Wireless Networking & Mobile Computing
CS 752/852 - Spring 2012
Lec #7: MAC Multichannel
Tamer Nadeem
Dept. of Computer Science
Multi-Channel MAC for Ad Hoc
Networks: Handling Multi-Channel
Hidden Terminals Using A Single
Transceiver *
(Jungmin So and Nitin Vaidya)
* Slides adapted from J. So
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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
1
1
defer
Single channel
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2
Multiple Channels
CS 752/852 - Wireless Networking and Mobile Computing
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
1
2
• 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
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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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Basics
802.11 Power Saving Mechanism
Multi-Channel Hidden Terminals
802.11 Power Saving Mechanism
Beacon
Time
A
B
C
ATIM Window
Beacon Interval
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802.11 Power Saving Mechanism
Beacon
A
Time
ATIM
B
C
ATIM Window
Beacon Interval
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802.11 Power Saving Mechanism
Beacon
A
Time
ATIM
B
ATIM-ACK
C
ATIM Window
Beacon Interval
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802.11 Power Saving Mechanism
Beacon
A
ATIM
Time
DATA
B
ATIM-ACK
Doze Mode
C
ATIM Window
Beacon Interval
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802.11 Power Saving Mechanism
Beacon
A
ATIM
Time
DATA
B
ATIM-ACK
ACK
Doze Mode
C
ATIM Window
Beacon Interval
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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
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Multi-Channel Hidden Terminals
Channel 1
Channel 2
A
RTS
C
B
A sends RTS
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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
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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
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Related Work
Previous work on multi-channel MAC
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
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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
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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
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Protocol Description
Multi-Channel MAC (MMAC) Protocol
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 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
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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
A
B
Selected Channel
ATIMATIM RES(1)
Beacon
ATIMACK(1)
C
D
Time
ATIM Window
Beacon Interval
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Channel Negotiation
Common Channel
A
B
ATIMATIM RES(1)
Beacon
ATIMACK(1)
ATIMACK(2)
C
D
Selected Channel
ATIM
ATIMRES(2)
Time
ATIM Window
Beacon Interval
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Channel Negotiation
Common Channel
A
B
ATIMATIM RES(1)
RTS
DATA
Channel 1
Beacon
Channel 1
ATIMACK(1)
ATIMACK(2)
C
D
Selected Channel
CTS
ACK
CTS
ACK
Channel 2
Channel 2
ATIM
ATIMRES(2)
RTS
DATA
Time
ATIM Window
Beacon Interval
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Performance Evaluation
Simulation Model
Simulation Results
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
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Aggregate Throughput (Kbps)
Wireless LAN - Throughput
2500
2500
MMAC
2000
DCA
1500
1000
MMAC
2000
1500
DCA
1000
802.11
500
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
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Aggregate Throughput (Kbps)
Multi-hop Network – Throughput
1500
MMAC
DCA
1000
2000
MMAC
1500
DCA
1000
500
802.11
0
802.11
0
1
10
100
1000
Packet arrival rate per flow (packets/sec)
3 channels
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500
Spring 2012
1
10
100
1000
Packet arrival rate per flow (packets/sec)
4 channels
CS 752/852 - Wireless Networking and Mobile Computing
Aggregate Throughput (Kbps)
Throughput of DCA and MMAC
(Wireless LAN)
4000
4000
6 channels
3000
3000
6 channels
2000
2000
3 channels
3 channels
1000
1000
802.11
802.11
0
0
Packet arrival rate per flow (packets/sec)
DCA
Packet arrival rate per flow (packets/sec)
MMAC
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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Partially Overlapped Channels Not
Considered Harmful *
(Arunesh Mishra, Vivek Shrivastava,
Suman Banerjee, William Arbaugh)
* Slides adapted from Ashwin Wagadarikar, Duke
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Spectral Bands and Channels
• Wireless communication uses electromagnetic signals over a
range of frequencies
• FCC has split the spectrum into spectral bands
• Each spectral band is split into channels
Example of a channel
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Typical usage of spectral band
• Transmitter-receiver pairs use independent channels
that don’t overlap to avoid interference.
Channel A
Channel B
Channel C
Channel D
Fixed Block of Radio Frequency Spectrum
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Ideal usuage of channel bandwidth
• Should use entire range of freqs spanning a channel
• Usage drops down to 0 just outside channel boundary
Channel B
Channel C
Channel D
Power
Channel A
Frequency
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Realistic usage of channel bandwidth
• Realistically, transmitter power output is NOT uniform at all
frequencies of the channel.
