WLAN SYSTEMS AND INTERWORKING Fast handoff scheme based on mobility prediction in public wireless LAN systems S. Pack and Y. Choi Abstract: Recently, wireless LAN systems have been widely deployed for public mobile Internet services. Public wireless LAN systems can provide high speed Internet connectivity using portable devices such as laptop computers, personal digital assistants (PDAs), etc. In public wireless LAN systems, reliable user authentication and mobility support are essential issues. However, re- authentication during handoff procedures causes long handoff latency and this affects the quality of service in real-time multimedia applications. A fast handoff scheme based on mobility prediction is proposed. In this scheme, a mobile host entering the area covered by an access point (AP) performs authentication procedures for multiple APs, rather than just the current AP. These multiple APs are selected by a prediction method called the frequent handoff region (FHR) selection algorithm, which takes into account users’ mobility patterns, service classes, etc. Since a mobile host is registered and authenticated for an FHR in advance, handoff latency resulting from re-authentication can be significantly reduced. Simulation results show that the proposed scheme is more efficient than other schemes in terms of handoff delay and buffer requirements. 1 Introduction Public wireless LAN systems based on IEEE 802.11 are becoming popular in hot spot areas. Unlike existing wireless Internet services, a public wireless LAN system can provide high-speed Internet connectivity of up to 11 Mbit/s. Originally, the wireless LAN was designed for indoor network solutions, so that deploying a wireless LAN in public areas requires a system of access control for unauthorised users. In addition, public wireless LAN systems should support different user mobility patterns. Therefore, user authentication and handoff support be- tween access points (APs) are two of the most important issues to be considered in the design of public wireless LAN systems. Generally, since user authentication should be performed at each AP, when a mobile host (MH) moves into a new AP area, it should perform a new user authentication procedure and receive a new wired equiva- lent privacy (WEP) key, which encrypts the transmitted data in the wireless link. These authentication mechanisms impact on the network performance. Since MHs need to be authenticated during and after handoff, the authentication mechanisms need to be responsive to the handoff time-scale required in micro- mobility environments [1]. Furthermore, since authentica- tion, authorising, and accounting (AAA) servers are located at locations far away from the AP, the current handoff scheme cannot meet certain requirements [2] for real-time multimedia applications. To reduce the handoff latency, several schemes have been proposed in the literature [3, 4]. However, these schemes focused on the reduction of the latency incurred in scanning, channel detection, and execution procedures and they did not consider any authentication latency, which is one of the inevitable latencies in public WLAN systems. In order to reduce the authentication delay, a pre-authentication scheme was proposed [5] . In the current WLAN standard, although MHs must authenticate with an AP before associating with it, no schemes in IEEE 802.11 require that authentication takes place immediately before association. On the other hand, in the pre-authentication scheme, MHs can authenticate with several APs during the scanning process so that when association is required, the MH is already authenticated. As a result of pre-authentication, MHs can reassociate with APs immediately upon moving into their coverage area, rather than having to wait for the authentication exchange. Although the pre-authentication scheme can reduce the authentication handoff delay, it requires a sufficient overlapping area and the modification of MHs. These points can be drawback in the deployment of the pre-authentication scheme. In this paper, we propose a fast handoff scheme that reduces the re-authentication latency in public wireless LANs. When a MH sends an authentication request, the AAA server authenticates not only the currently used AP, but also multiple APs, and sends multiple WEP keys to the MH. These multiple APs are selected by the frequent handoff region (FHR) selection algorithm based on mobility prediction. The FHR selection algorithm utilises the handoff weight in each link between APs, which is collected and calculated by the centralised system. 2 Fast handoff scheme in public WLAN systems The objective of the proposed handoff scheme is to reduce the handoff latency caused by authentication procedures at The authors are with the School of Computer Science & Engineering, Seoul National University Seoul, Korea r IEE, 2004 IEE Proceedings online no. 20040834 doi:10.1049/ip-com:20040834 Paper first received 29th July 2003 and in revised form 18th May 2004. Originally published online: 15th September 2004 IEE Proc.-Commun., Vol. 151, No. 5, October 2004 489