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 PCF 802.11e
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)
DCF More robust than PCF Basic operation of 802.11 DCF
802.11 Multicasting
No ACK mechanism for multicasting in 802.11
Excessive multicasting packet loss due to collision is a fundamental problem in WLAN
VoIP over a 802.11 WLAN
Problems Low VoIP call capacity Unacceptable VoIP performance
Longer delay Coexisting with other applications
Packet loss rate
Solution
Multiplexing-multicasting (M-M) scheme Multiplexing packets from several VoIP streams in
to one multicasting packet # stream: 2n →n+1 (n: # VoIP sessions)
M-M Scheme
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
Improvement of VoIP Call Capacity
M-M scheme can nearly double the capacity for most of the codecs
Is M-M Scheme Enough?
Delay Coexisting with TCP interference traffic
Packet loss Buffer overflow Downlink multicast stream may collide with uplink
unicast stream
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 c
apacity The performance gain for VoIP is not at the expen
se of TCP throughput
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 W
LAN 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)
Performance Improvement of VoIP
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
Superframe structure
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.
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)
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.
Goodput comparison
DCF PCF
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
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 conceal
ment issues are not addressed Many researches adaptively tune the paramete
r settings The setting may require to change as # VoIP sessio
ns changes
802.11e parameters
Contention Window (CW) Tradeoff between delay and retransmission
Transmission opportunity (TXOP) Balance uplink/downlink performance
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.
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
End-to-end delay over HCCA
Uplink delay over EDCA
Downlink delay over EDCA
Conclusion Many approaches are proposed to improve QoS or i
ncrease 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 M
MP-scheme) Could apply to various voice codecs, CBR and VBR VoIP s
treams Efficiently improve the VoIP capacity, delay and packet-los
s rate
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)
WiFi Future
Heterogeneous asynchronous tandem networks Different codecs Different network protocols Different channel behaviors Different bit error/packet loss mechanisms
Wireless VoIP Phone