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