| Wireless LAN technology makes the network access not limited by physics circuit any more for the. The embedded technology makes the communication device become more microminiaturized with lower power consumption and stronger function. The subway wireless communication system researched in this paper is a typical integrated application model of the wireless LAN and the embedded technology.The subway wireless communication system is used to monitor the status information of subway dynamically. The WLAN according to 802.11b standard is composed of STA and AP. Subway will go through several networks. STA won't be able to communicate with other nodes if the STA's IP address can't be updated to new IP address in the new network immediately. But the change of STA's IP address will cause another problem that is how STA announce its new IP address to other nodes in the subway system.In this paper, a set of mobile and roaming protocals were designed to ensure that STA can communicate with other nodes in the communication system while going through networks without changing its IP address, in the process of subway running. The principles are as follows: Tunnels are established between the home agent located in home network and the foreign agents located in foreign networks. Data packets whose destination IP address is the same with the mobile node's IP address are sent to the foreign network through the tunnel. Then the foreign agent sends the data packets to the mobile node.At first, each function module of the mobile and roaming protocol is designed, Then the specific implementation was given. At last, the fast moving test results were carried out using Ping program and Ixchariot software. The test results prove that, in the circumentstances of STA fast moving(50~60km/h), the time consumed in the swithing process between APs and STA is short enough, and the mobile and roaming functions can be ensured in the wireless network environment based on 802.11b. |
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