ECE 256, Spring 2009__________
Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals
Using A Single Transceiver __________________
Paper by Jungmin So & Nitin VaidyaUniversity of Illinois at Urbana-Champaign
ACM MobiHoc ‘04
Presenter: Sandip Agrawal, Duke University
Acknowledgments
Slides courtesy:
Jungmin So and Nitin Vaidya
http://www.crhc.uiuc.edu/wireless/groupPubs.html
Topics Introduction
o Motivationo Problem Statement
Preliminarieso 802.11 DCF structureo 802.11 PSM mode
Issues in multi-channel environment Other works in multi-channel MAC Proposed MMAC Simulation results Discussions
Motivation Multiple Channels available in IEEE 802.11
802.11b – 14 channels in PHY layer – 3 of them are used (1,6,11)
802.11a – 12 channels – 8 in the lower part of the band for indoor use and rest in higher for outdoor us
‘Exploit multiple channels to improve network throughput’ … why ?
Allow Simultaneous Transmissions
1
defer
1
2
Single channel Multiple Channels
Problem Statement The ideal scenario – use k channels to improve
throughput by a factor of k• Reality is different…Nodes listening on different
channels cannot talk to each other
Constraint : Single Transceiver - Can listen to only one channel at a time
Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance (at least possible cost and complexity)
1 2
Topics Introduction
Motivation Problem Statement
Preliminaries 802.11 DCF structure 802.11 PSM mode
Issues in multi-channel environment Other works in multi-channel MAC Proposed MMAC Simulation results Discussions
802.11 DCF (Distributed Coordinate Function)
Designed for sharing a single channel between the hosts
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 DCF
A B C D
A
B
C
D
Time
802.11 DCF
A B C DRTS
A
B
C
D
RTS
Time
802.11 DCF
A B C DCTS
A
B
C
D
RTS
CTS
SIFS
NAV Time
802.11 DCF
A
B
C
D
A B C D
RTS
CTS
DATA
SIFS
NAV
NAV
Time
DATA
802.11 DCF
A
B
C
D
A B C D
RTS
CTS
DATA
SIFS
ACK
NAV
NAV
Time
ACK
802.11 DCF
A
B
C
D
A B C D
RTS
CTS
DATA
SIFS
ACK
NAV
NAV
Time
Contention Window
802.11 PSM (Power Saving Mechanism)
A
B
C
Time
Beacon
ATIM Window
Beacon Interval
Doze mode – less energy consumption but no communication• ATIM – Ad hoc Traffic Indication Message
802.11 PSM (Power Saving Mechanism)
A
B
C
Time
Beacon
ATIM
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mechanism)
A
B
C
Time
Beacon
ATIM
ATIM-ACK
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mechanism)
A
B
C
Time
Beacon
ATIM
ATIM-ACK
ATIM-RES
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mechanism)
A
B
C
Time
Beacon
ATIM
ATIM-ACK
DATAATIM-RES
Doze Mode
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mechanism)
A
B
C
Time
Beacon
ATIM
ATIM-ACK
DATA
ACK
ATIM-RES
Doze Mode
ATIM Window
Beacon Interval
In essence … All nodes wake up at the beginning of a beacon interval
for a fixed duration of time (ATIM window)
Exchange ATIM 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
Topics Introduction
Motivation Problem Statement
Preliminaries 802.11 DCF structure 802.11 PSM mode
Issues in multi-channel environment Other works in multi-channel MAC Proposed MMAC Simulation results Discussions
Multi-channel Hidden Terminals
Multi-channel Hidden Terminals
Observations
1. Nodes may listen to different channels2. Virtual Carrier Sensing becomes difficult3. The problem was absent for single channel
Possible approaches
1. Use multiple transcievers2. Exploit synchronization technique available from IEEE
802.11 PSM
Topics Introduction
Motivation Problem Statement
Preliminaries 802.11 DCF structure 802.11 PSM mode
Issues in multi-channel environment Other works in multi-channel MAC Proposed MMAC Simulation results Discussions
Nasipuri’s Protocol
N transceivers per host- Capable of listening all channels
simultaneously
• Find an idle channel and transmit – sender’s policy
• Channel selection should be based on channel condition on receiver side
• High hardware cost
Wu’s Protocol
2 transceivers per host One transceiver always listens on control channel
• Sender includes preferred channel list in RTS, receiver picks one and tells in CTS
• Sender sends DATA on the selected data channel
No synchronization required
Control channel’s BW becomes 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
Topics Introduction
Motivation Problem Statement
Preliminaries 802.11 DCF structure 802.11 PSM mode
Issues in multi-channel environment Other works in multi-channel MAC Proposed MMAC Simulation results Discussions
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
- Multi-hop synch is achieved by other means
MMAC
Idea similar to IEEE 802.11 PSM
- 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 agreed upon channel after the ATIM window duration
MMAC Preferred Channel List (PCL)
- For a node, PCL records usage of channels inside Transmission 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
- In ATM window, sender transmits ATIM …. Includes its PCL
- 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
MMAC
A
B
C
DTime
ATIM WindowBeacon Interval
Common Channel Selected Channel
Beacon
MMAC
A
B
C
D
ATIM
ATIM-ACK(1)
ATIM-RES(1)
TimeATIM Window
Common Channel Selected Channel
Beacon
MMAC
A
B
C
D
ATIM
ATIM-ACK(1)
ATIM-RES(1)
ATIM-ACK(2)
ATIM ATIM-RES(2)
Time
ATIM Window
Common Channel Selected Channel
Beacon
MMAC
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
Topics Introduction
Motivation Problem Statement
Preliminaries 802.11 DCF structure 802.11 PSM mode
Issues in multi-channel environment Other works in multi-channel MAC Proposed MMAC Simulation results Discussions
Parameters
Transmission rate: 2MbpsTransmission range: 250mTraffic type: Constant Bit Rate (CBR)Beacon interval: 100ms
Packet size: 512 bytesATIM window size: 20msDefault number of channels: 3 channels
Compared protocols802.11: IEEE 802.11 single channel protocolDCA: Wu’s protocolMMAC: Proposed protocol
WLAN - Throughput
MMAC
DCA
802.11
MMAC
DCA
802.11
30 nodes64 nodes
MMAC shows higher throughput than DCA and 802.11
Multihop 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
Throughput of DCA and MMAC
DCA MMAC
2 channels
802.11
MMAC shows higher throughput compared to DCA
6 channels
802.11
2 channels
6 channels
Agg
rega
te T
hrou
ghpu
t (K
bps) 4000
3000
2000
1000
0
4000
3000
2000
1000
0
Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec)
Analysis of Results 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
Topics Introduction
Motivation Problem Statement
Preliminaries 802.11 DCF structure 802.11 PSM mode
Issues in multi-channel environment Other works in multi-channel MAC Proposed MMAC Simulation results Discussions
Discussions 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
Instead of counting source-destination pair for calculating channel usage, counting the number of pending packets may be a better idea
Starvation can occur with common source and multiple destinations
Questions??? While criticizing Wu’s protocol – control channel
‘prevents the data channel from being fully utilized’ … why ?
Source and Destinations may not be in one hop distance and may not be communicated within a beacon interval
Thank You!