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

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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

Slides written by original authors, modified by
Yong Yang and Ray K. Lam
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Overview

• Goal Design a MAC protocol that utilizes
  multiple channels to improve overall performance
• Modify 802.11 DCF to work in multi-channel
  environment
• Constraint Each node has only a single
  transceiver
• Capable of listening to one channel at a time
• MMAC (Multi-channel MAC)
• Divide time into fixed-time interval using
  beacons
• Have a small window at the start of each interval
• Senders and receivers negotiate channels for this
  interval

Common Channel
Selected Channel
Negotiate Channel
RTS
DATA
A
Beacon
B
CTS
ACK
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Multi-Channel Hidden Terminals

• Consider the following naïve protocol
• Each node has one transceiver
• One channel is dedicated for exchanging control
  msg
• Reserve channel as in IEEE 802.11 DCF
• Sender indicates preferred channels in RTS
• Receiver selects a channel and includes it in CTS
• Sender and Receiver switch to the selected
  channel
• This protocol is similar to DCA (Dynamic Channel
  assignment) Wu00 ISPAN
• RTS/CTS cant solve Hidden Terminal Problem

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Proposed Protocol (MMAC)

• Assumptions
• Each node is equipped with a single transceiver
• The transceiver is capable of switching channels
• Multi-hop synchronization is achieved by other
  means
• Out-of-band solutions (e.g. GPS)
• In-band solutions (e.g. beaconing)

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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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Some facts of MMAC

• Outside the ATIM window, the default channel is
  also used for data transmission
• To avoid ATIM collision, from the start of the
  ATIM window, each node waits for a random backoff
  interval before trans-mitting an ATIM packet
• Two closed transmissions may choose the same
  channel
• RTS/CTS are still used
• Nodes refrain from transmitting a packet if the
  time left in the current beacon interval is
  not long enough

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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
• Two Network Scenarios wireless LAN, multi-hop
  network
• Compared protocols
• 802.11 IEEE 802.11 single channel protocol
• DCA Wus protocol (1 control channel, 2 data
  channels)
• 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
  as network becomes saturated

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Wireless LAN Throughput vs. Channels
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

• The number of channels DCA can fully utilize is
  limited by the capa-city of the control channel
• When network load is high, the control channel
  could be the bottleneck

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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

• Performance difference is smaller
• Not every region of the network needs 3 channels
• A node being multiple sources / destinations

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Conclusion

• 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
• Requirement for synchronization