B91902058 葉仰廷 B91902078 陳柏煒 B91902088 林易增 B91902096 謝秉諺

1. A Multiplex-Multicast Scheme that Improves System Capacity of Voice-over-IP on Wireless LAN by 100% * B91902058 葉仰廷 B91902078 陳柏煒 B91902088 林易增 B91902096 謝秉諺

2. Outline • Introduction • VoIP Multiplex-Multicast Scheme • Capacity Analysis • Delay Performance • Conclusions

3. Introduction • This paper considers the support of VoIP over 802.11b WLAN. • WLAN capacity can potentially support more than 500 VoIP sessions when using GSM 6.10 codec. • But various overheads bring WLAN capacity only 12 VoIP sessions when using GSM 6.10 codec.

4. Introduction • 802.11b, which can support data rates up to 11Mbps. • A VoIP stream typically requires less than 10Kbps. • 11M/10K = 1100, which corresponds to about 550 VoIP sessions, each with two VoIP streams.

5. Introduction The efficiency at the IP layer for VoIP: • A typical VoIP packet at the IP layer consists of 40-byte IP/UDP/RTP headers. • A payload ranging from 10 to 30 bytes, depending on the codec used. • less than 50%!!

6. Introduction At the 802.11 MAC/PHY layers: • Attributed to the physical preamble, MAC header, MAC backoff time, MAC acknowledgement, and inter-transmission times of packets and acknowledgements…. • The overall efficiency drops to less than 3%!!

7. Outline • Introduction • VoIP Multiplex-Multicast Scheme • Capacity Analysis • Delay Performance • Conclusions

8. 每日一詞 • Unicast • Broadcast • Multicast

9. Multiplex-Multicast Scheme • An 802.11 WLAN is referred to as the basic service set (BSS) in the standard specification. • There are two types of BSSs: Independent BSS and Infrastructure BSS.

10. Multiplex-Multicast Scheme • Independent(ad hoc) BSS

11. Multiplex-Multicast Scheme • Infrastructure BSS

12. Multiplex-Multicast Scheme • This paper focuses on infrastructure BSSs. • We assume that all voice streams are between stations in different BSSs. • Each AP has two interfaces, an 802.11 interface which is used to communicate with wireless stations, and an Ethernet interface which is connected to the voice gateway.

13. Multiplex-Multicast Scheme

14. Multiplex-Multicast Scheme • Within a BSS, there are two streams for each VoIP session. • M-M Scheme idea : Combine the data from several downlink streams into a single packet for multicast over the WLAN to their destinations.

15. Multiplex-Multicast Scheme

16. Multiplex-Multicast Scheme • multiplexer(MUX), demultiplexer(DEMUX) • Add miniheader • In miniheader, there is an ID used to identify the session of the VoIP packet.

17. Multiplex-Multicast Scheme Header data1 MUX Header Header data2 Minih.+Data1+Minih.+data2+Minih.+data3 DEMUX Header data3

18. Multiplex-Multicast Scheme • Reduce the number of VoIP streams in one BSS from 2n to 1 + n, where n is the number of VoIP sessions. • The MUX sends out a multiplexed packet every T ms, which is equal to or shorter than the VoIP inter-packet interval. • For GSM 6.10, the inter-packet interval is 20 ms.

19. Multiplex-Multicast Scheme DEMUX DEMUX MUX

20. Multiplex-Multicast Scheme • Problem: Security!?

21. Outline • Introduction • VoIP Multiplex-Multicast Scheme • Capacity Analysis • Delay Performance • Conclusions

22. Capacity Analysis • consider the continuous-bit-rate(CBR) voice sources • voice packets are generated at the voice codec rate • focus on the GSM 6.10 codec • the payload is 33 bytes • the time between two adjacent frames is 20 ms

23. Capacity Analysis • n : maximum number of sessions that can be supported • Tdown& Tup: transmission times for downlink and uplink packets • Tavg: average time between the transmissions of two consecutive packets in a WLAN • NP: number of packets sent by one stream in one second • 1/Tavg = number of streams * NP

