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

This paper discusses the use of Dynamic PCF to enhance the capacity of VoIP traffic in IEEE 802.11 networks, addressing the limitations of current MAC protocols. The proposed solution increases the number of simultaneous calls supported in scenarios such as stadiums, concerts, and conventions.

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

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  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. Need to support many simultaneous calls • stadium • concert • conventions 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. CSMA/CA Contention Window DIFS DIFS Next frame backoff medium busy defer access Slot Virtual carrier sense mechanism (four-way handshake) CW SIFS SIFS SIFS 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

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

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

  7. poll poll poll poll poll poll poll poll poll poll poll poll poll poll poll poll Null Null Null Null 10 ms AP 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 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

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

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

  10. 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

  11. poll poll poll poll poll poll poll poll 1 1 2 1 2 2 1 1 1 +more +more +more 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. 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

  13. 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

  14. 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 (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 MN1 MN5 MN2 MN6 AP MN3 MN7 MN4 MN8

  16. 0.9 0.4 0.23 1.0 0.5 1.3 1.5 Simulations • VoIP traffic model • ITU-T P59 • Our Model

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

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