Journal of Computer Science 1 (1): 24-27, 2005 ISSN 1549-3636 © Science Publications, 2005 24 Performance Analysis of Downward Handoff Latency in a WLAN/GPRS Interworking System Mandalaparty Bhaskara Rama Murthy and Fredson Alfred Phiri Faculty of Engineering and Technology, Multimedia University Jalan Ayer Keroh Lama, 75450 Melaka, Malaysia Abstract: This study presents the performance analysis of a new tight coupling based WLAN/GPRS interworking architecture. The effects of network traffic on downward handoff latency are investigated. The results indicate that increasing WLAN traffic increases the overall downward handoff latency more than increasing GPRS traffic. On the other hand, increasing GPRS traffic results in higher packet buffering requirements at the Serving GPRS Support Node (SGSN). Key words: Vertical Handoff, Handoff Latency, GPRS, WLAN, Interworking Networks INTRODUCTION The increasing popularity of Wireless Local Area Network (WLAN) enabled devices has triggered interest in the development of systems that integrate WLAN and cellular wide area data network technologies with support for vertical handoffs between the different networks. A WLAN-GPRS interworking system is one such system. Vertical handoffs, in a GPRS-WLAN system, refer to the transferring of a data communication session from GPRS access network to the WLAN access network (downward handoff) or from the WLAN to the GPRS network (upward handoff). Handoff latency, handoff data loss and ease of implementation are some of the most prominent parameters that need to be considered in the design of an interworking system to support vertical handoffs [1, 2]. Other requirements include security [3], Authentication, Authorisation and Accounting (AAA) [4]. In this study we analysed the effect of network traffic on downward vertical handoff latency. Handoff latency is an important metric, the value of which can be a measure of the degree of smoothness of the overall handoff process. High handoff latency values may result in excessive data loss and lead to the termination of the data communication session. Interworking System Model: A WLAN-GPRS interworking scheme based on tight coupling architecture [5] has been proposed by the authors [6]. For sake of completeness the topology of the proposed scheme is shown in Fig. 1. The scheme employs a Protocol Translation Gateway (PTGW) to interface the WLAN/LAN system to the GPRS network through the Gb interface. The Mobile Station (MS) has both WLAN and GPRS interfaces as shown in Fig. 2. A Network Switching Function (NSF) is employed to direct packets going down the protocol stack through the appropriate interface at the GPRS LL layer. An Ethernet Adaptation Function (EAF) is used to ensure that packets transmitted by the MS through the WLAN/LAN are identified by the PTGW and forwarded to the GPRS core network. The PTGW hides the presence of the WLAN network from the GPRS core network and converts packets from the Serving GPRS Support Node (SGSN) into a format compatible with the WLAN/LAN network and vice versa using the Ethernet adaptation function. It appears to the SGSN as a Base Station Subsystem (BSS). Fig. 3 illustrates the protocol stack of the PTGW. The complete details of the handoff process are explained by [6]. Handoff Latency: Handoff latency refers to the amount of time that it takes for the MS to be granted access in the new network, measured from the instance it crosses a pre-defined handoff threshold power level point. The algorithm employed for vertical handoffs in the proposed system enables breaking down of the overall downward latency (L d ) into four components. Detection Delay (L D ): This is the time it takes for the mobile station to detect that it has moved into a WLAN coverage area, measured from the point that it crosses a predefined handoff threshold power level point. VMS Delay (L V ): The MS sends a request to the PTGW, on detection of the WLAN network, to register with the PTGW. The PTGW creates a record, referred to as the Virtual Mobile Station (VMS), of this MS with all required details. The time that elapses between the MS sending the message and receiving a response from the PTGW, indicating that a VMS has been created, is referred to as the VMS delay. Handoff Request Delay (L H ): This is the time between the MS sending a handoff request message to the GPRS network through the BSS and the MS receiving a handoff command message from the SGSN through the BSS.