Voice Capacity analysis over 802.11

Introducing VoIP and WLans IEEE 802.11 based Wireless Local Area Networks

(WLANs) are becoming popular

While WLANs continue to be predominantly data centric, there is growing interest in using WLANs for voice, especially in enterprise markets.

Seamless wireless data and voice communication is fast becoming a reality

One key capability in the next-generation wireless world will be Voice over Internet Protocol (VoIP) using 802.11 wireless local area networks (WLANs)

Voip and WLan

System set up:

Assumptions:

VoIP stations in the wire line n/w are connected to the AP via a p - p link with negligible delay.

Channel is error free Stations are within transmission range

therefore not considering RTS/CTS mode

802.11 WLANs

DCF based on CSMA/CA

PCF based on polling

VOIP and Protocol Stack

Voice Compression Techniques

Mapping Voice over 802.11

802.11b 11Mbps 128 kbps- full duplex Approx. 85 calls

supported However no more than

12 calls can be supported !!!

VoIP and WLan overhead

DCF mode of 802.11b

As packetization interval increases the capacity of voice calls increases

Efficiency increases by decreasing the no of packets per second (reducing the no of times the overhead is incurred)

G726 : 13Kbps 10ms Payload:16bytes Total:74bytes 74 – 10ms -> 59.6Kbps

20ms Payload:33bytes Total:91bytes 91 – 20ms -> 36.4Kbps

Voice capacity of DCF CBR VoIP client generates one VoIP packet every

packetization interval No of packets that can be sent during a packetization

interval is the maximum no of voice calls supported N = Tp / (2 * Tt) Tt = Tdifs +Tv +Tb +Tsifs+ Tack

Voice capacity in PCF

VoIP STAs need to be polled every packetization interval

CFP < = packetization interval N = (Tcfb –Tb – Tcp – Tcfpend) / (2*Tt) Tt = Tv + Tsifs

Problems with DCF and PCF

PROBLEM WITH DCF: Hard to implement QOS Poor performance under heavy load conditions Low bandwidth Limited number of VoIP connections

PROBLEM WITH PCF: AP keeps polling regardless of whether data is available for

transmission When no of stations in the BSS is large –polling overhead is large. Without service differentiation-poor performance Support for PCF is not so commonly available

Dynamic Point Coordination Function (DPCF) Differentiation of Traffic Types Dynamic Polling list

Maintains active nodesRemoving an STA from the polling listAdding an STA to the polling list

Dynamic CFP interval and more data field

Packetization interval = 10ms; CFP = 20 ms

Synchronization Problem

- CF polls are wasted and most packets are sent in CP

- Aggravated if more STAs

- CFP will be shortened and CP will be increased

Modified DPCF

prevents STAs from sending VoIP packets in CP when there is only one VoIP packet in their queue so that the packet can be sent in the next CFP

Tries to put voice packets into CFP as much as possible to reduce the no of CF – polls and null packets

VoIP packets – CFP

Non-VoIP packets – CP

Some other solutions:

802.11 e provides low end to end delay MDCF-Does not have mechanisms to

reduce collisions among same priority traffic

no effort to reduce the no of null packets Multiplex- Multicast (M-M)

802.11a,b,g CAPACITY IN 802.11a,g is greater than in 802.11b

This can be attributed to the fact that the peak rate of 802.11a and 802.11g is approximately 5 times higher than 802.11b,

the average backoff time is approximately 1/4th (CWmin is 15 as opposed to 31

SLOT duration is 9 micro s as opposed to 20 micro s and the physical layer header is about 8 times smaller.

However, since the minimum contention window is half the size in 802.11b the probability of collision is greater

Conclusions:

VoIP quality is fine as long as network throughput limit is not exceeded (packet loss, delay and jitter) Admission control needed

Payload size affects the throughput of the WLAN Increase audio data length per packet

PCF,DPCF, Modified DPCF is better than DCF

I The capacity in PCF,DPCF AND DPCF2 is larger than in

DCF because packets will collide with each other more in the CP as no of STAs increases, while polling mechanism reduces collisions.

DPCF And DPCF2 reduce the no of wasted CF-polls and null packets which results in improving the capacity by 20%

References:

Using Dynamic PCF to improve capacity for VoIP traffic in 802.11 Networks –Henning Schulzrinne, Sangho Shin, Andrea Forte , Takehiro Kawata

Supporting VoIP Traffic in IEEE 802.11WLAN in PCF mode – Donyan Chen,Sachin garg, Martin Kappes and Kishor Trivedi

An Experimental study of throughput for UDP and VoIP traffic in 802.11 Networks

Voice Performance in WLAN Networks –An experimental Study – Telcordia Technologies and Toshiba american research.

Can I add a VoIP call? –Avaya Labs.

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