Research Of Node Handoff Technology In WLAN

In recent years, Wireless Local Area Network (WLAN) technology has developed rapidly, but the performance of voice over WLAN is not satisfactory, especially the long latency brought by node handoff between Access Points (AP) in WLAN has a great impact on Quality of Service(QoS) of voice service. When a user moves from one Extended Service Set(ESS) or subnet to the other in IEEE 802.11 WLAN, handoff happens not only at link-layer, but also at network-layer. The link-layer handoff often achieves by Inter- Access Point Protocol (IAPP), while mobile Internet Protocol(IP) technology is used widely to achieve the network-layer handoff. But, these technologies bring more handoff latency and more packets lost that influence the QoS of real-time communications such as voice service seriously.IAPP of link layer defines that the old AP don't transfer the data sent to the mobile node to the new AP until the mobile node associates with the new AP. This process increase the transmit latency undoubtedly. As to this problem, a link-layer handoff method named synchronized transmission handoff technology was proposed to provide a mechanism that the process of node association and the process of the data transmission execute simultaneity. Using this method can reduce the handoff latency and the packets lost sharply at the cost of little more network expending.The network-layer handoff adopt Mobile IP technology widely. Mobile IP handoff technology is independent of link-layer handoff method, including mobile detection phase and registration phase. The detection latency is commonly long correspondingly in the handoff procedure. What's more, the mobile node didn't receive the packets until the registration phase finished. These factors bring long handoff latency and high packets lost.For the sake of solve mobile IP handoff problem, a fast network-layer handoff scheme is proposed, this scheme extend the information element of the request frame in re-association phase. While the new AP detect a re-association request of mobile node, the network-layer is triggered by the access point management entity. Then the link-layer handoff and the network-layer handoff carry out simultaneity to ensure zero-delay movement detection phase and low packets lost to satisfy the requirement of real-time application such as voice service.