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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. 1. 1. 2. defer. Motivation. Multiple Channels available in IEEE 802.11 3 channels in 802.11b

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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


Motivation

1 Multi-Channel Hidden Terminals Using A Single Transceiver

1

2

defer

Motivation

  • 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

Single channel

Multiple Channels


Problem statement

1 Multi-Channel Hidden Terminals Using A Single Transceiver

2

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


802 11 distributed coordination function
802.11 Distributed Coordination Function Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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 function1
802.11 Distributed Coordination Function Multi-Channel Hidden Terminals Using A Single Transceiver

A

B

C

D

Time

A

B

C

D


802 11 distributed coordination function2
802.11 Distributed Coordination Function Multi-Channel Hidden Terminals Using A Single Transceiver

RTS

A

B

C

D

Time

A

RTS

B

C

D


802 11 distributed coordination function3

NAV Multi-Channel Hidden Terminals Using A Single Transceiver

CTS

802.11 Distributed Coordination Function

CTS

A

B

C

D

Time

A

RTS

B

C

SIFS

D


802 11 distributed coordination function4

NAV Multi-Channel Hidden Terminals Using A Single Transceiver

NAV

DATA

CTS

802.11 Distributed Coordination Function

DATA

A

B

C

D

Time

A

RTS

B

C

SIFS

D


802 11 distributed coordination function5

NAV Multi-Channel Hidden Terminals Using A Single Transceiver

NAV

ACK

DATA

CTS

802.11 Distributed Coordination Function

ACK

A

B

C

D

Time

A

RTS

B

C

SIFS

D


802 11 distributed coordination function6

NAV Multi-Channel Hidden Terminals Using A Single Transceiver

NAV

ACK

CTS

DATA

802.11 Distributed Coordination Function

A

B

C

D

Time

A

RTS

B

C

Contention Window

SIFS

D

DIFS


802 11 power saving mechanism
802.11 Power Saving Mechanism Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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 mechanism1
802.11 Power Saving Mechanism Multi-Channel Hidden Terminals Using A Single Transceiver

Beacon

Time

A

B

C

ATIM Window

Beacon Interval


Issues in multi channel environment

Issues in Multi-Channel Environment Multi-Channel Hidden Terminals Using A Single Transceiver

Multi-Channel Hidden Terminal Problem


Multi channel hidden terminals

A Multi-Channel Hidden Terminals Using A Single Transceiver

C

B

Multi-Channel Hidden Terminals

Channel 1

Channel 2

RTS

A sends RTS


Multi channel hidden terminals1

A Multi-Channel Hidden Terminals Using A Single Transceiver

C

B

Multi-Channel Hidden Terminals

Channel 1

Channel 2

CTS

B sends CTS

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


Multi channel hidden terminals2

A Multi-Channel Hidden Terminals Using A Single Transceiver

B

Multi-Channel Hidden Terminals

Channel 1

Channel 2

DATA

RTS

C

C switches to channel 1 and transmits RTS

Collision occurs at B


Related work

Related Work Multi-Channel Hidden Terminals Using A Single Transceiver

Previous work on multi-channel MAC


Nasipuri s protocol
Nasipuri’s Protocol Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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
Wu’s Protocol [Wu00ISPAN] Multi-Channel Hidden Terminals Using A Single TransceiverDynamic 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
Wu’s Protocol (cont.) Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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 Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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 Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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 Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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 Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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
Channel Negotiation Multi-Channel Hidden Terminals Using A Single Transceiver

Common Channel

Selected Channel

A

Beacon

B

C

D

Time

ATIM Window

Beacon Interval


Channel negotiation1
Channel Negotiation Multi-Channel Hidden Terminals Using A Single Transceiver

Common Channel

Selected Channel

ATIM-

RES(1)

ATIM

A

Beacon

B

ATIM-

ACK(1)

C

D

Time

ATIM Window

Beacon Interval


Channel negotiation2
Channel Negotiation Multi-Channel Hidden Terminals Using A Single Transceiver

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

Beacon Interval


Channel negotiation3
Channel Negotiation Multi-Channel Hidden Terminals Using A Single Transceiver

Common Channel

Selected Channel

ATIM-

RES(1)

RTS

DATA

Channel 1

ATIM

A

Beacon

Channel 1

B

CTS

ACK

ATIM-

ACK(1)

ATIM-

ACK(2)

CTS

ACK

Channel 2

C

Channel 2

D

ATIM

DATA

RTS

Time

ATIM-

RES(2)

ATIM Window

Beacon Interval


Performance evaluation

Performance Evaluation Multi-Channel Hidden Terminals Using A Single Transceiver

Simulation Model

Simulation Results


Simulation model
Simulation Model Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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
Wireless LAN - Throughput Multi-Channel Hidden Terminals Using A Single Transceiver

2500

2000

1500

1000

500

2500

2000

1500

1000

500

MMAC

MMAC

DCA

DCA

Aggregate Throughput (Kbps)

802.11

802.11

1 10 100 1000

1 10 100 1000

Packet arrival rate per flow (packets/sec)

Packet arrival rate per flow (packets/sec)

30 nodes

64 nodes

MMAC shows higher throughput than DCA and 802.11


Multi hop network throughput
Multi-hop Network – Throughput Multi-Channel Hidden Terminals Using A Single Transceiver

2000

1500

1000

500

0

1500

1000

500

0

MMAC

MMAC

DCA

DCA

Aggregate Throughput (Kbps)

802.11

802.11

1 10 100 1000

1 10 100 1000

Packet arrival rate per flow (packets/sec)

Packet arrival rate per flow (packets/sec)

3 channels

4 channels


Analysis
Analysis Multi-Channel Hidden Terminals Using A Single Transceiver

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

  • - 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


Conclusion
Conclusion Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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
Future Work Multi-Channel Hidden Terminals Using A Single Transceiver

  • 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

Thank you! Multi-Channel Hidden Terminals Using A Single Transceiver

Sanhita Ganguly


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