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Improving Capacity of VoIP in IEEE 802.11 Networks. Takehiro Kawata Sangho Shin Andrea G. Forte Henning Schulzrinne. Need for support of many simultaneous calls Stadium Concert. Motivation. VoIP in Wireless Networks WIFI Phone, VoIP Client App for PDAs. Limited Capacity

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Improving capacity of voip in ieee 802 11 networks

Improving Capacity of VoIP in IEEE 802.11 Networks

Takehiro Kawata

Sangho Shin

Andrea G. Forte

Henning Schulzrinne


Motivation

Motivation

  • VoIP in Wireless Networks

    • WIFI Phone, VoIP Client App for PDAs

  • Limited Capacity

    • 802.11a/b/g 11 Mb/s ~ 54 Mb/s

    • Limited APs due to interferences


Outline
Outline

  • Medium Access Control (MAC) in IEEE 802.11 LANs

  • Theoretical Capacity of VoIP in IEEE 802.11 LANs

  • Enhanced MAC Protocol : DPCF

  • Simulation and results

  • Conclusions


Mac protocol in ieee 802 11

CSMA/CA

Contention Window

DIFS

DIFS

Next frame

Backoff

Busy Medium

Defer Access

Slot

Virtual Carrier Sense Mechanism (Four way handshake)

CW

SIFS

SIFS

SIFS

DIFS

DIFS

RTS

DATA

Backoff Slots

CTS

ACK

RTS: Request To Send, CTS: Clear To Send

MAC Protocol in IEEE 802.11

  • Distributed Coordination Function (DCF)

    • Default MAC protocol


Mac protocol in ieee 802 111
MAC Protocol in IEEE 802.11

  • Point Coordination Function (PCF)

    • For real time traffics

    • Supports QoS (rudimentary)

    • Optional, almost not implemented commercially

Contention Free Repetition Interval (Super Frame)

Contention Free Period (CFP)

Contention Period (CP)

SIFS

SIFS

SIFS

SIFS

SIFS

SIFS

PIFS

DCF

poll

Beacon

D1+poll

D2+Ack

+poll

CF-End

U1+ACK

U2+ACK

Null

PIFS < SIFS < DIFS


Theoretical capacity of voip
Theoretical Capacity of VoIP

1

N

2

3

Packetization Interval

….

N

2

3

N

3

….

….

1

2

N

1

3

2

1

1

Time

1

N

….

2

3

1

N

3

2

N

….

….

3

1

2


Theoretical capacity of voip1
Theoretical Capacity of VoIP

  • Constant Bit Rate (CBR)

  • No Silence Suppression

1

N

2

N+1

3

Delay

Delay

Delay

Packetization Interval

2

3

….

N+1

….

N

N

N+1

1

N+1

2

3

N

….

1

1

2

1

3

Time

1

2

3

N+1

N

1

N

N+1

….

….

N+1

2

3

1

N

2

….

3


Theoretical capacity of voip2

DIFS

BackOff

SIFS

ACK

Voice

PLCP

MAC

IP

UDP

RTP

Payload

Theoretical Capacity of VoIP

  • DCF

TTotal = Time for sending a voice packet

Packetization Interval

….

N

2

3

N

3

….

….

1

2

N

1

3

2

1

1

Time

1

N

….

2

3

1

N

3

2

N

….

….

3

1

2

N calls = Packtization Interval / ( 2 * TTotal)

TTotoal = TDIFS+Tbackoff+Tvoice+TSIFS+TACK

Average backoff time = CW/2 * Slot time


Theoretical capacity of voip3
Theoretical Capacity of VoIP

  • DCF (IEEE 802.11b)


Theoretical capacity of voip4
Theoretical Capacity of VoIP

  • DCF (IEEE 802.11b)


Theoretical capacity of voip5

SIFS

Voice

PLCP

MAC

IP

UDP

RTP

Payload

Theoretical Capacity of VoIP

  • PCF

PIFS

Contention Free Period

CP

B

C

B

C

B

N

N

2

3

2

3

2

….

….

1

1

1

Time

1

N

1

N

1

2

3

2

3

2

….

….

N calls = (CFP -TBeacon-TSIFS-TCF-End-TPIFS)/(2 * TTotal)

TTotoal = TSIFS+Tvoice

Packtization Interval = CFP Interval

CP = 0


Theoretical capacity of voip6
Theoretical Capacity of VoIP

  • PCF (IEEE 802.11b)


Theoretical capacity of voip7

DCF

SIFS

Voice

PCF

Voice

DIFS

BackOff

SIFS

ACK

Theoretical Capacity of VoIP

  • DCF vs PCF

PCF has so good performance also in VoIP traffic

(w Silence Suppression) ?

Ncalls = 25 /active ratio = 25 / 0.4 = 62 calls ??


