| Summary: | 碩士 === 國立交通大學 === 資訊學院碩士在職專班資訊組 === 96 === In this thesis, we present the experimental results and analysis of the handover behaviors of the IEEE 802.11 wireless network (WLAN) and the cross-layer interaction between WLAN adapters and Microsoft Windows operating systems during handoffs using SIP-based VoIP applications. In wireless networks when a mobile node (MN) detects poor link performance (e.g., low received signal strength or signal-to-noise ratio, or high frame error rate), the MN may have to change its point of attachment to the Internet from one access point (AP) to another in order to retain its connection. The link-switch process is called a link layer handover, and involves AP probe, authentication, and association phases in 802.11 WLANs.
Mishra et al. have conducted an empirical study on link layer handover and shown that probe delay constitutes most of the link layer handover latency. However, they did not explore the handover delays caused by different WLAN adapters implementations and network deployments, and the effects of the link layer handovers on the behaviors of the upper layer protocols.. The link layer handover behaviors may vary with different WLAN adapters, resulting in different handover delays. Even worse, a link layer handoff may further cause a layer-3 network layer handover if the new AP is situated in another subnet. Such handover delays may disturb or even break an ongoing session, especially for real-time services, such as Voice over Internet Protocol (VoIP) applications, Furthermore, the implementation of the application software itself also affects the handover delays. Therefore, in this thesis, we conduct a layer-by-layer analysis of handovers and investigate, systematically, the cross-layer effects of WLAN adapters, network configurations and application software on handover delays.
Our experimental study discovers several interesting results. First, link layer handover detection has a significant effect on handover delays and different WLAN adapters adopt different heuristic approaches to link layer handover detection. Second, different WLAN adapters employs different probe behaviors, and most WLAN adapters first look for the APs in the same extended service set (ESS) as the AP the adapter currently associates with during handovers. The adherence to the same ESS may contribute a significant amount of probe delay when a WLAN adapter makes an inter-ESS movement, that is, from the coverage area of an AP to the one of another AP in a different ESS. Third, different configuration tools, such as Windows Zero-configuration or Intel PROSet, of WLAN adapters have different probe behaviors. Fourth, the probe-before-associate of WLAN adapters makes direct connection inefficient, especially when WLAN adapters do not support unicast probes. Fifth, the most interesting observation, link layer event triggers have both their pro and con if not accompanied by sufficient information. Finally, the capability for applications to receive/send data from/to the new IP address of a corresponding node will shorten the overall handover latency.
|
|---|
