Elsevier

Computer Communications

Volume 29, Issues 13–14, 21 August 2006, Pages 2601-2611
Computer Communications

Location assisted fast vertical handover for UMTS/WLAN overlay networks

https://doi.org/10.1016/j.comcom.2006.01.013Get rights and content

Abstract

UMTS/WLAN integration offers considerable benefits for the mobile users as well as for the mobile network. As soon as coverage is available, a mobile user should switch seamlessly from the ubiquitous low bandwidth UMTS network to a high bandwidth WLAN. This will improve his quality of service, but also increases capacity in the cellular network. To be able to change between radio access technologies, an inter-RAT handover protocol is needed. If this handover protocol is not fast enough, the vehicular user will penetrate deep into the cell before packets get delivered to the right access point, underutilizing the available bandwidth with a lower overall performance as a result. In this paper, we introduce a new vertical handover protocol which uses location information from the vehicle to predict where and when the handover should occur, thus optimizing user throughput and network performance. We consider its deployment on three UMTS/WLAN integration scenarios.

Introduction

Beyond 3G systems are considered to be heterogeneous networks with multiple radio access technologies (RATs) as well as reconfigurable user terminals, in order to allow mobile users to enjoy seamless wireless services irrespective of their location, speed or time of the day. They will allow users to choose its access technology according to his or her own needs. This concept of “Always Best Connected” (ABC) [1] is shared by the ETSI 3rd Generation Partnership Program (3GPP) and it has laid the groundwork for a UMTS/WLAN interworking specification. The cellular operators therefore no longer see Wireless LAN networks as a competitive radio access technology, but rather embrace its benefits to build Internet access hotspots and offer roaming agreements for their mobile customers. In the near future, WLANs will be considered a true complementary technology to the 3G cellular networks (e.g., UMTS) and will be deployed to offer the same services, but with higher bandwidth whenever coverage is available, typically in town or business centers.

While Wireless Local Area Networks can offer high bandwidth, cellular networks provide a (nearly) full coverage. It is obviously advantageous for the mobile user to connect to the WLAN network as soon as coverage and capacity are available. The user terminal will therefore regularly scan the corresponding frequency bands and try to detect beacons of UMTS or WLAN cells. If signal strength is adequate, a handover to the other network technology is attempted. The handover of connections between different networks with different radio access technologies is also called vertical handover, but these inter-RAT handover procedures have not yet been defined by the 3GPP, except for GSM–UMTS/UMTS–GSM handover.

Most of the papers in the literature discuss the aspects related to the common management of radio resources among heterogeneous networks from an architectural point of view, describing constraints, potential advantages and drawbacks of various integration levels [2]. The 3GPP research on UMTS/WLAN RAT integration has also concentrated on authentication, authorization and accounting (AAA) aspects. However, fast mobility management for vehicular users in heterogeneous networks is in a very early stage and still requires many research efforts [3], [4], [5].

Mobility management in heterogeneous networks controls the handover from one radio access technology to the other, such that ongoing connections are not dropped. It needs to work in close collaboration with the local mobility management entities in the corresponding access networks.

In the UMTS Radio Access Network (UTRAN) the Radio Network Controller (RNC) forms the bridge between the UMTS physical layer and the link layer. A GPRS tunnel (GTP-U) is used to encapsulate the IP packets and to forward them from the RNC via the Serving GPRS Support Node (SGSN) to the Gateway GPRS Support Node (GGSN). A Packet Data Protocol (PDP) context defines this tunnel for each UE. Inter-RNC handover is handled at the SGSN at the physical level (soft handover). The Inter-SGSN handover procedure consists of tunneling (GTP-C) the packets for the UE from the old SGSN to the new SGSN using a PDP context transfer. Within a UMTS domain, the mobile user does not need to change its IP address. On the other hand, in WLANs, mobility management has not yet been standardized and is outside the scope of this paper. Here, we assume a suitable link layer handover protocol maintains ongoing connections without changing the IP address of the UE (e.g., 802.11f [6]).

Some research suggest Mobile IP [7] or and end-to-end mobility scheme like SIP [8] as a mobility management scheme for heterogeneous networks. Mobile IP is a well-known IP network layer mobility management scheme, in which packets from and to the mobile host are tunneled through a home agent at its home network so that the corresponding node that communicates with this mobile host is unaware of the mobility of the mobile host. Unfortunately, Mobile IP suffers from high delay and packet loss and is therefore not suited for fast moving mobile users. End-to-end based mobility management protocols can handle mobility without additional support from the network elements. The drawback of this end-to-end approach is that, especially in the case of fast moving users, requiring fast handover, this protocol suffers from delay as both parties can be geographically spread. Additionally, this mobility scheme is only valid for applications that are SIP-aware.