24. Capacity of Ordinary VoIP over WLAN • OHhdr= HRTP + HUDP+ HIP+ HMAC • OHsender • if unicast packet:OHreceiver • Tdown= Tup=(Payload + OHhdr) * 8 / dataRate + OHsender + OHreceiver

25. Capacity of Ordinary VoIP over WLAN • n downlink and n uplink unicast streams • Tavg= (Tdown+ Tup) / 2 • 1/Tavg= 2n *Np • n = 11

26. Capacity of Multiplex-Multicast Scheme over WLAN • the RTP, UDP and IP header of each packet is compressed to 2 bytes • Tdown= [(Payload + 2) *n + HUDP+ HIP+ HMAC] * 8 / dataRate + OHsender • Tavg= (Tdown+ n *Tup) / (n + 1) • 1/Tavg= (n + 1) *Np • n = 21.2

27. VoIP Capacities assuming Different Codecs

28. Simulations • increase the number of VoIP sessions until the per stream packet loss rate exceeds 1% • system capacity = max number of sessions • assume that the retry limit for each packet is 3

29. Simulations • for ordinary VoIP over WLAN, the system capacity is 12 • exceeding the system capacity leads to a large surge in packet losses for the downlink streams

30. Analysis vs. Simulation Capacity of Ordinary VoIP and Multiplex- Multicast Schemes assuming GSM 6.10 codec

31. Outline • Introduction • VoIP Multiplex-Multicast Scheme • Capacity Analysis • Delay Performance • Conclusions

32. Delay Performance • voice quality:packet-loss rates & delay performance • with ordinary VoIP: • local delay: only the access delay within the WLAN • at the AP: time between the packet’s arrival until it’s successfully transmitted or dropped • at the client: time from when the packet is generated until it leaves the interface card

33. Delay Performance • with the M-M scheme: • local delay: access delay & the MUX delay incurred at the VoIP multiplexer (only downlink) • MUX delay: time from the packet’s arrival until the next one is generated • we set a requirement: no more than 1% of packets should suffer a local delay of more than 30 ms

34. Access Delay • ordinary VoIP scheme (12 sessions): • in the AP: average delay and delay jitter are 2.5 ms and 1.4 ms • in the wireless station: average delay & delay jitter are 1.2 ms and 1.0 ms • if normally distributed:less than 0.27% of the packets would suffer local delays larger than 30 ms

35. Access Delay Access Delays in AP and a Station in Original VoIP over WLAN when there are 12 Sessions

36. Access Delay • M-M scheme (22 sessions): • in the AP: average delay and delay jitter are 0.9 ms and 0.2 ms • in the wireless station: average delay & delay jitter are 2.0 ms and 1.5 ms • no link layer retransmissions for the packets whencollisions occur

37. Access Delay Access Delay in AP and a Station in M-M Scheme when there are 22 Sessions

38. Extra Delay Incurred by the Multiplex-Multicast Scheme • when a VoIP packet waits for the MUX to generate the next multiplexed packet • we set the multiplexing period to be at most one audio-frame period • 20 ms if GSM 6.10 codec is used • random variable M : the MUX delay • assume M to be uniformly distributed between 0 and 20 ms

39. Delay Distribution for Ordinary VoIP When System Capacity of 12 is Fully Used

40. Delay Distributions for Multiplex-Multicast Scheme When System Capacity of 22 is Fully Used

41. Outline • Introduction • VoIP Multiplex-Multicast Scheme • Capacity Analysis • Delay Performance • Conclusions

42. Conclusions • M-M scheme can reduce the large overhead when VoIP traffic is delivered over WLAN • it requires no changes to the MAC protocol at the wireless end stations • more readily deployable over the existing network infrastructure. • it makes the voice capacity nearly 100% higher than ordinary VoIP