Research And Application On The Handoff Scheme In IEEE 802.11

As a perfect combination of computer networks and wireless telecommunication technology, Wireless Local Area Network (WLAN) is gradually used for data communications in rail transit, simultaneously WLAN handoff becomes one of the hottest research issues. Due to the limited WLAN radio power, the range of radio coverage is within hundreds of meters, and since that vehicle-mounted mobile radio needs frequently handoff in different access points. It will not only cause packet loss, reducing reliability, but can also lead to certain time network outages. So it plays a critical impact on wireless network performance.Firstly this paper introduces the data communication system in the rail transportation, then gives an overview of IEEE 802.11 WLAN and some limitations in the rail transit, finally analyzes and summarizes the WLAN data link layer handoff research progress at home and abroad.Second, in view of the smooth handoff, combined with the common store and forward thinking in wireless network, this paper gets the wireless link events through the Netlink socket for user and kernel space communication, captures and forwards frames by packet socket, implemented a smooth handoff mechanism on the Linux wireless AP which is independent of dviver. Fping, Ixchariot and other test tools verify that the new mechanism greatly reduces the packet loss rate and improves the network performance.Third, in the fast handoff, the same channel is used in the track side adjacent AP for the special rail transit circumstances. This paper obtains different BSS’s beacons by way of modifying MadWifi. Fast handoff is opportunely triggered by comparison of RSSI to support stable and reliable link. Extensive experimental results bear out that the handoff delay is less than 20ms.Finally, this paper gives the improved mechanisms for smooth and fast handoff in the rail transit. In the introduced data communication system framework, the redundancy and retransmission mechanism is adopted to overcome the problem of wireless channel unreliability, at the same time, nil packet loss is achieved; while using wireless admission control, firewall and IPSec policies make up for the open authentication measures in order to reduce the zapping.