In this paper, we propose a proactive inter-RAT vertical mobility management protocol for UMTS/WLAN interworking access networks called APACHE (A ProACtive Handover Enhancing protocol). It uses location information from the mobile user, for example acquired by a GPS system, and tries to pro-actively determine to which WLAN access point or UMTS base station the mobile user will switch and when this handover should occur. Our goal is to optimize the use of the available bandwidth for the user, keeping handover delay as low as possible and avoiding packet loss. In the next section we discuss possible UMTS/WLAN interworking architectural scenarios that allow inter-RAT mobility. Section 3 discusses the requirements for a location assisted proactive handover approach and introduces the APACHE protocol. The implementation of APACHE on the different interworking scenarios is also investigated. Following this section, we compare the performance of the APACHE protocol with a hard and a forwarding based handover scheme. We finish this paper with some conclusions.

Section snippets

UMTS/WLAN inter-RAT handover

In a UMTS/WLAN overlay network, one has typically large UMTS cells that provide nearly ubiquitous coverage, while in areas with high user density and high band-width demand, small WLAN cells will be deployed (e.g., in a town center, Fig. 1). Vehicular users, coming from the main road and entering this town center, will have the choice of handing over from the low bandwidth UMTS cell to the higher bandwidth WLAN cell. The mobile user can use the additional bandwidth for new services, like

Implementation requirements

In the following sections, we discuss the implementation of the APACHE protocol in the different scenarios of UMTS/WLAN interworking. We consider the UMTS to WLAN handover. APACHE procedures can be applied analogously to WLAN to UMTS handover.

Results

In this section, we show that our location assisted vertical handover approach (APACHE) is able to achieve a higher performance level than current solutions in terms of network load and user experienced throughput. The APACHE protocol was implemented in the NS-2 [11] simulator.

In Fig. 9, we compare the access network load for varying vertical handover durations: from (almost) instantaneously, up to 500 ms. In the classical forwarding approach, packets that were misrouted to the previous SGSN

Conclusion

In this paper, we have introduced the APACHE handover protocol for vertical handover in UMTS/WLAN overlay networks. Using location information from the vehicular user, we can achieve fast and seamless handover from a low to a high bandwidth radio access technology and vice versa. We have discussed its possible deployment in three UMTS/WLAN integration scenarios: the loose, the tight and the very tight coupled approach. The APACHE handover protocol approaches the speed of a hard vertical

Acknowledgements

Part of this research is funded by the Belgian Science Policy Office through the IAP (phase V) Contract No. P5/11 and by the Flemish IWT through the GBOU Contract 20152 ‘End- to-end QoS in an IP Based Mobile Network’.

Ir. Tom Van Leeuwen was born in Gent, Belgium in 1979. He received his masters degree in Computer Engineering from the Ghent University, Gent, Belgium in 2002. Since 2002, he has been working with Department of Information Technology of Ghent University (INTEC) as a doctoral researcher. In 2004 he received a Ph.D. grant from the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen). His current research interests are in broadband wireless

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Cited by (2)

Ir. Tom Van Leeuwen was born in Gent, Belgium in 1979. He received his masters degree in Computer Engineering from the Ghent University, Gent, Belgium in 2002. Since 2002, he has been working with Department of Information Technology of Ghent University (INTEC) as a doctoral researcher. In 2004 he received a Ph.D. grant from the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen). His current research interests are in broadband wireless communication for vehicular users.

Prof. Dr. Ir. Ingrid Moerman was born in Gent, Belgium, in 1965. She received the degree in Electro-technical Engineering and the Ph.D. degree from the Ghent University, Gent, Belgium in 1987 and 1992, respectively. Since 1987, she has been with the Interuniversity Micro-Electronics Centre (IMEC) at the Department of Information Technology (INTEC) of the Ghent University, where she conducted research in the field of optoelectronics. In 1997, she became a permanent member of the Research Staff at IMEC. Since 2000 she is part-time professor at the Ghent University. Since 2001 she has switched her research domain to broadband communication networks. She is currently involved in the research and education on broadband mobile & wireless communication networks and on multimedia over IP. The main research topics related to mobile & wireless communication networks are: wireless access to vehicles (high bandwidth & driving speed), adaptive QoS routing in wireless ad hoc networks, body area networks, protocol boosting on wireless links, design of fixed access/metro part, traffic engineering and QoS support in the wireless access network. Ingrid Moerman is author or co-author of more than 300 publications in the field of optoelectronics and communication networks.

Prof. Dr. Ir. Piet Demeester received the Masters degree in Electro-technical engineering and the Ph.D. degree from the Ghent University, Gent, Belgium in 1984 and 1988, respectively. In 1992 he started a new research activity on broadband communication networks resulting in the IBCN-group (INTEC Broadband communications network research group). Since 1993 he became professor at the Ghent University where he is responsible for the research and education on communication networks. The research activities cover various communication networks (IP, ATM, SDH, WDM, access, active, mobile), including network planning, network and service management, telecom software, internetworking, network protocols for QoS support, etc. Piet Demeester is author of more than 300 publications in the area of network design, optimization and management. He is member of the editorial board of several international journals and has been member of several technical program committees (ECOC, OFC, DRCN, ICCCN, IZS).

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Research funded by a Ph.D. grant for Tom Van Leeuwen of the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen).

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