ACK

C

ATIM Window

Beacon Interval

802.11 PSM (Power Saving Mode)

ECE 256 - Spring 2010

802 11 psm power saving mode3

Beacon

Time

ATIM

ATIM-RES

A

B

ATIM-ACK

C

ATIM Window

Beacon Interval

802.11 PSM (Power Saving Mode)

ECE 256 - Spring 2010

802 11 psm power saving mode4

Beacon

Time

ATIM

ATIM-RES

DATA

A

B

ATIM-ACK

Doze Mode

C

ATIM Window

Beacon Interval

802.11 PSM (Power Saving Mode)

ECE 256 - Spring 2010

802 11 psm power saving mode5

Beacon

Time

ATIM

ATIM-RES

DATA

A

B

ATIM-ACK

ACK

Doze Mode

C

ATIM Window

Beacon Interval

802.11 PSM (Power Saving Mode)

ECE 256 - Spring 2010

802 11 psm power saving mode summary
802.11 PSM (Power Saving Mode) Summary
  • 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

ECE 256 - Spring 2010

multi channel hidden terminals
Multi-channel Hidden Terminals

ECE 256 - Spring 2010

multi channel hidden terminals1
Multi-channel Hidden Terminals
  • Observations
  • Nodes may listen to different channels
  • Virtual Carrier Sensing becomes difficult
  • The problem was absent for single channel
  • Possible approaches
  • Exploit synchronization technique available from IEEE 802.11 PSM
  • Use multiple transceivers

ECE 256 - Spring 2010

slide20
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

ECE 256 - Spring 2010

slide21
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 agreed upon channel after the ATIM window duration

ECE 256 - Spring 2010

slide22
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

ECE 256 - Spring 2010

slide23
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

ECE 256 - Spring 2010

slide24

Common Channel

Selected Channel

A

Beacon

B

C

D

Time

ATIM Window

Beacon Interval

MMAC

ECE 256 - Spring 2010

slide25
MMAC

Common Channel

Selected Channel

ATIM-

RES(1)

ATIM

A

Beacon

B

ATIM-

ACK(1)

C

D

Time

ATIM Window

ECE 256 - Spring 2010

slide26
MMAC

Common Channel

Selected Channel

ATIM-

RES(1)

ATIM

A

Beacon

B

ATIM-

ACK(1)

ATIM-

ACK(2)

C

D

ATIM

Time

ATIM-

RES(2)

ATIM Window

ECE 256 - Spring 2010

slide27
MMAC

Common Channel

Selected Channel

ATIM-

RES(1)

ATIM

RTS

DATA

Channel 1

A

Beacon

Channel 1

B

CTS

ACK

ATIM-

ACK(1)

ATIM-

ACK(2)

CTS

Channel 2

ACK

C

Channel 2

D

DATA

ATIM

Time

ATIM-

RES(2)

RTS

ATIM Window

Beacon Interval

ECE 256 - Spring 2010

experimental parameters
Experimental Parameters
  • 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

ECE 256 - Spring 2010

wlan throughput
WLAN - Throughput

ECE 256 - Spring 2010

multi hop network throughput
Multi-hop Network - Throughput

ECE 256 - Spring 2010

analysis
Analysis
  • - For MMAC:
  • ATIM window size significantly affects performance
  • ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon interval – reduced overhead
  • ATIM window size can be adapted to traffic load

ECE 256 - Spring 2010

discussions
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
  • Beaconing mechanism may fail to synchronize in a multi-hop network – probabilistic beaconing may help
  • Starvation can occur with common source and multiple destinations

ECE 256 - Spring 2010

related works
Related Works
  • 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

ECE 256 - Spring 2010

related works1
Related Works
  • Wu et. al talks about a scheme with 2 transceivers per host
  • RTS/CTS/RES packets sent on control channel
  • Sender includes PCL list in RTS, receiver picks one and tells in CTS
  • Sender transmits RES and sends data on agreed channel
  • No synch is required
  • Per packet channel switching can be expensive
  • Control channel’s BW becomes an issue

ECE 256 - Spring 2010