Multi Channel Problem Analysis - cgu.edu.tw
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Transcript of Multi Channel Problem Analysis - cgu.edu.tw
Introduction
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CH 1
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CH 6
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CH 11
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2MHz (Wi-Fi) IEEE 802.11b/g
2401MHz 2483MHz
CH
(ZigBee) IEEE 802.15.4
Wi-Fi
Using 2.4 GHz ISM Band
Channel CH11~CH26: 16 channels
Channel interval: 5MHz
Channel bandwidth: 2MHz
IEEE 802.11
ZigBee
Using 2.4 GHz ISM Band
Channel CH1~CH13: 13 channels
Channel interval: 5MHz
Channel bandwidth: 22MHz
IEEE 802.15.4
22MHz
IEEE 802.11 Distributed Coordination
Function (DCF)
• In DCF: Before transmitting a packet, a node has to wait for a small duration of time even if the channel is idle.
• This is called interframe spacing.
• SIFS, PIFS, DIFS, and EIFS are the four interframe spacings.
• A node waits for a DIFS before transmitting an RTS.
• Waits for a SIFS before sending a CTS or an ACK.
• SIFS duration is smaller than a DIFS.
• Distributed Coordination Function (DCF)
– Use CSMA/CA to sense the medium is busy or idle.
– Exchange short control frames (RTS/CTS) to further minimize collisions.
– Use a random backoff procedure to resolve contention conflicts
Introduction
DIFS
DIFS
DIFS
SIFS SIFS
A B C D
A
B
C
D
RTS
CTS
NAV
DATA
SIFS
ACK
CWbackoff = 3
1 2 3
Time
NAV Contention Window
Contention Window
Channel Comparison
• Single channel:
• control channel and data channel on the same channel.
• Multi channel:
– Reduce contention
– Improve throughput
Multi-channel: Case 1
Control Channel (CC)
Data Channel (DC_1)
Data Channel (DC_2)
Data Channel (DC_3)
Multi-channel: Case 2
CC
CC
CC
CC
DC_1
DC_2
DC_3
DC_4
Multi-channel: Case 3
DC CC DC
CC DC DC
DC DC CC
DC CC DC
Challenges for multi-channel MAC protocols
• Multi-channel hidden terminal problem
• Missing receiver problem
• Broadcast support problem
• Channel switching delay problem
• Multi-channel Hidden Terminal Problem
• Channel 1 is the common control channel
A B C D
RTS
CTS(2)
Channel 1
Channel 2
Channel 3
Channel 2
CTS(2)
DATA
Channel 2
DATA
Collision
ACK
Time
RTS
B C A D
.
.
.
Channel 2 Channel 1
Channel 3
Channel
switching
• Channel 1 is the common control channel
Broadcast support problem
A B C D E
CC
DC1
DC2
Sleep
S
broadcast broadcast
X X X
Channel switching delay problem
• Switching amongst channels may take
considerable time
• Hence may increase delay and degrade
throughput.
Dedicated control channel approach
CH4(Data)
CH3(Data)
CH2(Data)
CH1(Control) RTS2 CTS2 RTS3 CTS3 RTS4 CTS4
DATA2 ACK
DATA3 ACK
DATA4
Dedicated control channel approach
• Advantage:
– it does not require time synchronization
• Disadvantage:
– requires a dedicated control channel
Channel hopping approach
CH4
CH3
CH2
CH1 RTS CTS
RTS CTS
RTS CTS
RTS
DATA1 ACK
DATA3 ACK
DATA4
control period transmission period
control period
transmission period
idle
idle
transmission period
Channel hopping approach
• Advantage:
– Only one transceiver per node
– can use all the channels for data transmission
• Disadvantage:
– Need time synchronization
Time division approach
CH4
CH3
CH2
CH1 RTS1 CTS1 RTS3 CTS3 DATA1 ACK
DATA3
RTS4 CTS4
DATA4
ACK
Control phase Control phase Data exchange phase ........
Time division approach
• Advantage:
– Only one radio per node
• Disadvantage:
– Need time synchronization
– Waste other channels In control phase
Multiple transceivers approach
Multiple transceivers approach
• Advantage:
– can receive packets on all channels the same time
• Disadvantage:
– increases the energy consumption
– High Hardware cost
Multi Channel Single Transceiver
A B C E D
Channel 1
Channel 3
Channel 2
• Channel 1 is the common control channel
A B C D E
Multi Channel Two Transceiver
Triple hidden terminals(THT)
• Multi channel hidden
• Sleep hidden terminal
• Multi hop hidden
Multi Channel Single Hop Hidden Terminal
Problem
A B C E D
CC
DC1
DC2
X
C
E
D A
B
A B C E D
CC
DC1
DC2
Sleep
X
C
E
D A
B
Multi Channel Single Hop Hidden Terminal
Problem due to Sleep State
X
