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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. Introduction. Motivation Problem Statement. 1. 1. 2. defer. Motivation.

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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

introduction

Introduction

Motivation

Problem Statement

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
  • 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
preliminaries

Preliminaries

802.11 Distributed Coordination Function (DCF)

802.11 Power Saving Mechanism (PSM)

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 function10

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 function11

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 function12

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 mechanism14
802.11 Power Saving Mechanism

Beacon

Time

A

B

C

ATIM Window

Beacon Interval

802 11 power saving mechanism15
802.11 Power Saving Mechanism

Beacon

Time

ATIM

A

B

C

ATIM Window

Beacon Interval

802 11 power saving mechanism16
802.11 Power Saving Mechanism

Beacon

Time

ATIM

A

B

ATIM-ACK

C

ATIM Window

Beacon Interval

802 11 power saving mechanism17
802.11 Power Saving Mechanism

Beacon

Time

ATIM

ATIM-RES

A

B

ATIM-ACK

C

ATIM Window

Beacon Interval

802 11 power saving mechanism18
802.11 Power Saving Mechanism

Beacon

Time

ATIM

ATIM-RES

DATA

A

B

ATIM-ACK

Doze Mode

C

ATIM Window

Beacon Interval

802 11 power saving mechanism19
802.11 Power Saving Mechanism

Beacon

Time

ATIM

ATIM-RES

DATA

A

B

ATIM-ACK

ACK

Doze Mode

C

ATIM Window

Beacon Interval

issues in multi channel environment

Issues in Multi-Channel Environment

Multi-Channel Hidden Terminal Problem

hidden terminal problem

A

C

B

Hidden Terminal Problem

DATA

C does not hear A’s transmission

hidden terminal problem22

A

C

B

Hidden Terminal Problem

DATA

C starts transmitting – collides at B

solution virtual carrier sensing

A

C

D

B

Solution: Virtual Carrier Sensing

RTS

A sends RTS

D overhears RTS and defers transmission

solution virtual carrier sensing24

A

C

D

B

Solution: Virtual Carrier Sensing

CTS

B sends CTS

C overhears CTS and defers transmission

solution virtual carrier sensing25

A

C

D

B

Solution: Virtual Carrier Sensing

DATA

A sends DATA to B

solution virtual carrier sensing26

A

C

D

B

Solution: Virtual Carrier Sensing

RTS

D overhears RTS and defers transmission

multi channel hidden terminals
Multi-Channel Hidden Terminals
  • Consider the following naïve protocol
    • Static channel assignment (based on node ID)
    • Communication takes place on receiver’s channel
      • Sender switches its channel to receiver’s channel before transmitting
multi channel hidden terminals28

A

C

B

Multi-Channel Hidden Terminals

Channel 1

Channel 2

RTS

A sends RTS

multi channel hidden terminals29

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 terminals30

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
  • Sender searches for an idle channel and transmits on the channel [Nasipuri99WCNC]
  • Extensions: channel selection based on channel condition on the receiver side [Nasipuri00VTC]
  • Disadvantage: High hardware cost
wu s protocol wu00ispan
Wu’s Protocol [Wu00ISPAN]
  • 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 transmits RES (Reservation)
    • 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
    • Per-packet channel switching can be expensive
    • 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
protocol description

Protocol Description

Multi-Channel MAC (MMAC) Protocol

proposed protocol mmac
Proposed Protocol (MMAC)
  • Assumptions
    • Each node is equipped with a single transceiver
    • The transceiver is capable of switching channels
    • Channel switching delay is approximately 250us
      • Per-packet switching not recommended
      • Occasional channel switching not to expensive
    • Multi-hop synchronization is achieved by other means
slide37
MMAC
  • Idea similar to IEEE 802.11 PSM
    • Divide time into beacon intervals
    • At the beginning of each beacon interval, all nodes must listen to a predefined common channel for a fixed duration of time (ATIM window)
    • Nodes negotiate channels using ATIM messages
    • Nodes switch to selected channels after ATIM window for the rest of the beacon interval
preferred channel list pcl
Preferred Channel List (PCL)
  • Each node maintains PCL
    • Records usage of channels inside the transmission range
    • High preference (HIGH)
      • Already selected for the current beacon interval
    • Medium preference (MID)
      • No other vicinity node has selected this channel
    • Low preference (LOW)
      • This channel has been chosen by vicinity nodes
      • Count number of nodes that selected this channel to break ties
channel negotiation
Channel Negotiation
  • In ATIM window, sender transmits ATIM to the receiver
  • Sender includes its PCL in the ATIM packet
  • 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
channel negotiation40
Channel Negotiation

Common Channel

Selected Channel

A

Beacon

B

C

D

Time

ATIM Window

Beacon Interval

channel negotiation41
Channel Negotiation

Common Channel

Selected Channel

ATIM-

RES(1)

ATIM

A

Beacon

B

ATIM-

ACK(1)

C

D

Time

ATIM Window

Beacon Interval

channel negotiation42
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 negotiation43
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

throughput of dca and mmac wireless lan
Throughput of DCA and MMAC(Wireless LAN)

4000

3000

2000

1000

0

4000

3000

2000

1000

0

6 channels

6 channels

2 channels

Aggregate Throughput (Kbps)

2 channels

802.11

802.11

Packet arrival rate per flow (packets/sec)

Packet arrival rate per flow (packets/sec)

MMAC

DCA

MMAC shows higher throughput compared to DCA

analysis of results
Analysis of Results
  • DCA
    • Bandwidth of control channel significantly affects performance
    • Narrow control channel: High collision and congestion of control packets
    • Wide control channel: Waste of bandwidth
    • It is difficult to adapt control channel bandwidth dynamically
  • MMAC
    • ATIM window size significantly affects performance
    • ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon interval – reduced overhead
      • Compared to packet-by-packet control packet exchange in DCA
    • 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!

jso1@uiuc.edu