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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.

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introducing voip and wlans
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 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
802.11 WLANs
  • DCF based on CSMA/CA
  • PCF based on polling
mapping voice over 802 11


128 kbps- full duplex

Approx. 85 calls supported

However no more than 12 calls can be supported !!!

Mapping Voice over 802.11
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


Payload:16bytes Total:74bytes

74 – 10ms -> 59.6Kbps


Payload:33bytes Total:91bytes

91 – 20ms -> 36.4Kbps

voice capacity of dcf
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
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
Problems with DCF and PCF
  • Hard to implement QOS
  • Poor performance under heavy load conditions
  • Low bandwidth
  • Limited number of VoIP connections
  • 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
Dynamic Point Coordination Function (DPCF)
  • Differentiation of Traffic Types
  • Dynamic Polling list
    • Maintains active nodes
    • Removing an STA from the polling list
    • Adding 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
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
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
  • 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
  • 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%
  • 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.