Research And Application On The Handoff Scheme In Subway Mobile Wireless Network

With the development of the wireless communication technology and urban rail transit, WLAN has been applied to subway communication systems. In slow-moving environments, WLAN can meet the requirements of internet access. But in fast-moving environments, WLAN can not support mobility well. How to improve the mobile network qaulity of service, especially decrease the handoff delay and packet loss rate have become problems to be solved urgently.The project named"Research on generalization of IEEE 802.11a/b/g radio station"is a subitem of"ATC system of urban rail transit"which is a transformation of scientific and technological achievements in Jiangsu Province. The train-ground communication is the core ingredient, which has a direct impact on network QoS. First, the special wireless network in subway communication system was introduced, and problems in current wireless mobile network were indicated. Then after studing and comparision of existed handoff schemes, two handoff schemes were proposed for single and dual wireless NICs separately. At last, detailed implementation and tests have been given in this paper.In single wireless NIC environment, this paper put forward an improved scheme in view of problems existed in handoff delay and packet loss rate. The number of used channels was reduced, which decreases the scan delay effectively. And an algorithm called Layer2-Buff was proposed to solve the problem of losing packets, which sets up a buffer for STA, used to store data failed to send. Using Ixchariot and smokeping software, taking handoff delay, throughput and packet loss rate as testing indexes, the results illustrated that this scheme can reduce the packet loss rate to zero in specific environment, and also have a good promotion to network throughput.In dual wireless NICs environment, this paper combined two wireless NICs to complete data transmission. Firstly, Channel Quality Indication was introduced into handoff, according to historical data, the trend of CQI was obtained to determine the best time to change the working NIC. Then the data transmission was monitored, once current CQI reduced to the threshold, the working NIC should be changed. Without exception, whenever one NIC triggered a handoff, another NIC was used to transfer data; otherwise, single NIC handoff scheme was used. The results indicated that, this handoff scheme, which is based on the single NIC handoff scheme, can futher reduce delay, and the communi- cation was ensured at the utmost.