1 Abstract— Experimental results are presented for end-point controlled handover of a stream of voice-like packet data traffic between two independent wireless networks. The handover was achieved by using a version of the Stream Control Transmission Protocol (SCTP) that had been modified to monitor link quality and switch the primary connection to the “best” quality link available, where quality is evaluated in terms of application performance. The results clearly show that mobility management based solely on received signal strength is not suitable for wireless systems with contention-based access policies such as WiFi. Index Terms— Mobile Communication, Network Testing, Packet Radio, Wide area networks, Wireless LAN, Handover, VoIP, Call Quality I. INTRODUCTION THE use of Voice over IP (VoIP) technology is moving from the low cost international call services market to become a serious contender in the home phone market, and is expected to continue its growth into the wireless market. There is a move towards a multitude of small inexpensive WiFi (Wireless LAN) “cells” overlaid by larger WiMAX or WiBRO cells offering competing and complementary offerings to the range of cellular systems that are available. This is leading towards a Heterogeneous Wireless Network Environment where an end user could be offered an instantaneous choice of network technologies that could provide connectivity and billing rate alternatives. These networks are expected to be owned by many different entities who may compete to provide each customer’s network connectivity. Mobility and handover have been addressed to date by using network-based entities to redirect packet streams, because any interruption to service on a voice call can have significant impact on the call quality. This approach is 1 L. Murphy, P. Perry and J. Murphy are with the Performance Engineering Laboratory, School of Computer Science and Informatics, University College Dublin, Ireland. J. Noonan is now with Avaya, although his work on this paper was performed while with the Performance Engineering Laboratory at UCD. The authors are grateful to Eircom for an equipment loan which helped make this work possible. This work was supported in part by the Informatics Research Initiative of Enterprise Ireland. exemplified by the Unlicensed Mobile Access (UMA) system, which ties WiFi hotspots into a 3G core network [1]. This paper reports the first known end-point controlled handover of a data transfer session from a WiMAX system to a WiFi system, and vice versa. The data stream in question was a packet stream emulating a VoIP call being sent from a mobile node via a WiFi access link. As the WiFi link quality degraded, the call was seamlessly transferred to a WiMAX link in the sense that no packets were lost and there was no interruption to the packet stream. This represents a significant practical demonstration of mobility management in the end points, rather than relying on network-based solutions. The results also show that received signal strength is a poor indicator of expected call quality in a congested WiFi environment. Our experiments were carried out using a private IEEE802.11b WiFi Access Point (AP) and a commercially available pre-WiMAX deployment. We developed an application that used a modified version of SCTP to trigger a handover between the two networks with no network intervention. To avoid firewall issues, the client and the server were both connected through the WiMAX network as shown in Figure 1. The rest of this paper is organized as follows: some related work is described in section II; causes of link quality degradation in wireless networks are discussed in section III. In the following two sections we present our experimental set- up and results, respectively. Finally we offer some conclusions and suggestions for future work in section VI. II. RELATED WORK Handover in cellular networks is well understood: a large geographic area is divided into smaller cells, and handover is performed between cells as required [2]. This choice of network and decision to handover is normally based on the strongest available Receive Signal Level (RSL) of the available cells. The handover is normally performed with the cooperation of both the end user and the network. This approach is valid under the assumption that the network cells provide a performance that only differs based on signal quality, measured using RSL and Bit Error Rate (BER). An Application-quality-based Mobility Management Scheme Liam Murphy, Member, IEEE, James Noonan, Student Member, IEEE, Philip Perry, Member, IEEE, and John Murphy, Senior Member, IEEE 1