Problems of pcf

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

Null

Null

Null

Null

10 ms

AP

Problems of PCF

  • Waste of polls

    • VoIP traffic with Silence Suppression

Talking Period

Mutual Silence Period

Listening Period

poll

poll

poll

poll

poll

poll

1

1

1

1

1

1

  • Various packetization intervals

ACK

Data

ACK

Data

ACK

Data

Null

Null

Null

Node 1 : 10 ms, Node 2 : 20 ms, AP: 10 msPCF Intervals

1

1

1

2

1

1

1

2

1

2

1

2


Problems of pcf1

poll

poll

poll

poll

poll

Null

Null

AP side

CFP

CP

CFP

CP

5

6

7

5

6

7

MAC

3

4

1

1

2

3

4

2

Null

Null

Polling time

Polling time

Packet generation time

Problems of PCF

  • Synchronization between polls and Data

Node side

App

CP

CFP

MAC


Dynamic pcf

poll

poll

poll

poll

poll

poll

poll

poll

9

1

7

5

3

1

Polling List

1

3

8

1

3

5

7

9

CP

CFP

MAC

3

8

Best effort

VoIP

VoIP

Dynamic PCF

  • Classification of traffics

    • Real-time traffic (VoIP)

      • Use PCF, also CF

    • Best effort traffic

      • Use only CF

    • Give higher priority to real-time traffics


Dynamic pcf1

7

6

5

2

3

4

7

8

2

3

4

5

6

7

8

1

1

5

6

7

5

6

7

8

1

1

3

8

3

Null

Null

ACK

ACK

ACK

Null

Null

ACK

ACK

ACK

Polling List

1

3

8

DPCF

poll

poll

poll

poll

poll

poll

5

6

7

5

6

7

8

8

1

3

1

3

ACK

ACK

ACK

ACK

ACK

ACK

Dynamic PCF

  • Dynamic Polling List

    • Store only “active” nodes

Queue

Polling List

1

2

3

4

5

6

7

8

7

PCF

6

CFP

CP

CFP

CP

5

MAC

CFP

CP

CFP

CP

MAC


Dynamic pcf2

Polling List

1

3

8

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

poll

CP

CFP

MAC

3

1

1

8

8

1

3

3

8

1

5

1

8

1

3

8

3

3

8

Polling List

1

3

8

CP

CFP

MAC

5

5

Dynamic PCF

  • Dynamic Polling List

    • Adding a node to a polling list

      • As soon as AP detects VoIP packet in CP

    • Removing a node from a polling list

      • After AP gets 2 consecutive Null packets

5

5

5


Dynamic pcf3

poll

poll

poll

poll

poll

poll

poll

poll

1

1

2

1

2

2

1

1

1

+more

+more

+more

Dynamic PCF

  • Dynamic CFP Interval and More data field

    • Use the biggest packetization interval as a CFP interval.

    • Set “more data field” when there are more than two packets to send.

    • Solution to the various packetization intervals problem

Node 1 : 10 ms, Node 2 : 20 ms, AP: 20 ms PCF Intervals

20 ms

poll

poll

poll

AP

1

2


Dynamic pcf4

poll

poll

poll

poll

Null

+more

Fail to send

Dynamic PCF

  • More data field

    • Solution to the synchronization problem

Node side

App

CP

CFP

poll

poll

MAC


Dynamic pcf5

Polling time

Dynamic PCF

  • Synchronization problem in DPCF

AP side

CFP

CP

PCF

7

8

MAC

5

1

2

Polling time

CFP

CP

DPCF

7

8

MAC

5

1

2


Dynamic pcf6

PCF

App

CP

CFP

poll

poll

poll

poll

poll

MAC

DPCF

App

CP

CFP

poll

poll

poll

poll

poll

poll

MAC

Null

+more

Dynamic PCF

  • Solution to the Synchronization problem

    • Allow VoIP packets to be sent in CP only when there are more than two VoIP packets in queue


Simulations
Simulations

  • QualNet Simulators

    • Commercial simulator, evaluation available

    • Better physical model than NS2

    • Easy graphical + text interface

  • Topology : Ethernet to Wireless

CN1

MN1

CN2

MN2

Router

AP

CN3

MN3

CN4

MN4


Simulations1

0.23

1.0

0.5

0.9

0.3

1.5

Simulations

  • VoIP traffic model

    • ITU-T P59

  • Our Model


Simulations2
Simulations

  • Deciding the capacity of VoIP

    • Threshold : 50 ms



Simulation results1
Simulation Results

  • Delay and throughput with FTP traffics

    • DCF (33 nodes)


Simulation results2
Simulation Results

  • Delay and throughput with FTP traffics

    • PCF (33 nodes)


Simulation results3
Simulation Results

  • Delay and throughput with FTP traffics

    • DPCF (33 nodes)


Simulation results4
Simulation Results

DCF 33 nodes

PCF 33 nodes

DPCF 33 nodes


Conclusions
Conclusions

  • Estimated capacity of VoIP theoretically and with simulation.

  • Proposed Dynamic PCF.

    • Dynamic Polling List

    • More data field

    • Synchronization

  • Improved VoIP capacity by 25%.

  • With FTP traffics, DPCF has lower delay and higher throughput.


Thank you

http://www.cs.columbia.edu/IRT/wireless



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