Multi Channel Problem Analysis - cgu.edu.tw

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Multi Channel Problem Analysis

Prasan Kumar Sahoo

pksahoo@mail.cgu.edu.tw

Introduction

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CH 6

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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