Research On Handoff And Roaming Scheme For Mobile Subnet With High Speed

Grown with internet service and network technology, more and more applications have been moved to network (wire network & wireless network). More and more people's life and work depend on the network environment. Wide bandwidth, low latency, more flexibility and mobility are goals of the network. Roaming network is not only one client terminal, but also a small network, i.e. a mobile subnet. So mobile subnet's connection strategy should be investigated and smart technical solution is requested.To make the mobile subnet roaming as a whole, two problems should be solved. The first is the fast and seamless handoff among access points, which aims at shortening the handoff latency and decreasing the data lose during handoff. The second is to allow the mobile subnet access network without interrupting data transmission and without changing its IP address. The dissertation researches core techniques of handoff and roam to solve the above problems. It analyzes the handoff scheme of single mobile node in data link layer, and researches the mobility management in network layer.The mobile subnet connects to the network through only one STA. Therefore the existing techniques supporting one node roaming can't support the subnet roaming as a whole. The main contributions of the dissertation are summarized as follows:1) A novel handoff opportunity decision scheme is proposed. It aims at triggering accurately to decrease the handoff latency and avoid the"Ping-Pang"handoff. It considers several important factors in the subnet roaming environment, including the distributions of access points, the attenuation law of radio signal, the moving speed and direction of the STA and the subnet. It decreases the handoff latency of a wireless link. And a layer2 Handoff-Buff algorithm independent with applications is proposed to avoid data lose during the handoff. It modifies the wireless driver, saves unsuccessfully sent data and resends them after link restoral. It diminishes the packet loss ratio and improves the QoS of the link.2) A novel soft-handoff scheme in WLAN realized by dual link is proposed. Channel allocating manners are designed for soft handoff in WLAN, and one of them is chosen for small STA and DSH. The proposed DSH scheme cooperates the working of dual wireless network adaptors in one STA by a global semaphore. STA maintains two active links to forward and backward APs respectively, and they never do the handoff simultaneously. The DSH ensures at least one data link to remote network at any moment. DSH is realized on the STA side, so AP can use any standard product. It achieves seamless data transmission during STA's fast motion and enhances the link reliability.3) A novel improved mobile IP (IMIP-RS) is proposed to support subnet's roaming. IMIP-RS extends the mobile IP to support muliti-hops connection. It allows the mobile network roaming as a whole. IMIP-RS integrates the home agent, foreign agent and intra-domain agent to transfer packets from remote nodes to mobile nodes in the mobile subnet. In IMIP-RS, STA and AP work as gateways at layer 3 to forward packets instead of as layer 2 device. While adopting IEEE 802.11g wireless devices, the mobile subnet applying IMIP-RS can achieve 29Mbps as the maximum application throughput by TCP data transmission.4) A mobile platform for rapid roaming subnet (MP-RRS) is realized in our developed AP and STA based on the infrastructure of WLAN. The discussed technologies are integrated into the platform, including fast handoff and buffer in data link, soft handoff scheme based on dual-link, and the IMIP-RS. The MP-RRS is applied to the data communication sub-system of the automatic train control system in urban subway. Testing results indicate that the core techniques in MP-RRS are feasible, and broad band connection is provided for fast moving subnet.The MP-RRS is applied to an important production transformation project in Jiangsu province.