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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. 1. 1. 2. defer. Motivation. Multiple Channels available in IEEE 802.11 3 channels in 802.11b - PowerPoint PPT Presentation
Citation preview
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
• 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
Motivation
1
defer
1
2
Single channel Multiple Channels
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– Requires modification of coordination schemes among the nodes
• 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
1 2
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 Function
A
B
C
D
A B C D
Time
802.11 Distributed Coordination Function
A
B
C
D
A B C D
RTS
Time
RTS
802.11 Distributed Coordination Function
A
B
C
D
A B C D
RTS
CTS
SIFS
NAV Time
CTS
802.11 Distributed Coordination Function
A
B
C
D
A B C D
RTS
CTS
DATA
SIFS
NAV
NAV
Time
DATA
802.11 Distributed Coordination Function
A
B
C
D
A B C D
RTS
CTS
DATA
SIFS
ACK
NAV
NAV
Time
ACK
802.11 Distributed Coordination Function
A
B
C
D
A B C D
RTS
CTS
DATA
SIFS
ACK
NAV
NAV
DIFS
Time
Contention Window
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
A
B
C
Time
Beacon
ATIM Window
Beacon Interval
Issues in Multi-Channel Environment
Multi-Channel Hidden Terminal Problem
Multi-Channel Hidden Terminals
A B CRTS
A sends RTS
Channel 1
Channel 2
Multi-Channel Hidden Terminals
A B CCTS
B sends CTS
Channel 1
Channel 2
C does not hear CTS because C is listening on channel 2
Multi-Channel Hidden Terminals
A B CDATA
C switches to channel 1 and transmits RTS
Channel 1
Channel 2
Collision occurs at B
RTS
Related Work
Previous work on multi-channel MAC
Nasipuri’s Protocol
• Assumes N transceivers per host– Capable of listening to all channels simultaneously– Always have information for all channels
• Disadvantage: High hardware cost
Wu’s Protocol [Wu00ISPAN]Dynamic Channel Assignment
• 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 sends DATA on the selected data channel
Wu’s Protocol (cont.)
• Advantage– No synchronization required
• Disadvantage– Each host must have 2 transceivers– 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
MMAC
Assumptions
- All channels have same BW and none of them are overlapping channels
- Nodes have only one transceiver
- Transceivers are capable of switching channels but they are half-duplex
- Channel switching delay is approx 250 us, avoid per packet switching
- Nodes synchronized: Begin their beacon intervals same time
MMAC
Steps –
- Divide time into beacon intervals
- At the beginning, nodes listen to a pre-defined channel for ATIM window duration
- Channel negotiation starts using ATIM messages
- Nodes switch to the selected channel after the ATIM window duration
MMAC
Preferred Channel List (PCL)
- For a node, PCL records usage of channels inside Tx range
- HIGH preference – always selected
- MID preference – others in the vicinity did not select the channel
- LOW preference – others in the vicinity selected the channel
MMAC
Channel Negotiation
- Sender transmits ATIM to the receiver and includes its PCL in the ATIM packet
- Receiver selects a channel based on sender’s PCL and its own PCL
- Receiver sends ATIM-ACK to sender including the selected channel
- Sender sends ATIM-RES to notify its neighbors of the selected channel
Channel Negotiation
A
B
C
DTime
ATIM Window
Beacon Interval
Common Channel Selected Channel
Beacon
Channel Negotiation
A
B
C
D
ATIM
ATIM-ACK(1)
ATIM-RES(1)
Time
ATIM Window
Beacon Interval
Common Channel Selected Channel
Beacon
Channel Negotiation
A
B
C
D
ATIM
ATIM-ACK(1)
ATIM-RES(1)
ATIM-ACK(2)
ATIM ATIM-RES(2)
Time
ATIM Window
Beacon Interval
Common Channel Selected Channel
Beacon
Channel Negotiation
A
B
C
D
ATIM
ATIM-ACK(1)
ATIM-RES(1)
ATIM-ACK(2)
ATIM ATIM-RES(2)
Time
ATIM Window
Beacon Interval
Common Channel Selected Channel
Beacon
RTS
CTS
RTS
CTS
DATA
ACK
ACK
DATA
Channel 1
Channel 1
Channel 2
Channel 2
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
Wireless LAN - Throughput
30 nodes 64 nodes
MMAC
DCA
802.11
MMAC shows higher throughput than DCA and 802.11
802.11
DCA
MMAC
Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec)
1 10 100 1000 1 10 100 1000
2500
2000
1500
1000
500
Agg
rega
te T
hrou
ghpu
t (K
bps)
2500
2000
1500
1000
500
Multi-hop Network – Throughput
3 channels 4 channels
MMAC
DCA
802.11802.11
DCA
MMAC
Packet arrival rate per flow (packets/sec)1 10 100 1000
Packet arrival rate per flow (packets/sec)1 10 100 1000
Agg
rega
te T
hrou
ghpu
t (K
bps)
1500
1000
500
0
2000
1500
1000
500
0
Analysis
For DCA: BW of control channel significantly affects the performance and it’s difficult to adapt control channel BW
- For MMAC:
1. ATIM window size significantly affects performance
2. ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon interval – reduced overhead
3. ATIM window size can be adapted to traffic load
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
• 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!
Sanhita Ganguly