Using Dynamic PCF to improve the capacity of VoIUsing Dynamic PCF to improve the capacity of VoIP traffic in IEEE 802.11 NetworksP traffic in IEEE 802.11 Networks

Takehiro Kawata (NTT, Japan)Sangho Shin, Andrea G. Forte, Henning Schulzrinne

Dept of Computer ScienceColumbia University

New York, NY

2

MotivationMotivation VoIP in wireless networks

WIFI phone, VoIP clients for PDAs

Limited capacity 802.11a/b/g: nowhere close to 11 Mb/s 54 Mb/s 802.11b/g: only 3 non-interfering channels

limited AP count Need to support many simultaneous calls

stadium concert conventions

3

OutlineOutline

Medium access control (MAC) in IEEE 802.11 LANs

Theoretical capacity of VoIP in IEEE 802.11 LANs

Modified MAC protocol: DPCF Simulation and results Conclusions

4

MAC Protocol in IEEE 802.11MAC Protocol in IEEE 802.11 Distributed Coordination Function (DCF)

Default MAC protocol

Backoff SlotsRTS

CTS

DATA

ACK

SIFS SIFS SIFS DIFS CW

Virtual carrier sense mechanism (four-way handshake)

RTS: Request To Send, CTS: Clear To Send

Contention Window

medium busy

DIFS DIFS

CSMA/CA

backoff Next frame

defer access Slot

5

MAC Protocol in IEEE 802.11MAC Protocol in IEEE 802.11 Point Coordination Function (PCF)

For real time traffic Supports QoS (rudimentary) Optional, usually not implemented commercially

Beacon D1+poll

U1+ACK

D2+Ack+poll

U2+ACK

CF-End

SIFS SIFS SIFS SIFS SIFS

Contention Free Period (CFP)

Contention Period (CP)

Contention Free Repetition Interval (Super Frame)

poll

Null

SIFSDCF

PIFS

SIFS < PIFS < DIFS

6

Theoretical Capacity for VoIPTheoretical Capacity for VoIP DCF vs. PCF

VBR (with silence suppression) = CBR / Active Ratio (3.8)

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8 9 10 11

Bit Rate (Mb/s)

Num

ber

of c

alls

CBRVBRDCFPCF

7

PCF ProblemsPCF Problems Waste of polls

VoIP traffic with Silence Suppression

1

poll

1

poll

Data

1

poll poll

NullData

poll

Null

poll

Null

1

ACK

1

ACK

1

ACK

Talking Period Mutual Silence Period Listening Period

Data

1

poll

1

poll

2Null

poll poll

1

poll

1

poll

2Null

poll poll

1

poll

1

poll

2Null

poll poll

1

poll

1

poll

2Null

poll poll

10 ms

AP

Various packetization intervalsNode 1: 10 ms, Node 2: 20 ms, AP: 10 ms PCF intervals

8

PCF Problems – PCF Problems – synchronizationsynchronization

Synchronization between polls and data

poll poll poll

Null

CFP CPpoll

Null

poll

App

MAC

Node side

1MAC

2 3 4

5 6 7

CFP CP

AP side

1 2 3 4

CFP CP

NullNullPolling time

Packet generation timePolling time

5 6 7

9

Our Proposal: Dynamic Our Proposal: Dynamic PCFPCF

Classification of traffic Real-time traffic (VoIP)

Use CFP, also CP Best effort traffic

Use only CP

1

poll

3

poll

5

poll

1 3 5 7 9

7

poll

9

poll

MAC

1

CFP CP

Polling List 3 8

1

poll

3

poll

8

poll

10

Dynamic PCFDynamic PCF

Dynamic Polling List Store only “active” nodes

MAC

CFP CP

Polling List 1 23 4

MAC

CFP CP

5678

1 3 8

65

Null NullACK

1 2 3 4

ACK

7 8

7

ACK

567

Queue

CFP CP

567

CFP CP

PCF

Polling List 1 3 8DPCF

5

1 3

1 2 3 4

Null

6 7 8

65 7

Null ACKACKACK

8

1 3 8

pollpoll poll

65

ACKACK

7

ACK

1 3 8

pollpoll poll

65

ACKACK

7

ACK

11

Dynamic PCFDynamic PCF More data field

Set “more data field” when there are more than two packets to send in the queue

Solution to the various packetization intervals problem

1

poll

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

2

poll poll

20 ms

1

poll

+more

21

poll

1

poll

1

poll

1

poll

2

poll

+more

2

poll

1

poll

+more

AP

12

poll poll

Dynamic PCFDynamic PCF More data field

Solution to the synchronization problem

poll

Null

CFP CPpoll

App

MAC

Node side

pollpoll

+more

Fail to send

13

Dynamic PCFDynamic PCF Synchronization problem in DPCF

MAC

AP side

1 2

7 8

CFP CP

Polling time

5

MAC1 2

7 8

CFP CP

Polling time

5

PCF

DPCF

14

App

MAC

DPCF

CFP CP

Dynamic PCF (DPCF2)Dynamic PCF (DPCF2) Solution to the Synchronization problem

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

poll poll

Null

poll pollCFP CP

poll poll

App

MAC

PCF

poll

poll

+more

poll pollpoll

15

SimulationsSimulations

QualNet Simulator Commercial simulator, evaluation available Easy graphical + text interface

