Research Of Fast Handoff In Rail Transit Based On MAC Layer

Along with the development of wireless communication technology, the IEEE802.11-based wireless local area network (WLAN) has been pervasively applied into our daily work and life. The frequent handoff due to the limitation of WLAN coverage greatly decreases the quality of service provided by the WLAN because of its poor support for high-speed mobility, especially in high-speed railway system. The widely used multimedia applications such as voice and video integrated program in WLAN environment call for the demand of real-time low handoff delay and minimal packet loss. It has great meaning to research on the fast radio handoff technology in railway transport system.This thesis first describes the protocol architecture of IEEE802.11, two access mechanisms and the carrier sense mechanism of MAC layer, the enhanced frame aggregation and block acknowledgment of802.11n MAC. It introduces three phases of handoff process, analyzes the handoff delay and draws the conclusion that the scan delay is the main reason leading to handoff delay. Handoff mechanism can be switched into single-card and dual-card. In the comparison on reliability, handoff efficiency, accomplishment complexity and economic costs, the result that dual-card handoff is the most comprehensive and effective approach is reached.Then, this thesis presents the basic principle of dual-card handoff, proposes SRSB and DR to realize dual-card handoff, describes their working principle and implementation method. SRSB is the mode that two cards share the same IP, take transmission and scanning in turn, and send ARP response packet timely to update route. So that data transmission and handoff processes can be done in parallel. The mode of DR uses caching idea. By two cards’ two-way transmission and one-way return, there is no one packet lost. At the same time, OSPF protocol is used to update routing table by flooding when network topology changes.Finally, this thesis simulates these two proposed modes, evaluates them from response time, throughput, rate of packet loss and delay jitter, validates both of them are fast handoff methods based on quality of service.