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

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

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

4. Background Codecs Access mechanisms
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 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
因為在無線網路的環境中，receiver 可以自由的加入或離開 multicast的團體，所以回ACK是不可行的

7. VoIP over a 802.11 WLAN Problems Low VoIP call capacity
Unacceptable VoIP performance
Longer delay
Coexisting with other applications
Packet loss rate
Low VoIP call capacity (for GSM 6.10, it is about 12) - Due to various protocol overheads

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
Mux/demux delay

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 Packet loss
Coexisting with TCP interference traffic
Packet loss
Buffer overflow
Downlink multicast stream may collide with uplink unicast stream
The nature of TCP is such that after a connection is established, it will continue to increase the data input rate until packet losses occur.
The relatively high level of the buffer occupancy and the oscillatory data input rate of TCP leads to the high delay and jitter performance for the downlink VoIP 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
The nature of TCP, commercial AP: all downlink traffic share a common FIFO queue

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 Designed for QoS
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

28. End-to-end delay over HCCA

29. Uplink delay over EDCA

30. Downlink delay over EDCA

31. Conclusion
Many approaches are proposed to improve QoS or increase call capacity of VoIP over Wi-Fi
Most of them need to modify 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 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

32. 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)

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