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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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slide1

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

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

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
  • 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 function4

NAV

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

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

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
  • 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

Beacon

Time

A

B

C

ATIM Window

Beacon Interval

issues in multi channel environment

Issues in Multi-Channel Environment

Multi-Channel Hidden Terminal Problem

multi channel hidden terminals

A

C

B

Multi-Channel Hidden Terminals

Channel 1

Channel 2

RTS

A sends RTS

multi channel hidden terminals1

A

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

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

Previous work on multi-channel MAC

nasipuri s protocol
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
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
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
slide21
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
slide22
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
slide23
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
slide24
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
Channel Negotiation

Common Channel

Selected Channel

A

Beacon

B

C

D

Time

ATIM Window

Beacon Interval

channel negotiation1
Channel Negotiation

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

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

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

Simulation Model

Simulation Results

simulation model
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
Wireless LAN - Throughput

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

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
  • 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
  • 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
  • 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!

Sanhita Ganguly