Topology : Wireless to Wireless

AP

MN5

MN6

MN7

MN8

MN1

MN2

MN3

MN4

16

SimulationsSimulations VoIP traffic model

ITU-T P59

Parameter Duration (s)

Rate (%)

Talk-spurt 1.004 38.53

Pause 1.587 61.47

Double-Talk 0.228 6.59

Mutual Silence

0.508 22.48

0.51.0

1.5

0.23 0.40.9

1.3

Duration (s)

Rate (%)

1.004 38.53

1.587 61.47

0.508 22.48

Our Model

17

SimulationsSimulations Measuring the capacity of VoIP

Acceptable delay threshold : 60msec

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 5 10 15 20 25 30 35 40

Number of VoIP

End

-to

-En

d D

ela

y /

Jitte

r (s

ec)

Delay uplink (90%)Jitter uplink (90%)Delay downlink (90%)Jitter downlink (90%)

18

Simulation ResultsSimulation Results

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6 7 8 9 10 11 12

Transmission Rate (Mbps)

Nu

mb

er

of

Vo

IP N

od

es

DPCF

PCF

DCF

SimulationCalculation

19

Simulation ResultsSimulation Results Delay and throughput with FTP traffic

DCF (30 nodes)

35.5

372.9

468.8

520.4

0

500

1000

1500

2000

2500

3000

0.00 1.00 2 3

Number of FTP Sessions

Th

rou

gh

pu

t (k

bp

s)

0

100

200

300

400

500

600

En

d-t

o-E

nd

De

lay

(ms)

FTP Throughput (upload)

VoIP Throughput

VoIP Delay (90%)

20

Simulation ResultsSimulation Results Delay and throughput with FTP traffic

PCF (30 nodes)

55.1

134.0

285.5

430.0

0

500

1000

1500

2000

2500

3000

0.00 1.00 2 3

Number of FTP Sessions

Th

rou

gh

pu

t (k

bp

s)

0

100

200

300

400

500

600

En

d-t

o-E

nd

De

lay

(ms)

FTP Throughput (upload)

VoIP Throughput

VoIP Delay (90%)

21

Simulation ResultsSimulation Results Delay and throughput with FTP traffic

DPCF (30 nodes)

51.348.245.028.9

0

500

1000

1500

2000

2500

3000

0.00 1.00 2 3

Number of FTP Sessions

Th

rou

gh

pu

t (k

bp

s)

0

100

200

300

400

500

600

En

d-t

o-E

nd

De

lay

(ms)

FTP Throughput (upload)

VoIP Throughput

VoIP Delay (90%)

22

Simulation ResultsSimulation Results Delay and throughput with FTP traffic

DPCF2 (30 nodes)

52.449.546.150.2

0

500

1000

1500

2000

2500

3000

0.00 1.00 2 3

Number of FTP Sessions

Th

rou

gh

pu

t (k

bp

s)

0

100

200

300

400

500

600

En

d-t

o-E

nd

De

lay

(ms)

FTP Throughput (upload)

VoIP Throughput

VoIP Delay (90%)

23

Simulation ResultsSimulation Results Delay and throughput with FTP traffic

DPCF (36 nodes)

54.974.6

97.4116.8

0

500

1000

1500

2000

2500

3000

0.00 1.00 2 3

Number of FTP Sessions

Th

rou

gh

pu

t (k

bp

s)

0

100

200

300

400

500

600

En

d-t

o-E

nd

De

lay

(ms)

FTP Throughput (upload)

VoIP Throughput

VoIP Delay (90%)

24

Simulation ResultsSimulation Results Delay and throughput with FTP traffic

DPCF2 (36 nodes)

55.980.7

110.1140.4

0

500

1000

1500

2000

2500

3000

0.00 1.00 2 3

Number of FTP Sessions

Th

rou

gh

pu

t (k

bp

s)

0

100

200

300

400

500

600

En

d-t

o-E

nd

De

lay

(ms)

FTP Throughput (upload)

VoIP Throughput

VoIP Delay (90%)

25

ConclusionsConclusions Dynamic PCF

Improved VoIP capacity by 20% When mixed with FTP traffic, higher throug

hput and lower delay

MAC Scheme DCF PCF DPCFCapacity (# of calls) 30 30 36

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