Channel B
Channel C
Channel D
Power
Channel A
Real Usage
Wastage of spectrum
• PROBLEM:
• Transmitted power of some freqs. < max. permissible limit
• Results in lower channel capacity and inefficient usage of the spectrum
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Consideration of the 802.11b standard
• Splits 2.4 GHz band into 11 channels of 22 MHz each
• Channels 1, 6 and 11 don’t overlap
• Can have 2 types of channel interferences:
• Co-channel interference
• Address by RTS/CTS handshakes etc.
• Adjacent channel interference over partially overlapping channels
• Cannot be handled by contention resolution techniques
 Wireless networks in the past have used only non-overlapping channels
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Focus of paper
• Paper examines approaches to use partially
overlapped channels efficiently to improve spectral
utilization
Channel A
Channel B
Channel A’
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Empirical proof of benefits of partial overlap
Link A Ch 1
Ch 1
Ch 3
Ch 6
Link B Ch 3
Link C Ch 6
Amount of Interference
• Can we use channels 1, 3 and 6 without interference ?
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Empirical proof of benefits of partial overlap
Link A Ch 1
Ch 1
Ch 3
Ch 6
Link B Ch 3
Link C Ch 6
Virtually non-overlapping
•
Typically partially overlapped channels are avoided
•
With sufficient spatial separation, they can be used
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Link A Ch 1
Link B Ch X
UDP Throughput (Mbps)
Empirical proof of benefits of partial overlap
6
5
4
3
0
10
20
30
40
50
Distance between the 2 links (meters)
LEGEND
Non-overlapping channels, A = 1, B = 6
Partially Overlapped Channels, A = 1, B = 3
Partially Overlapped Channels, A = 1, B = 2
Same channel, A = 1, B = 1
60
5
2
1
0
Channel Separation
• Partially overlapped channels can provide much
greater spatial re-use if used carefully!
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Interference factor
• To model effects of partial overlap, define:
• Interference Factor or “I-factor”
• Transmitter is on channel j
• Pj denotes power received on channel j
• Pi denotes power received on channel i
I-factor(i,j) =
Pi
Pj
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Theoretical Estimate for I-Factor
Channel B
Channel A
-30 dB
-50 dB
-22 Mhz
-11 Mhz
FcA
FcB
• Theoretically, I-factor = Area of intersection between
two spectrum masks of transmitters on channels A
and B
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Normalized I-factor
Estimating I-Factor at a receiver on channel 6
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1
0.8
I(theory)
0.6
I(measured)
0.4
0.2
0
0
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2
4
6
8 10
Receiver Channel
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WLAN Case study
• WLAN comparison between:
• 3 non-overlapping channels, and
• 11 partially overlapping channels
• over the same spectral band
• WLAN consists of access points (APs) and clients
• AP communicates with clients in its basic service set on a single
channel
• GOAL: allocate channels to AP’s to maximize
performance by reducing interference
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Why use partial overlap?
Consider a case where you have 300 APs
Non-overlap
3 channels, 100 APs each
Partial overlap
5 channels, 60 APs each
60
100
100
60
60
60
60
100
Worst case
Worst case
Interference by all 100 APs
on same channel
Interference by all 60 APs
on same channel + some
interference from POV
channels
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Channel assignment w/ non-overlap
• Mishra et al. previously proposed “client-driven”
approach for channel assignment to APs
• Use Randomized Compaction algorithm
• Optimization criterion: minimize the maximum interference
experienced by each client
• 2 distinct advantages over random channel
assignment:
• Higher throughput over channels
• Load balancing of clients among available APs
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Channel assignment w/ non-overlap
• (X,C) = WLAN
• X = set of APs and C = set of all clients
• How to assign APs to these 3 channels?
• MUST LISTEN TO THE CLIENTS!
• To evaluate a given channel assignment
• Compute interference for each client: cf c   ( ( x)  1)
• Sum taken over APs on same channel since channels are
independent
• Create vector of cfc’s (CF) and sort in non-increasing order
• Optimal channel assignment minimizes CF
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Channel assignment w/ partial overlap
=
+
• Each client builds I-factor model using scan operation
• POV(x,xch,y,ych) = 1 if nodes x and y on their channels
interfere with each other
• To evaluate a given channel assignment
• Compute interference for each client: cf c   ( ( x)  1)
• Sum taken over APs that interfere on own channel + all POV
channels
• Create vector of cfc’s (CF) and sort in non-increasing order
• Optimal channel assignment minimizes CF
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Results for high interference topologies
• 28 randomly generated topologies with 200 clients and 50 APs
– 14 high interference topologies (average of 8 APs in range for
client)
– 14 low interference topologies (average of 4 APs in range for client)
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Results for low interference topologies
• Using partially overlapped channels and I-factor, clients
can experience less contention at the link level.
 Higher layers have better throughput
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Questions
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