Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver

1
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

2
Introduction

• Motivation
• Problem Statement

3
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
4
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

5
Preliminaries

• 802.11 Distributed Coordination Function (DCF)
• 802.11 Power Saving Mechanism (PSM)

6
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)

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802.11 Distributed Coordination Function
A
B
C
D
Time
A
B
C
D
8
802.11 Distributed Coordination Function
RTS
A
B
C
D
Time
A
RTS
B
C
D
9
802.11 Distributed Coordination Function
CTS
A
B
C
D
Time
A
RTS
B
C
SIFS
D
10
802.11 Distributed Coordination Function
DATA
A
B
C
D
Time
A
RTS
B
C
SIFS
D
11
802.11 Distributed Coordination Function
ACK
A
B
C
D
Time
A
RTS
B
C
SIFS
D
12
802.11 Distributed Coordination Function
A
B
C
D
Time
A
RTS
B
C
Contention Window
SIFS
D
DIFS
13
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

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802.11 Power Saving Mechanism
Beacon
Time
A
B
C
ATIM Window
Beacon Interval
15
802.11 Power Saving Mechanism
Beacon
Time
ATIM
A
B
C
ATIM Window
Beacon Interval
16
802.11 Power Saving Mechanism
Beacon
Time
ATIM
A
B
ATIM-ACK
C
ATIM Window
Beacon Interval
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802.11 Power Saving Mechanism
Beacon
Time
ATIM
ATIM-RES
A
B
ATIM-ACK
C
ATIM Window
Beacon Interval
18
802.11 Power Saving Mechanism
Beacon
Time
ATIM
DATA
ATIM-RES
A
B
ATIM-ACK
Doze Mode
C
ATIM Window
Beacon Interval
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802.11 Power Saving Mechanism
Beacon
Time
ATIM
DATA
ATIM-RES
A
B
ATIM-ACK
ACK
Doze Mode
C
ATIM Window
Beacon Interval
20
Issues in Multi-Channel Environment

• Multi-Channel Hidden Terminal Problem

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Hidden Terminal Problem
DATA
C does not hear As transmission
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Hidden Terminal Problem
DATA
C starts transmitting collides at B
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Solution Virtual Carrier Sensing
RTS
A sends RTS
D overhears RTS and defers transmission
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Solution Virtual Carrier Sensing
CTS
B sends CTS
C overhears CTS and defers transmission
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Solution Virtual Carrier Sensing
DATA
A sends DATA to B
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Solution Virtual Carrier Sensing
RTS
D overhears RTS and defers transmission
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Multi-Channel Hidden Terminals

• Consider the following naïve protocol
• Static channel assignment (based on node ID)
• Communication takes place on receivers channel
• Sender switches its channel to receivers channel
  before transmitting

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Multi-Channel Hidden Terminals
Channel 1
Channel 2
RTS
A sends RTS
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Multi-Channel Hidden Terminals
Channel 1
Channel 2
CTS
B sends CTS
C does not hear CTS because C is listening on
channel 2
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Multi-Channel Hidden Terminals
Channel 1
Channel 2
DATA
RTS
C
C switches to channel 1 and transmits RTS
Collision occurs at B
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Related Work

• Previous work on multi-channel MAC

32
Nasipuris 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

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Wus 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

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Wus 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

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Protocol Description

• Multi-Channel MAC (MMAC) Protocol

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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

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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

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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

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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 senders PCL
  and its own PCL
• Order of preference HIGH gt MID gt LOW
• Tie breaker Receivers 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

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Channel Negotiation
Common Channel
Selected Channel
A
Beacon
B
C
D
Time
ATIM Window
Beacon Interval
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Channel Negotiation
Common Channel
Selected Channel
ATIM- RES(1)
ATIM
A
Beacon
B
ATIM- ACK(1)
C
D
Time
ATIM Window
Beacon Interval
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Channel Negotiation
Common Channel
Selected Channel
ATIM- RES(1)
ATIM
A
Beacon
B
ATIM- ACK(1)
ATIM- ACK(2)
C
D
ATIM
ATIM- RES(2)
Time
ATIM Window
Beacon Interval
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Channel Negotiation
Common Channel
Selected Channel
ATIM- RES(1)
RTS
DATA
Channel 1
ATIM
A
Beacon
Channel 1
B
ATIM- ACK(1)
CTS
ACK
ATIM- ACK(2)
CTS
ACK
Channel 2
C
Channel 2
D
ATIM
DATA
ATIM- RES(2)
Time
RTS
ATIM Window
Beacon Interval
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Performance Evaluation

• Simulation Model
• Simulation Results

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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 Wus protocol
• MMAC Proposed protocol

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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
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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
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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
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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

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Conclusion Future Work
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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

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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

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Thank you!

• jso1_at_uiuc.edu

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