Research Of MAC Layer Handoff In Wireless LANs

IEEE802.11Wireless Local Network (WLAN) provides highly scalable mobility andflexible deployment. It can meet the needs of high speed wireless network access for people atanytime, anywhere. So it’s widely deployed. It is developing towards the goal of providinghigher speed, more convenient and better mobility. With the growing demand for WLAN, itneeds to deploy multiple wireless access points (AP) to construct a ubiquitous wirelessnetwork environment. As the coverage of the WLAN AP is limited, the radius of coverage isgenerally less than100meters. However, the mobile users may move freely between accesspoints, so it leads to the technology problem called handoff. Handoff will cause thecommunication latency, jitter and packet loss, even network outages. It affects the quality ofservice for real-time multimedia business. Therefore, it’s of great significance to reduce thehandoff delay, protect the quality of services and improve system performance.This thesis first introduces the IEEE802.11protocol standards, networking architecture,the MAC layer and physical layer. Then the handoff process of IEEE802.11is studied and thehandoff delay is analyzed. The WAVE (Wireless Access in the Vehicular Environment)protocol and the handoff of IEEE802.11p are also studied.This thesis describes the criteria of handoff trigger, analyzes and summarizes the triggeralgorithm based on the received signal strength, introduces the RSSI prediction algorithmbased on gray model GM (1,1). As the calculation of GM (1,1) is relative complexity, thisthesis presents a differential RSSI prediction algorithm; the GM (1,1) and the differentialRSSI prediction algorithm are simulated using MATLAB. The theory and the modeling of thealgorithm are simple, but it can predict the trends of RSSI accurately.In order to reduce handoff delay, achieve load balancing and improve quality of service,this thesis takes many factors into account when handoff decision and presents an improvedhandoff algorithm based on neighbor graph (NG) and fuzzy logic. Based on the Neighborgraph algorithm, the channel on which the AP is working on, the available bandwidth of theAP and the current user number of the AP are added. The pre-scan stage is also proposed inorder to reduce packet loss when scanning. Considering the needs of quality of service and load balancing, this thesis take RSSI, available bandwidth and user numbers three factors intoaccount when handoff decision. These factors are considered as inputs to the fuzzy logicsystem. Through the fuzzy logic system, it can find the best target AP and make reasonablehandoff decision.Finally, the thesis simulated the proposed algorithm by using MATLAB and NS2.Thesimulation results show that the proposed approach is able to reduce the handoff latency andpacket loss when scanning. It can balance work load of AP and improve system performancewhen considering many factors.