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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. 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

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  1. Improving Capacity of VoIP in IEEE 802.11 Networks Takehiro Kawata Sangho Shin Andrea G. Forte Henning Schulzrinne

  2. Need for support of many simultaneous calls • Stadium • Concert 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. Theoretical Capacity of VoIP • DCF (IEEE 802.11b)

  10. Theoretical Capacity of VoIP • DCF (IEEE 802.11b)

  11. 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

  12. Theoretical Capacity of VoIP • PCF (IEEE 802.11b)

  13. 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 ??

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. 0.23 1.0 0.5 0.9 0.3 1.5 Simulations • VoIP traffic model • ITU-T P59 • Our Model

  25. Simulations • Deciding the capacity of VoIP • Threshold : 50 ms

  26. Simulation Results

  27. Simulation Results • Delay and throughput with FTP traffics • DCF (33 nodes)

  28. Simulation Results • Delay and throughput with FTP traffics • PCF (33 nodes)

  29. Simulation Results • Delay and throughput with FTP traffics • DPCF (33 nodes)

  30. Simulation Results DCF 33 nodes PCF 33 nodes DPCF 33 nodes

  31. 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.

  32. Thank you http://www.cs.columbia.edu/IRT/wireless

  33. Simulation Results

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