The Study Of Voice Performance Based On IEEE 802.11b Wireless LAN

With the development of the wireless LAN technology and voice over IP (VoIP)technology,using wireless network to transmit voice over IP becomes a hot research direction in the recent years.However,the traditional IP network's 'best-effort' service mechanism and the limited bandwidth of wireless network make the wireless LAN unable to guarantee the QoS of the real-time applications such as voice.This thesis evaluates the transmission efficiency of voice over wireless LAN,proposes a viewpoint that 802.11b networks is not suitable for transmiting IP voice,calculates and validates the VoIP capacity from both theories and experiments,and gives a new call admission control scheme based on the experimental results.The study in the thesis has some guiding meaning to the use of VoWLAN,the CAC scheme has practical signification on guaranteeing the QoS of the call.Although the technology of using IEEE 802.11b wireless networks to transmit IP voice has been widely used,its transmission efficiency is very low.Most of 802.11b devices adopt DCF to contend for the channel.DCF treats the voice and data equally, and does not provide any access priority to the voice application.These make it difficult to guarantee the QoS of the voice application which is sensitive to delay and packets lost ratio.The MAC layer of IEEE 802.11b uses Carrier Sense Multiple Access with Collision Avoidance protocol and this result in the low utilization rate of the system. The present thesis analyzes the factors which affect the efficiency of the VoWLAN. Considering the low utilization rate and the high costs of constructing a wireless LAN, the thesis draws a conclusion that it is not suitable to transmit voice over IP that is based on the principles of basic access scheme under DCF in the IEEE 802.11b networks.In order to prove the viewpoint,the thesis gets the VoIP capacity in the ideal network environment by theoretical analyzing.The various kinds of background flow in the real networks will affect the performance of the voice over WLAN.This thesis further calculates the capacity of VoIP under the background flow with different rate,from the perspective of delay and packets lost ratio.In order to validate the results from the theoretical analysis,the thesis measures the capacity of the VoWLAN with different background flow by setting up a testing network.The experimental results match the theory.And this proves the conclusion of the thesis. The thesis proposes a call admission control scheme that is based on the communication capacity,using the results of the experiment.The CAC scheme can maintain the OoS of the users who are under service effectively,and avoid a decline in the OoS for too many users.The CAC scheme solves the conflict between the service satisfaction of user and system resource utilization,keeps the system stable,improves the system capacity,and avoids the system overload.The present thesis contains 20 pictures,5 tables and 28 references.