Research On Handoff Mechanism In Heterogeneous Wireless Mesh Network

With the development of wireless communication techniques, variety of wireless access networks can be seen everywhere in the form of cellular networks, WLANs, WiMAX networks, UWB networks. With different coverage, transmission rate, network load and costs, these networks offer people various services. But with the limit of the network architecture and the technology natural characteristics, none of these networks can meet all the users'needs. In addition, the terminals have been increasingly equipped with multiple communication interfaces. Therefore the future communication network has to exploit this multitude of connectivity alternatives resulting from diverse wireless communication systems and different access technologies to provide useful services with guaranteed quality to users.In traditional wireless networks such as WLAN, WPAN, WMAN, wireless terminals access the Internet through a one-hop wireless connection to the access point. As one-hop wireless transmission is distance limited, the wireless network coverage can not be very large. In tradional networks, the access points must connect to the wired network, so the deployment costs could be very high. The wireless mesh network(WMN) which is different from the traditional ones can cover these problems. Multi-hop is a key character of wireless mesh networks, that means, the terminals can transfer packets through retransmission by several other networks nodes. A multi-hop wireless access can increase the coverage of wireless networks and decrease the dependency on the wired networks so that the wireless terminals can roam more freely and the wireless network can be deployed more quickly and with much less cost.With the advantages of the Wireless Mesh Networks, variety of wireless access technologies have been improved their functions to support the mesh network architecture. The heterogeneous wireless mesh network is an important trend of the future wireless communication network. The future terminal can be equipped with several different wireless interfaces, so that they are capable of accessing information from different access points at anytime and anywhere. Handoff mechanism is a key chanllenge to reach the target of users roaming among these networks. Most of the researchers have focused their handoff researches on the environment of two integrated networks of cellular network and the WLAN. These handoff mechanisms for one-hop integrated networks are not applicable for the hetegeneous wireless mesh networks with a multi-hop transmission. Most of the handoffs are triggered by decreasing RSSI or the network quality of services, this reactive trigger will cause long handoff latency and poor quality of services. In addition, as the networks have so many overlay areas, the handoff should not be triggered only by the decrease of quality of services. Users are expected to look for new networks for better services at any time. We analyze the movement prediction models and propose an improved HMM (Hidden Markov Model) to predict the next postion of the wireless terminal. With the movement prediction, the terminal and the networks can do the pre-handoff work such as the resource reservation and the context transfer so as to decrease the handoff latency and increase the system performance.This thesis envisions a hierarchical handoff scheme for the hetegeneous wireless mesh networks. This scheme divides the handoff into two kinds, the intra-domain handoff and the inter-domain handoff; and supports the handoff with different procedures. The simulation results show that this handoff scheme can decrease the average handoff latency, and guarantee a very small packet loss by the data buffering strategy.