Wireless Lan Voip Qos Research

The development of Wireless LAN technology and VoIP technology makes it possible to transmit voice over Wireless LAN. But because of the IP network's best-effort service and the limited bandwidth of wireless network, QoS requirements of real-time applications such as voice can't be met. The main factors that affect the performance of VoIP are delay, jitter, packet loss rate as well as bandwidth.Currently, the most widely used WLAN is IEEE 802.11b. Most 802.11b devices adopt DCF to contend for the channel. DCF treats voice and data equally, and doesn't provide any access priority to the real-time applications. This can't guarantee the QoS of delay-sensitive voice application. This paper analyzes the principles of basic access scheme and RTS/CTS scheme under DCF in detail, draws a conclusion that the four handshakes RTS/CTS scheme is not suitable for VoIP through simulation, and concludes that the delay of VoWLAN can be reduced by the control to the queueing delay, IFS and the random backoff time. On this basis, another method to contend for the channel-EDCA is presented. After the analysis of EDCA's QoS support, experiment is performed. The result shows that EDCA greatly improves VoIP's performance.Considering that CRC of MAC requires error free packets, otherwise MAC retransmission will occur, but voice transmitted in the form of small packets can tolerate some bits error, so a modified CRC scheme is brought forward. The modified CRC doesn't protect voice data any more, so MAC retransmission is less. This results in reduced delay and jitter of VoIP. Simulation result shows that the modified CRC scheme can improve delay and jitter.Capacity of VoWLAN is related with QoS of VoWLAN, and it is also a very important problem. The thesis analyzes VoIP capacity of 802.11b network theoretically in ideal channel condition, and simulation result matches the theory.