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Voice over WiFi

Voice over WiFi. R94922049 張素熒 R94922050 朱原陞 R94922127 王振宇 R94944012 許雅鈴. Outline. Introduction M-M scheme over DCF Implicit signaling over PCF 802.11e future. Introduction. WiFi Wireless Fidelity Radio technology : 802.11 a/b/g Voice over WiFi meet QoS requirement ? DCF

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Voice over WiFi

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  1. Voice over WiFi R94922049 張素熒 R94922050 朱原陞 R94922127 王振宇 R94944012 許雅鈴

  2. Outline • Introduction • M-M scheme over DCF • Implicit signaling over PCF • 802.11e • future

  3. Introduction • WiFi • Wireless Fidelity • Radio technology : 802.11 a/b/g • Voice over WiFi • meet QoS requirement ? • DCF • PCF • 802.11e

  4. Background • Codecs • Voice signals are encoded and compressed into a low-rate packet streams by codecs. • Access mechanisms • Point coordination function (PCF) • Distributed coordination function (DCF)

  5. DCF • More robust than PCF • Basic operation of 802.11 DCF

  6. 802.11 Multicasting • No ACK mechanism for multicasting in 802.11 • Excessive multicasting packet loss due to collision is a fundamental problem in WLAN

  7. VoIP over a 802.11 WLAN • Problems • Low VoIP call capacity • Unacceptable VoIP performance • Longer delay • Coexisting with other applications • Packet loss rate

  8. Solution • Multiplexing-multicasting (M-M) scheme • Multiplexing packets from several VoIP streams into one multicasting packet • # stream: 2n →n+1 (n: # VoIP sessions)

  9. M-M Scheme

  10. MUX / DEMUX delay • Target: no more than 1% of the downlink/uplink VoIP packets should suffer a local delay of more than 30ms • MUX / DEMUX delay is negligible • If delays were to be normally distributed, less than 0.27% of the packets would suffer local delays larger than 30ms

  11. Improvement of VoIP Call Capacity • M-M scheme can nearly double the capacity for most of the codecs

  12. Is M-M Scheme Enough? • Delay • Coexisting with TCP interference traffic • Packet loss • Buffer overflow • Downlink multicast stream may collide with uplink unicast stream

  13. Solution to Delay Problem • Priority queuing (PQ) • Voice packets are given priority over TCP packets within the AP buffer • Limiting # VoIP sessions to below the VoIP capacity • The performance gain for VoIP is not at the expense of TCP throughput

  14. Solution to Packet-loss Problem • MAC-layer multicast priority scheme (MMP) • AP waits for a MIFS before transmission • Restrict to only one multicasting node within the WLAN • MIFS • Larger than SIFS • It will not collide with control frames (ex. ACK) • Smaller than DIFS • It will not collide with uplink unicast packets (DCF)

  15. Performance Improvement of VoIP

  16. PCF • Centralize polling scheme • Priority-IFS (PIFS) • Used by the AP to gain and retain control of the wireless channel • SIFS<PIFS<DIFS • Contention free period (CFP) • CFPRate • CFPMaxDuration

  17. Superframe structure

  18. Problems of PCF • Stretching effect on CFP • CFP is not long enough to poll all stations in the polling list. Stations have not been polled must wait the next CFP. Which causes an additional delay.

  19. PCF with implicit signaling (1/2) • Using PCF mode raises a penalty in overall throughput. • Extra overhead in centralized polling process • AP uses available information from higher layer. (ex: RTP)

  20. PCF with implicit signaling (2/2) • When TALKSPURT ends, the next polling attempt fails, and AP removes the station from polling list. • When the station continues sending audio packets in DCF mode, AP detects and adds it to polling list. • Using this approach can avoid unsuccessful polling attempts.

  21. Goodput comparison DCF PCF

  22. Other Approaches to Improve QoS • Codecs choices • Based upon channel conditions • Packet loss concealment (PLC) • Try to generate a synthesized packet that has lost instead of retransmission • Selective error checking of classified bits • Repetition of perceptually important packets

  23. 802.11e • Access mechanism • Enhanced Distributed Channel Access (EDCA) • HCF Controlled Channel Access (HCCA) • Designed for QoS • But still can’t solve the capacity problem • Voice codecs selections or packet loss concealment issues are not addressed • Many researches adaptively tune the parameter settings • The setting may require to change as # VoIP sessions changes

  24. 802.11e parameters • Contention Window (CW) • Tradeoff between delay and retransmission • Transmission opportunity (TXOP) • Balance uplink/downlink performance

  25. 802.11e analysis • HCCA is more suitable than EDCA • AP is usually a heavily loaded node In EDCA , voip pkts may be queued at AP if AP cannot gain TXOP.

  26. Simulation environment • number of best effort traffic source is 5 • Best effort traffic is exponentially distributed with mean 7.8ms • G.711 a-Law codec is used

  27. End-to-end delay over HCCA

  28. Uplink delay over EDCA

  29. Downlink delay over EDCA

  30. Conclusion • Many approaches are proposed to improve QoS or increase call capacity of VoIP over Wi-Fi • Most of them need to modify 802.11 MAC protocol • Some solutions consider purely VoIP packets, which is not practical • The M-M scheme + PQ scheme + MMP scheme • Require no changes to the 802.11 MAC protocol (without MMP-scheme) • Could apply to various voice codecs, CBR and VBR VoIP streams • Efficiently improve the VoIP capacity, delay and packet-loss rate

  31. Conclusion • 802.11e provides better QoS than DCF/PCF • EDCA : prioritized QoS , home • WME (Wireless Media Extensions) • HCCA : parameterized QoS , WLAN • WSM (Wireless Scheduled Media)

  32. WiFi Future • Heterogeneous asynchronous tandem networks • Different codecs • Different network protocols • Different channel behaviors • Different bit error/packet loss mechanisms • Wireless VoIP Phone

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