Research On Voice Transmission Over WLAN With High Quality

With the rising of Internet and Local Area Network (LAN), Wireless Local Area Network (WLAN) technology and standards develops quite rapidly. It is getting more popular in people's life and work. As in the Internet and LAN, people's requirement is growing fast to transmit multimedia traffic (such as audio and video) on WLAN, which makes the problem of quality of service (QoS) become much more important. The technology of WLAN is not quite suit for transmitting of real time services such as audio and video for present. Considering the evident difference in network performance and QoS between WLAN and LAN, the mechanism of access controlling is MAC layer is one of the most important reasons besides the characteristic of wireless media.In the thesis, the author does particularly research and analysis with the IEEE 802.11 and IEEE 802.11e standards. In IEEE 802.11 standard, the stations of WLAN compete equably for the chance of accessing the wireless media. In this model AP is competing with stations, which is not fit for the need of transmitting audio service with symmetry traffic in both direction. Moreover, the real time services can't obtain the deserved service quality guarantee with the scarcity of service class mechanism. IEEE 802.11e standard does not resolve the problem of transmitting symmetry traffic all the same, and its traffic class mechanism need the support of up layer.To make up for the insufficiency of WLAN to support VoIP traffic transmission, the thesis proposes a strategy. Two mechanisms is brought forward to modify the MAC layer of WLAN, and control the total traffic over wireless channel, in order to enhance the support of QoS guarantee with WLAN.One strategy is flows separating, which separates the transmitting time of upload and download traffic in the frame period. Burst transmitting is involved in download traffic transmitting to avoid the equal competition of AP and stations in symmetry traffic model.In each frame period,MAC entity will change the download send window dynamically according to the total load of the network, to adapt to the bandwidth need of VoIP service.The other strategy is to apply the method of flow controlling in both directions based on service priority. A modified RED algorithm dominated by voice service quality parameters is employed to control the packets sending of stations. With these methods, the performance and QoS requirement of real time services over WLAN is satisfied well.