Broadband Communications via Satellite Presented by Eric Zucker

Evolution towards Broadband Networks Over the last years, massive investments have been devoted to the development of terrestrial broadband networks based on fiber optic transmissions and on the use of cell switching techniques. For broadband ISDN (B-ISDN), Asynchronous Transfer Mode (ATM) has been selected as the switching technique. It is seen as the target technique for all sorts of networking, e.g. LAN interconnection, Metropolitan Area Networks, Wide Area Networks. In the trunking part of the network, ITU-T has also standardized the Synchronous Digital Hierarchy (SDH) as the target multiplexing technique between cross- connects or switches in the network. Satellite systems providing trunking services and interfacing with public networks have traditionally been supporting terrestrial interfaces compatible with the PDH hierarchy. However, the rapid roll out of SDH networks has prompted Intelsat and Eutelsat, among others, to become more and more interested in the compatibility of their networks with SDH interfaces.

Objectives of the Study The study, which was carried out for the European Space Agency in the period by CSEM supported by PTT Telecom of Switzerland and Logica, is oriented towards the integration of communications satellites in SDH networks and fast packet switching (ATM) networks, and it involves the identification of the strategic areas to be addressed by satellite communications systems, leading to the identification of the development of equipment or systems necessary for this purpose. The study work has been split into two complementary and non-overlapping parts: part 1, SDH related part 2, ATM related.

Scope of the Study The study has included, for both part 1and part 2: an analysis of the evolution of the market and the direction in which the European Operators are moving, with the identification of network scenarios which include the satellite; a part covering the design of the architecture of the relevant components of the satellite system, which satisfies the requirements corresponding to the scenarios identified. Furthermore, part 2 also includes a performance evaluation of the design by means of simulation, in order to verify that the design chosen can guarantee on one hand that no compatibility issues exist related to the transport of ATM over satellite links, and on the other hand to show that an acceptable degree utilization of the satellite link can be achieved.

SDH Network Rollout in Europe SDH equipment began to be introduced in European networks in By 1992 the Deutsche Bundespost Telekom had already embarked on a major plan to overlay their network with an SDH transmission infrastructure. Other European countries were not so quick with their plans, but by 1994 all European TOs had begun to plan and implement SDH as a major benefit to the efficient running of their networks. Start up TOs which have been permitted under UK liberalization installed SDH transmission equipment from the outset. Thus the current position is that an SDH network is nearing completion of its initial phase in Germany, and will reach a similar stage in France, Italy and the UK by The Scandinavian countries will have an extensive SDH network within their countries by The other TOs will by that time also have SDH links within the main transmission links in their networks. Full deployment i.e. SDH to any exchange or to any customer which needs it will not be available in all countries until at least the turn of the century.

SDH Network Topology SDH bit rates range from 155 Mbps (STM-1), 622 Mbps (STM-4) to 2.5 Gbps (STM-16).Typically, STM-16 is used in the highest levels of the network interconnecting all the main trunk exchanges. At lower levels in the network, STM-4 or STM-16 is used, with STM-1 is used in the access network. A distinction can be made between the network topologies of new TOs and those of established national TOs. In the case of start-up networks such as COLT (City of London Telecommunications) the network is implemented as a number of STM-1 rings interconnected by STM-16 rings as shown in the Figure 1. For established networks such as the German network, the Italian network, or BT, this basic concept is modified to meet their specific conditions. As shown in the Figure 2, the highest levels in the national network, all major exchanges are fully mesh connected – that is every exchange has a direct connection to every other exchange. At lower levels in the network the ring principle is used.

STM Chart

SDH Chart

Bit Rates STM-1 has so far been used at the gateway between networks but if traffic levels justify higher rates could be used. STM-1 network interconnection by satellite will be used by some TOs where satellite is cheaper than cable, for fast network rollout or to bypass other operators' networks. There are several medium/thin inter-continental routes from Europe where cable does not yet exist. Where STM-1 or greater cannot be conveyed by the current generation of satellites, sub-STM-1 medium rates of 34 Mbps or 51 Mbps could serve these routes. However, no TOs stated plans for such medium rate SDH implementations and three expressed strongly that they did not want to encourage the proliferation of different rates by proposing this. Sub-STM-1 (2 Mbps) over satellite is needed to convey tributary units to remote large corporate customers.