Using Dynamic PCF to improve the capacity of VoIP traffic in IEEE 802 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

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 Need to support many simultaneous calls stadium concert conventions

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

MAC Protocol in IEEE 802.11 Distributed Coordination Function (DCF) Default MAC protocol Contention Window medium busy DIFS CSMA/CA backoff Next frame defer access Slot Backoff Slots RTS CTS DATA ACK SIFS DIFS CW Virtual carrier sense mechanism (four-way handshake) RTS: Request To Send, CTS: Clear To Send

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) PIFS SIFS SIFS SIFS SIFS SIFS SIFS DCF Beacon D1+poll D2+Ack +poll poll CF-End U1+ACK U2+ACK Null SIFS < PIFS < DIFS

Theoretical Capacity for VoIP DCF vs. PCF VBR (with silence suppression) = CBR / Active Ratio (3.8)

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 Data Data Null Null Null ACK ACK ACK Node 1: 10 ms, Node 2: 20 ms, AP: 10 ms PCF intervals 10 ms AP poll poll poll poll poll poll poll poll poll poll poll poll poll poll poll poll 1 2 1 Null 1 2 1 Null 1 2 1 Null 1 2 1 Null

PCF Problems – synchronization Synchronization between polls and data Node side App CFP CP poll poll poll poll poll MAC Null Null AP side CFP CP CFP CP 5 6 7 5 6 7 MAC Polling time Packet generation time 1 2 3 4 Polling time 1 2 3 4 Null Null

Our Proposal: Dynamic PCF Classification of traffic Real-time traffic (VoIP) Use CFP, also CP Best effort traffic Use only CP MAC 1 CFP CP Polling List 3 8 poll 1 poll 3 5 7 9

Dynamic PCF Dynamic Polling List Store only “active” nodes Queue 5 6 7 7 Polling List 1 2 3 4 5 6 7 8 PCF 6 CFP CP CFP CP 5 1 3 8 6 5 Null ACK 2 4 7 5 1 3 2 4 Null 6 7 8 5 Null ACK MAC Polling List 1 3 8 DPCF CFP CP CFP CP 1 3 8 poll 6 5 ACK 7 1 3 8 poll 6 5 ACK 7 MAC

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 1 poll +more poll 2 1 poll 1 poll 2 +more AP 1 2

Dynamic PCF More data field Solution to the synchronization problem Node side poll Null poll +more poll App CFP CP poll poll MAC Fail to send

Dynamic PCF Synchronization problem in DPCF AP side PCF MAC DPCF MAC CFP CP PCF 7 8 MAC 1 2 5 Polling time CFP CP DPCF 7 8 MAC Polling time 1 2 5

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 CFP CP App MAC PCF App MAC DPCF CFP CP poll Null poll +more poll

Simulations QualNet Simulator Topology : Wireless to Wireless Commercial simulator, evaluation available Easy graphical + text interface Topology : Wireless to Wireless MN1 MN5 MN2 MN6 AP MN3 MN7 MN4 MN8

Simulations VoIP traffic model ITU-T P59 Our Model 0.5 1.0 1.5 0.23 0.4 0.9 1.3 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

Simulations Measuring the capacity of VoIP Acceptable delay threshold : 60msec

Simulation Results

Simulation Results Delay and throughput with FTP traffic DCF (30 nodes)

Simulation Results Delay and throughput with FTP traffic PCF (30 nodes)

Simulation Results Delay and throughput with FTP traffic DPCF (30 nodes)

Simulation Results Delay and throughput with FTP traffic DPCF2 (30 nodes)

Simulation Results Delay and throughput with FTP traffic DPCF (36 nodes)

Simulation Results Delay and throughput with FTP traffic DPCF2 (36 nodes)

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