Multi-Channel MAC for Ad Hoc
Networks:
Handling Multi-Channel Hidden
Terminals Using A Single Transceiver
Jungmin So and Nitin Vaidya
Dept. of Computer Science
University of Illinois Urbana_Champaign
Motivation



Multiple channels in 802.11 standard.
802.11 MAC is designed for one channel.
To design a new MAC to exploit multiple channels.



Many benefit, throughput gain
Assume only one transceiver.
Goal: to improve overall performance
1
1
defer
Single channel
2
Multiple Channels
Multi-Channel Hidden Terminals
Channel 1
Channel 2
A
RTS
B
A sends RTS
C
Multi-Channel Hidden Terminals
Channel 1
Channel 2
A
CTS
B
C
B sends CTS
C does not hear CTS because C is listening on channel 2
Multi-Channel Hidden Terminals
Channel 1
Channel 2
A
DATA
B
RTS
C
C switches to channel 1 and transmits RTS
Collision occurs at B
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 Mechanism
Beacon
Time
A
B
C
ATIM Window
Beacon Interval
802.11 Power Saving Mechanism
Beacon
A
Time
ATIM
B
C
ATIM Window
Beacon Interval
802.11 Power Saving Mechanism
Beacon
A
Time
ATIM
B
ATIM-ACK
C
ATIM Window
Beacon Interval
802.11 Power Saving Mechanism
Beacon
A
ATIM
Time
DATA
B
ATIM-ACK
Doze Mode
C
ATIM Window
Beacon Interval
802.11 Power Saving Mechanism
Beacon
A
ATIM
Time
DATA
B
ATIM-ACK
ACK
Doze Mode
C
ATIM Window
Beacon Interval
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)

Each node maintains PCL


Records usage of channels inside the
transmission range
High preference (HIGH)


Medium preference (MID)


Already selected for the current beacon interval
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



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 Negotiation
Common Channel
Selected Channel
A
Beacon
B
C
D
Time
ATIM Window
Beacon Interval
Channel Negotiation
Common Channel
A
B
Selected Channel
ATIMATIM RES(1)
Beacon
ATIMACK(1)
C
D
Time
ATIM Window
Beacon Interval
Channel Negotiation
Common Channel
A
B
C
D
Selected Channel
ATIMATIM RES(1)
Beacon
ATIMACK(1)
ATIMACK(2)
ATIM
ATIMRES(2)
Time
ATIM Window
Beacon Interval
Channel Negotiation
Common Channel
A
B
C
D
ATIMATIM RES(1)
Selected Channel
RTS
DATA
Channel 1
Beacon
Channel 1
ATIMACK(1)
ATIMACK(2)
CTS
ACK
CTS
ACK
Channel 2
Channel 2
ATIM
ATIMRES(2)
RTS
DATA
ATIM Window
Beacon Interval
Time
Aggregate Throughput (Kbps)
Wireless LAN - Throughput
2500
2500
MMAC
2000
DCA
1500
1000
500
MMAC
2000
1500
DCA
1000
802.11
1
10
100
1000
Packet arrival rate per flow (packets/sec)
30 nodes
500
1
802.11
10
100
1000
Packet arrival rate per flow (packets/sec)
64 nodes
MMAC shows higher throughput than DCA and 802.11
Discussions
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Clock Synchronization
Overhead due to message exchange
Delay
Some fundamental problems in multi-hop
wireless networks remain unsolved