| Current trends in wireless LANs indicate that people are not satisfied with the simple data service, like web browse. Many kinds of services, such as VoIP application and flexible access to networks are also their demands. To meet these demands, it is very important to study the handoff mechanism of mobile node and VoIP application in WLAN, which is the topic of this thesis.Firstly, by analyzing the handoff process of a mobile node, it shows that the drop rate and the handoff latency are the main aspects that influence the QoS of this node. But the handoff methods that a node now use, including the hard handoff and smooth handoff, either have higher drop rate or have longer handoff latency. To this problem, a novel handoff method based on AP Pre-Forward is proposed in this thesis and simulation of it is also carried out. The result shows that the whole performance of this proposed algorithm is higher than the others.Then in order to reduce the overheads that the proposed handoff method brings to, two-tie cell coverage method is used . Besides this, the degree that the size of two cell overlap and the mobile velocity which influence the handoff performance is also studied, the results further validating our proposed handoff method .Next, the VoIP application in 802.11 wireless LANs is also studied in this thesis. In order to supply QoS to VoIP, the PCF channel access method , which is based on AP polling stations, is used, but the beacon sent by AP is sometimes delayed using this method, which will enlarge the jitter time of polled VoIP stations. To deal with it, a dynamic polling-list method is proposed and the experiment results show the proposed method improves the performance greatly.On the other hand, the number of polled stations in a polling list has its maximum. In order to fulfill the connection admission control(CAC) easily, the maximum must be calculated firstl so a deducing method of variable bit rate(VBR) codec is introduced into this thesis, which makes use of May or Brady's conclusion that people's conversation feature meets with 2-state Markov chain .A VoIP station can move freely from one BSS to another in wireless LANs, but after handoff , whether it can get the identical QoS before is also very important. At the base of the present resource reservation schemes and the wireless LAN's specialties, a PCF station resource reservation scheme is proposed in this thesis too, which when a mobile finds a new AP, can request its adjacent AP to reserve resource for it and at the same time, the adjacent AP also adjusts the size of reserved resource adaptively. A simulation of the proposed scheme shows that the proposed scheme can make the handoff drop rate lower than other reservation schemes under the condition of not wasting the wireless bandwidth resource.At last, the load distribution of AP in hot area is also studied in this thesis. In an infrastructure wireless LANs , one station always selects AP to connect is based on the received signal strength indicator(RSSI). This approach may make a large number of active stations connect to a few APs...
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