تجزیه و تحلیل عملکرد از روش حرکت در یک محیط IPفضای ترکیبی زمینی

The emphasis of this paper is on investigating the performance of signalling protocols designed for a mobility management scheme, which uses Mobile IP for inter-segment mobility in a hybrid space and terrestrial environment. Initially, the system architecture, which consists of three wireless access networks attached to an IP backbone, is presented. This is followed by a description of the proposed mobility procedures employed in the system, which aim at minimising modifications to existing satellite and terrestrial network protocols. The mobility procedures are simulated in order to evaluate their performance and determine their effectiveness in an operational environment. Results verify the efficiency of the protocols and show that the additional signalling time introduced by the procedures is minimal.

The last decade proved to be a turbulent time for the mobile-satellite industry. Original market predictions for the demand for mobile-satellite services have proven to be somewhat optimistic, following the widespread, phenomenal take-up of second-generation cellular communication services, such as GSM. Consequently, there has been a drastic re-alignment of the mobile-satellite industry, towards serving smaller, niche markets. The start of the nineties was very much geared towards global, non-geostationary satellite solutions. However, by the start of the new millennium, the introduction of regional geostationary satellite networks, notably by ACeS [26] and THURAYA, plus the announcement of the new generation of INMARSAT-4 satellites, has seen the technological pendulum swing back towards the geostationary satellite solution. We are now on the verge of the introduction of the third-generation (3G) of mobile communications, which will be known in Europe as the UMTS. The introduction of 3G technologies by 2002 will open up new market opportunities for the mobile-satellite industry. The nature of service delivery will change with the introduction of 3G networks from existing circuit-switched to a combination of packet- and circuit-switched delivery. Eventually, an all packet-oriented environment can be envisaged, in time for the introduction of fourth-generation (4G) technologies, sometime towards the end of the decade [20]. Demand for voice services is likely to remain dominant over data-services in the short-term. However, as users become more au fait with the mobile multimedia environment, there will come a time when data, transmitted in the form of packets in a Mobile IP environment, will dominate the available resources of the network. This, of course, will require more bandwidth per user. The broadcast nature of geostationary satellites makes them ideally suited for the delivery of particular, mobile multimedia services, such as video-on-demand, tele-conferencing, and so on. As services evolve, the anticipated demand for higher data rate, broadband services will render the need to move up in transmission frequency from the allocated S-UMTS L/S-bands to the next suitable band available for broadband transmission, the Ka-band. However, one disadvantage of mobile-satellite systems is their inability to provide sufficient coverage to urban areas and indoor environments due to severe shadowing of the signal and power limitations. This is particularly the case at Ka-band, where the mobile channel takes on an on–off characteristic [22] with fades in excess of 20 dB. This problem can be alleviated if the satellite system is complemented by a terrestrial system in areas where deployment of satellite systems is not possible [8]. The only difficulty with such an approach is that the specifications of both terrestrial and satellite systems mostly emphasise the operations of each individual system. Therefore, for the systems to inter-work, a new set of signalling protocols for mobility management has to be introduced to ensure smooth inter-segment operation. Issues related to mobility management for converged networks have been addressed on numerous occasions in the literature. As a result, several proposals for the convergence of networks have emerged in recent years, particularly from the European Union's ACTS programme. In the ACTS project INSURED [4], an inter-working function was incorporated to emulate both terrestrial and satellite segment, whereas in SINUS [11], the use of a flexible functional model in a radio independent environment was implemented to integrate the terrestrial and satellite components. Other work related to mobility management in heterogeneous networks include the scheme presented in Ref. [18], where the concept of inter-system boundary cells is introduced to facilitate fast handover to a new system. The work presented in this paper differs from the work described above, as the schemes proposed in Refs. [4], [11] and [18] include several modifications to the existing wireless access segments. From a commercial perspective, this may not be very desirable since the mobility procedures for certain segments, in particular terrestrial systems such as GPRS and UMTS, have already been standardised and any modifications would prolong the commercial deployment of an integrated system. In order to solve this problem, a signalling protocol which utilises the Mobile IP [21] protocols is introduced in this paper, which aims to reduce the modifications performed on the access segments. Essentially, the signalling protocols presented in this paper are applicable to any mobile network, since the protocols are designed to operate in the higher layers of the protocol stack. In fact, no modification is foreseen in the core network of each access segment. Using Mobile IP, a mobile node is given a CoA when it is away from home. Therefore, packets destined for the node can be re-directed to the CoA by a home agent, which is essentially a router on the home network. Using this approach, the mobility problem is basically transformed into a routing problem. In recent years, the use of Mobile IP to manage inter-segment mobility has also gained popularity among researchers. For example, in Ref. [17], interworking of DECT with Mobile IP to support mobility is described, whereas in Ref. [24], Mobile IPv4 functionalities were introduced to perform vertical handovers between different media such as infrared, radio LANs, wide-area cellular, and satellite depending upon error rates and bandwidth availability. This work is later elaborated in Ref. [25], where a policy-enabling mechanism to estimate the system's dynamics is included in the infrastructure to improve the efficiency of the handover algorithm. Using this system, users are allowed to influence the selection of the best wireless system using network characteristics and dynamics, such as performance, cost and power consumption. The primary objective is, therefore, to make it possible to balance the bandwidth load across networks with comparable performance. Even though Mobile IP principles are still utilised in this paper, the methodology described here is different from the one developed for Refs. [24] and [25] in terms of the system architecture and the method in which the performance of the protocols are measured. In this paper, several inter-working units are introduced, which can be managed and owned by a network or service provider. In addition, the design of the functional model for the integrated system is presented, which can later be used when implementing the signalling protocols on a different wireless access segment. Another important difference in this paper is the use of Mobile IPv6 concepts rather than those of Mobile IPv4. The paper is organised as follows: in the next section, the main components of the system architecture are briefly explained. This is followed by a description of the mobility management scheme, which includes the design of the signalling protocols and functional model employed for inter-segment mobility. In Section 4, the simulation approach and the operational scenario of the system are outlined. The results obtained are then presented and analysed in Section 5 before concluding the paper.

The convergence of mobile and IP technologies offers the potential to develop and deliver a vast array of new services and applications. The technological alliance formed by a network comprising of space and terrestrial components will enable services to be made available to the end user, anywhere and at anytime. In this paper, a possible integration scenario comprising of a broadband satellite network and packet-oriented cellular segments has been discussed. The rationale of the paper was from the perspective of incorporating Mobile IP features in order to facilitate the interworking between already standardised mobile networks. A functional model to manage mobility procedures, which is compatible with the IMT-2000 functional model, has been presented. From the functional model, the signalling protocols have subsequently been derived. The protocols, which use Mobile IP for inter-segment mobility, under ideal propagation conditions, appear to offer a promising solution with minimal additional signalling delay detected in both satellite and terrestrial segments and are comparable to other proposed protocols in this field. Additionally, these protocols have the added advantage of being flexible and adaptable to different wireless networks without requiring excessive modification to the existing systems. Such a solution will facilitate the early deployment of an integrated network, in which mutually compatible space and terrestrial technologies will enable the global availability of a mobile Internet.