Mobility Study of 802.11 networks.
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As WLAN technology emerge to smaller and more mobile clients, the mobility capabilities of the 802.11 stack have come in focus. The evolution of the 802.11 family of standards is constantly ongoing, and new features and mechanisms are added. As often is the case in wireless networks, security is the one area that receives the most attention. Analysts have uncovered shortcomings in the security provided in 802.11 WLANs, and new security schemes have been developed as a result of these shortcomings. The result of this process is that the phase where the client authenticate and associates with the AP, namely the 'handover' have become more complex and the mobility efficiency have suffered. The 802.11 family has been around for more than 5 years, but methods for testing their performance are not fully developed. This report presents a method to measure the mobility performance during handovers in 802.11 WLANs. The test-bed proposed here is generic and can support testing the performance under many different configurations. The mobility performance offered in several 802.11 standards have been tested under the proposed test framework, and the results are presented in this report. Average handover delay ranges from 58 ms under the most basic configuration with no authentication and no encryption, to 452 ms in more robust security configuration with WPA enterprise configuration with TKIP and EAP-TLS authentication. The results are analyzed and discussed to uncover such things as what the main contributors to the handover delay are, what improvements can be made to the protocol etc. Special cases occur, when handover delays are much longer than the average. Two main problems occur again and again when these special cases happen. There is one problem occurring before the handover, which is called the 'triggering problem' in this report. This is a problem with the roaming algorithm at the mobile station, as the station triggers the handover too late. The second problem occurs after the handover, and is named the 'data-flow problem' in this report. The issue here is that the station manages to associate efficiently to the new AP, but the data transmission refuses to resume straight away. These and other problems are discussed in this report, and some solutions are presented as well.