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

1
Using Dynamic PCF to improve the capacity of VoIP
traffic in IEEE 802.11 Networks

• Takehiro Kawata (NTT, Japan)
• Sangho Shin, Andrea G. Forte, Henning Schulzrinne
• Dept of Computer Science
• Columbia University
• New York, NY

2
Motivation

• 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

3
Outline

• 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 802.11

• Distributed Coordination Function (DCF)
• Default MAC protocol

5
MAC Protocol in IEEE 802.11

• Point Coordination Function (PCF)
• For real time traffic
• Supports QoS (rudimentary)
• Optional, usually not implemented commercially

Contention Free Repetition Interval (Super Frame)
Contention Free Period (CFP)
Contention Period (CP)
SIFS
SIFS
SIFS
SIFS
SIFS
SIFS
PIFS
DCF
Beacon
poll
D1poll
D2Ack poll
CF-End
U1ACK
U2ACK
Null
SIFS lt PIFS lt DIFS
6
Theoretical Capacity for VoIP

• DCF vs. PCF

VBR (with silence suppression) CBR / Active
Ratio (3.8)
7
PCF Problems

• 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

Data
ACK
ACK
Null
Data
ACK
Data
Null
Null
Node 1 10 ms, Node 2 20 ms, AP 10 ms PCF
intervals
1
1
1
1
1
2
1
2
1
1
2
2
8
PCF Problems synchronization

• Synchronization between polls and data

Node side
App
CFP
CP
MAC
AP side
CFP
CP
CFP
CP
5
6
7
5
6
7
MAC
1
1
3
4
2
3
4
2
Null
Null
9
Our Proposal Dynamic PCF

• Classification of traffic
• Real-time traffic (VoIP)
• Use CFP, also CP
• Best effort traffic
• Use only CP

10
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
11
Dynamic 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

Node 1 10 ms, Node 2 20 ms, AP 20 ms PCF
Intervals
20 ms
poll
poll
poll
AP
1
2
12
Dynamic PCF

• More data field
• Solution to the synchronization problem

Node side
App
CP
CFP
poll
poll
MAC
13
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
1
5
2
14
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

15
Simulations

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

• Topology Wireless to Wireless

MN5
MN1
MN6
MN2
AP
MN7
MN3
MN8
MN4
16
Simulations

• VoIP traffic model
• ITU-T P59

• Our Model

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
Duration (s) Rate ()
1.004 38.53
1.587 61.47

0.508 22.48
17
Simulations

• Measuring the capacity of VoIP
• Acceptable delay threshold 60msec

18
Simulation Results
19
Simulation Results

• Delay and throughput with FTP traffic
• DCF (30 nodes)

20
Simulation Results

• Delay and throughput with FTP traffic
• PCF (30 nodes)

21
Simulation Results

• Delay and throughput with FTP traffic
• DPCF (30 nodes)

22
Simulation Results

• Delay and throughput with FTP traffic
• DPCF2 (30 nodes)

23
Simulation Results

• Delay and throughput with FTP traffic
• DPCF (36 nodes)

24
Simulation Results

• Delay and throughput with FTP traffic
• DPCF2 (36 nodes)

25
Conclusions

• Dynamic PCF
• Improved VoIP capacity by 20
• When mixed with FTP traffic, higher throughput
  and lower delay

MAC Scheme DCF PCF DPCF
Capacity ( of calls) 30 30 36
http//www.cs.columbia.edu/IRT/wireless