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Wireless VoIP

Wireless VoIP

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Wireless VoIP

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  1. Wireless VoIP C3 R94922096 謝明龍 R94922088 關尚儒

  2. Outline • Problems to use VoIP on wireless network • Voice over WLAN • MAC method • 802.11e • Dual queue scheme • VoIP and 802.11x standards

  3. VoIP on Wireless Network • Wireless network – lower speed , noise • Upgrade physical speed , reduce noises (PHY) • Real-time packet prioritize (MAC) • 1AP-to-many Station • Upgrade the capacity of single AP • Admission control • Roaming • Mobile device power • Wireless security

  4. Voice over WLAN

  5. 802.11 supplements glossary • 802.11a – 5GHz OFDM PHY layer • 802.11b – 2.4GHz CCK PHY layer • 802.11c – bridging tables • 802.11d – international roaming • 802.11e – quality of service MAC • 802.11f – inter-access point protocols • 802.11g – 2.4GHz OFDM PHY • 802.11h – European regulatory extensions • 802.11i – enhanced security • 802.11n – MIMO ODFM PHY

  6. PHY  802.11n • 2.4GHz+5GHz (a/b/g) • MIMO+OFDM • MIMO (Multiple-In, Multiple-Out)

  7. IEEE 802.11 MAC

  8. Dual Queue Strategy

  9. Dual Queue Strategy • The 802.11e MAC implementation cannot be done by just upgrading the firmware of an existing MAC controller chip only • It is difficult to Upgrade (replace) the existing APs

  10. Dual Queue Strategy • above 802.11 the MAC controller • Original NIC driver  FIFO queue • New NIC driver  RT + NRT queue • Strict priority queuing • Effect of MAC HW Queue

  11. Dual Queue Strategy

  12. VOIP AND ADMISSION CONTROL • VoIP • codec  G.711 • 64 kbps stream • 8-bit pulse coded modulation (PCM) • sampling rate : 8000 samples/second • A VoIP Packet per 20ms 160-byte DATA + 12-byte RTP header + 8-byte UDP header+ 20-byte IP header + 8-byte SNAP header = 208 bytes per VoIP packet

  13. VOIP AND ADMISSION CONTROL • VoIP Admission Control • assumptions • ACK Packet transmitted with 2Mbps • Long PHY preamble • Packet transmission MAC • DIFS deference • Backoff • Packet transmission • SIFS deference • ACK transmission

  14. VOIP AND ADMISSION CONTROL • VoIP packet transmission time ≒ 981μs • VoIP MAC packet transmission time • 192-μs PLCP preamble/header + (24-byte MAC header + 4-byte CRC-32 + 208-byte payload) / 11 Mbits/s = 363 μs • ACK transmission time at 2 Mbits/s • 192-μs PLCP preamble/header + 14-byte ACK packet / 2Mbits/s = 248 μs • Average backoff duration • 31 (CWmin) * 20 μs (One Slot Time) / 2 = 310 μs

  15. VOIP AND ADMISSION CONTROL • Every VoIP sessioin • inter-active 2 senders • one voice packet transmitted every 20ms • Every 20ms time interval • 20 (= 20 ms / 981 μs) voice packets • Maximum number of VoIP sessions over a 802.11 LAN is 10

  16. COMPARATIVE PERFORMANCE EVALUATION • Using the ns-2 simulator • 802.11b PHY • Traffic • Voice  two-way constant bit rate (CBR) session according to G.711 codec • Data unidirectional FTP/TCP flow with 1460-byte packet size and 12-packet (or 17520-byte) receive window size.

  17. COMPARATIVE PERFORMANCE EVALUATION

  18. EVALUATION RESULT • Pure VoIP • Effect of VoIP with different TCP session numbers • Performance with Dual queue • Unfairness of NRT Packet • Effect of MAC HW Queue

  19. Observation • Compare to our Evaluation • packet drop rate • 50 packets for the RT queue size • Downlink is disadvantaged • Simulation results are based on 11 Mbps

  20. EVALUATION RESULT • Pure VoIP • Effect of VoIP with different TCP session numbers • Performance with Dual queue • Unfairness of NRT Packet • Effect of MAC HW Queue

  21. Observation • Effect of queue size

  22. EVALUATION RESULT • Pure VoIP • Effect of VoIP with different TCP session numbers • Performance with Dual queue • Unfairness of NRT Packet • Effect of MAC HW Queue

  23. Observation • worst case delay 11ms • Queuing delay with the single queue • MAC HW queue wireless channel access • NRT queues • Size = 50 or 100  increase as the number of TCP flows increases • Size = 500  almost no change in delay

  24. EVALUATION RESULT • Pure VoIP • Effect of VoIP with different TCP session numbers • Performance with Dual queue • Unfairness of NRT Packet • Effect of MAC HW Queue

  25. Observation • Unfairness • between upstream and downstream TCP flows with the queue sizes of 50 and 100 • Queue size for the AP should be large enough - This is good for us

  26. EVALUATION RESULT • Pure VoIP • Effect of VoIP with different TCP session numbers • Performance with Dual queue • Unfairness of NRT Packet • Effect of MAC HW Queue

  27. Observation • Delay of downlink voice packets • increases linearly proportional to the MAC HW queue size • Another effect • with the MAC HW queue size of 8, the worst delay is observed with a single VoIP session • Large MAC HW queue size is still aceptable • <25ms

  28. Brief Summary • Driver of the 802.11 MAC controller • Strict priority queuing • Bottleneck of TCP in WLAN  downlink

  29. VoIP and 802.11e QoS standards

  30. What’s the difference between Wireless/Wired VoIP? • Mobility • Roaming • Security • Hidden UA • Quality of Service • Guarantee of voice quality

  31. Hidden Node Problem

  32. Quality of Service • QoS problems • 802.11e QoS standard • A non-standard solution – Dual Queue Strategy

  33. QoS Problems • Dropped Packets • Delay • Jitter • Out-of-order Delivery • Error • VoIP requires strict limits on jitter and delay

  34. Quality of Service • QoS problems • 802.11e QoS standard • A non-standard solution – Dual Queue Strategy

  35. IEEE 802.11e • A draft standard of July 2005 • It defines a set of QoS enhancements for WLAN applications • and enhances the IEEE 802.11 Media Access Control (MAC) layer

  36. Coordination Function • For stations to decide which one has the right to deliver its packets • 802.11: DCF & PCF • 802.11e: EDCF & HCF

  37. Original 802.11 MAC • Distributed Coordination Function (DCF) • Point Coordination Function (PCF)

  38. Distributed Coordination Function (DCF) • Share the medium between multiple stations • Rely on CSMA/CA and optional 802.11 RTS/CTS

  39. How DCF works?

  40. DCF Limitations • When many collisions occur, the available bandwidth will be lower • No notion of high or low priority traffic • A station may keep the medium • If the station has a lower bitrate, all other stations will suffer from that • No QoS guarantees

  41. Original 802.11 MAC • Distributed Coordination Function (DCF) • Point Coordination Function (PCF)

  42. Point Coordination Function (PCF) • Available only in "infrastructure" mode • Optional mode, only very few APs or Wi-Fi adapters actually implement it • Beacon frame, Contention Period, and Contention Free Period

  43. How PCF works?

  44. 802.11 MAC Layer Framework

  45. 802.11e MAC Protocol Operation • Enhanced DCF (EDCF) • Hybrid Coordination Function (HCF)