Hybrid satellite/terrestrial networks integration

Abstract

The satellite component is expected to operate alongside that of the terrestrial component and provide a complementary rather than a competitive service. In this paper, the requirements of inter-working and integration of these networks are discussed at different levels. There are currently several consortiums that are working on global voice communications via satellite (e.g., Ellipso, Odyssey, ICO, GlobalStar, OrbComm, and Iridium). Some consortiums are working on global broadband networks via satellite (e.g., Cyberstar, Astrolink, Teledesic, Spaceway, and Skybridge). Almost all of these architectures do not consider advanced capabilities for satellite and terrestrial integration with mobile and fixed networks. Special consideration of intelligent payload architectures and management aspects is required. For the long term architecture, intelligent on-board processing (OBP) capabilities would enable adequate distribution of control for an integrated satellite and terrestrial network. This paper presents some techniques to help the evolution toward this scenario.

Introduction

Convergence of satellite and terrestrial networks is now becoming a key factor in forming the foundation for an efficient global information infrastructure (GII) that is going to be supported by the wealth of information sources offered by content providers. The focal point of this integration is to enlarge and complete service coverage, and in so doing, provide the mobile user with the ability to roam globally and internetwork efficiently across all domains. This inter-operability requires dynamic and transparent connectivity to join local and wide area, public and private networks which are characterised by a diverse mix of topologies, physical media, transmission speeds, carrier services, geographical coverage, interfaces, etc. Several European (Acts SINUS, SUMO, VANTAGE) and American (ACTS) projects, and standardisation groups (ETSI, ITU) [2], [6] have been dealing with mobile and satellite networks. Because of the large number of issues to be dealt with, no uniform and unique solution is available yet.

To address different satellite and terrestrial integration issues, consider several degrees of mobility:

• No mobility.

• Service portability. Roaming users can seamlessly access and use network and terminal-independent services through a number of access networks and devices. They would be offered customised virtual home environments.

• Movability. The moving entities (users, terminals and networks) can change their attachment points to the network between calls. They must log-in to the network for the registration of their location.

• Mobility. Mobile entities (users, terminals, and networks) can change their attachment points to the network during a call. The management centre identifies the user, can locate the serving resource at that moment, and can hand the call over to another resource, when the user moves across resource boundaries.

The mobility and movability may be “embedded”, where the users, for example, employ a mobile terminal in a mobile customer premises network (CPN); or “interlinked”, where one user is connected to the system via satellite and the other via a mobile cellular link.

Satellite service types may be divided into subservices according to the following classification:

• Fixed satellite services. Fixed satellite services concern all radio communication services between earth stations at given positions that can be at a specified fixed point or any fixed point within specified areas. This type of service provides analogue and digital transmission nationally or internationally on the basis of a network topology that can be of transit, distribution or contribution type. (Note: these types are related to the different levels of mobility described above: fixed satellite services are related to the case of no mobility, and mobile satellite services are related to portability, movability and mobility.) Broadcast services, i.e., video, television and sound transmission, are included in this case. The following basic types of users are identified:

  • Users fixed within local “broadband islands” access remote broadband islands through areas with “satellite coverage only”. They need terminals for broadband access only. Inter-operation of islands is achieved through satellites.

  • Users access services within satellite coverage only. They need terminals for satellite access only.

  • Users access a remote broadband island from areas with satellite coverage only (or vice versa). They need terminals for satellite access only. (Note: for the “vice versa” case, terminals should have broadband access only.)

• Mobile satellite services. Mobile services include all radio communications between a mobile earth station and the space station, or between mobile stations via one or more intermediate space stations. They provide the means of uni- or bi-directional exchange of information between users or between users and hosts, where at least one user is mobile. That means either the user's equipment is moving during communication, or it is not moving during communication but is transportable to another location (for example: a reporter’s “suitcase”-terminal with an “umbrella”-antenna). In both cases, the mobility may be global (world-wide) or local (not world-wide). Interactive and distribution services are included in this case. The following basic types of users are identified:

  • Users travelling from one “cellular island” to another via areas with satellite coverage only (using trains, aircraft or ships). They need terminals with access to satellite services via dedicated interfaces (local radio link).

  • Users moving within satellite coverage only. They need terminals for satellite access only.

  • Users travelling from cellular islands to areas with satellite coverage only and vice versa. They need dual or multimode terminals. In this case, consider also that the remote users may be in cellular islands as well as fixed broadband islands.

For a mobile architecture supporting moving and portable end devices, support for location management (i.e., registration, paging, roaming, and routing), and handover of mobile connections between access points, should be provided. The gateway earth stations (GESs) should then support interaction with mobile enabled entities (switches, terminals) and are used for interconnection of various island networks. The associated air interface may require a media access control (MAC) protocol since multiple user terminals may be trying to access the satellite channel. While the topology of terrestrial routing can be considered quasi-static, this assumption does not hold with some mobile satellite constellations with inter-satellite links, where satellite movement leads to a dynamic, although deterministic, topology. Furthermore, taking into account pointing, acquirement and tracking requirements of antennas, some inter-satellite links are not permanent as are the links between stations and satellites. The dynamics of the constellation can then cause handover of a connection, which should be handled seamlessly from a user standpoint. Taking into account all these aspects, routing algorithms for mobile constellations must face two challenges: handling numerous route computations caused by handovers as well as numerous topology changes.

This paper discusses the satellite/terrestrial integration architectures at the service level in Section 2, satellite orbit choice in Section 3, space segment technology aspects in Section 4, and network level aspects in Section 5. Mobility typically infers entities for transport, control and management. Architecture evolution should lead to more intelligent, scalable capabilities. Agents could be exploited to improve the adaptiveness of on-board systems while decreasing the latency of control and management of satellite systems. A new technology-based on DPGA may be considered to support reconfigurable and programmable on-board systems.