Virtual topology dynamics and handover mechanisms in Earth-fixed LEO satellite systems q Ömer Korçak * , Fatih Alagöz Department of Computer Engineering, Computer Networks Research Laboratory (NETLAB), Bog ˘aziçi University, Istanbul, Turkey article info Article history: Received 11 March 2008 Received in revised form 23 October 2008 Accepted 29 January 2009 Available online 8 February 2009 Responsible Editor: E. Ekici Keywords: Earth-fixed satellite systems Virtual topology Virtual node Handover mechanisms Soft handover abstract Handling network mobility in a highly dynamic LEO satellite network is a critical issue to achieve seamless and efficient integration of satellite and terrestrial networks. In Earth- fixed satellite systems, this task could be simplified by representing the network with a more static virtual topology. Virtual node (VN) approach is widely explored in satellite net- works research; however, it has some deficiencies due to necessity of one-to-one corre- spondence between virtual nodes and physical satellites. In this work, a generic virtual topology model, namely multi-state virtual network (MSVN) architecture that alleviates these deficiencies, is proposed. A new mathematical model for MSVN is introduced along with its potential contribution to the overall system availability. Furthermore, possible handover mechanisms in Earth-fixed satellite systems are investigated, and new efficient handover mechanisms for both VN-based systems and MSVN-based systems are proposed. Comparing the handover mechanisms, we conclude with the benefits of using an MSVN- based Earth-fixed satellite system. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Satellite networks are an attractive option to provide broadband integrated Internet services to globally scat- tered users, due to their potential advantages such as extensive geographic coverage, high bandwidth availabil- ity, and inherent broadcast capabilities. Satellites rotating in geostationary orbit (GEO) are well suited for broadcast services; however, they suffer from high free space attenu- ation and long delays. On the contrary, non-geostationary systems consisting of Medium Earth Orbit (MEO) and Low Earth Orbit (LEO) satellites offer lower latency, lower free space loss, and better re-use of available ground-space communication frequencies. Therefore, they are more suit- able for most applications, especially, for those running in real-time. However, these advantages come with a price: Footprints of satellites at lower altitudes are smaller, and global coverage can be provided by higher number of sat- ellites connected with inter-satellite links (ISL). Moreover, lower orbit satellites move with higher speeds relative to the Earth’s surface, resulting in high dynamic in the net- work topology. This topological phenomenon constitutes a major challenge in providing quality of service (QoS) for rapidly growing real-time multimedia services. Connectionless protocols may use the network re- sources efficiently; however, providing QoS guarantees is difficult in connectionless networks, in particular due to the difficulty in accounting for the delay aspects of QoS and sequencing. Reliable and powerful traffic engineering methods and QoS provisioning mechanisms are usually provided through connection orientation. However, con- nection-oriented protocols face an important challenge in satellite networks: Established connections must be main- tained as the network topology changes. To address this challenge, satellite topology dynamics should be handled properly. Fortunately, although the topology of a satellite 1389-1286/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.comnet.2009.01.010 q Part of this work has been presented at IEEE GLOBECOM, Washington, DC, 2007 [1]. * Corresponding author. Tel.: +90 212 3596652; fax: +90 212 2872461. E-mail addresses: omer.korcak@cmpe.boun.edu.tr (Ö. Korçak), fatih. alagoz@boun.edu.tr (F. Alagöz). Computer Networks 53 (2009) 1497–1511 Contents lists available at ScienceDirect Computer Networks journal homepage: www.elsevier.com/locate/comnet