Research And Implementation On Handoff Technique For WiFi Networks

In recent years,Wireless Local Area Networks(WLAN) have developed rapidly and gained common acceptance, with numerous successful deployments in local network environments. The IEEE 802.11 standard (WiFi) has enabled fast, effective and low-cost WLAN services. Wireless networks offer access to the Internet for delivery of various services such as VoIP, multimedia, and data transmission. Continuous network connectivity is highly desirable for most services. However, the coverage of a WiFi access point (AP) is normally 50 to 300 meters and the small coverage can only support users with low mobility. Handoff happens frequently when the mobile station is roaming between different APs. The handoff latency significantly influences the qualities of real-time applications such as VoIP. Thus, handoff had always been a critical and challenging issue in related areas.This paper focuses on handoff techniques research and its implementation in WiFi networks. First of all, the significance and current status of WiFi handoff research are presented. Then the procedure of IEEE802.11 handoff is introduced in detail including its performance analysis. Moreover,we propose a fast handoff scheme based on background scanning, which helps the mobile station to acquire the information of the APs around and reduces the scanning latency at the same time. Otherwise, scan threshold and handoff threshold are introduced to make the handoff decision efficient and reliable. When selecting the target AP, we choose the RSSI as a dominant factor as well as security, stability and customer demands. At last,we implement handoff scheme on Windows platform, according to the NDIS standard. Experiments result very low handoff latency which should satisfy the requirement of real-time applications such as VoIP and video streaming transmission.