Broadband Satellite Multimedia Networks

Introduction

Satellite communication systems represent an adequate solution for providing high bit-rate services to users over wide areas. Important advantages of the satellite approach are: (i) easy support for both broadcast and multicast high bit-rate multimedia services; (ii) backup communication services for third-generation (3G) cellular users on a global scale; (iii) efficient support of high-mobility users (e.g., users on trains, planes, etc.). For many isolated areas on earth, satellites are the only solution to be connected to local Internet service providers.

When interconnected together with local or geographical networks, satellites can be the bottleneck of the entire system because of the delay and throughput that they entail. For these reasons, getting the maximum performance out of the satellite segment is very important.

The ETSI TC-SES/BSM (satellite earth stations and systems / broadband satellite multimedia) working group had the task to focus on IP layer interworking for satellite networks. This working group has defined a reference broadband satellite multimedia (BSM) network architecture as in Figure 1. The interest here is on geostationary orbit (GEO) satellites. They are on an equatorial plane at an altitude of about 35,800 km. They have synchronous motion with respect to a point on the earth (i.e., 24-hour orbital period), so that they are stationary with respect to a user on the earth. Three GEO satellites would be enough to cover all the earth except Polar Regions. Due to their distance from earth, communications with these satellites is affected by a significant delay for the propagation of the electromagnetic signal (at least 250 ms for each hop).

Figure 1.

BSM reference network architecture

From the protocol stack standpoint, a BSM network can involve different layers (ETSI - TR 101 985, 2002):

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  The BSM network interconnects with ground network elements at layer 2, like a bridge.

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  The BSM network interconnects with ground network elements at layer 3, so that the satellite earth stations are routers.

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  The BSM network operates at a layer above the 3^rd one: the satellite earth stations are gateways. In this case, these stations can perform a more accurate routing based not only on the IP datagram header, but also on information of the higher layer headers. The earth station can implement special functions, like performance enhancing proxies (PEP) that are important in order to improve the transmission control protocol (TCP) performance in satellite networks (note that a significant problem in the provision of TCP/IP services through GEO satellites is the propagation delay of the signal from the earth station to the satellite and back).

The DVB-S standard (and its variants) has gained momentum for the provision of different services in satellite networks. DVB-S has been conceived for primary and secondary distribution (fixed satellite service, FSS) and broadcast satellite service (BSS), operated in Ku (11/12 GHz) and Ka (20/30 GHz) bands (ETSI - EN 300 421, 1997). Moreover, the DVB-RCS standard (ETSI - EN 301 790, 2002) defines a two-way DVB satellite system (i.e., also a return path is considered). We refer here to a start (or mesh) topology where terminals [sometimes known as satellite interactive terminals (STs) or return channel satellite terminals (RCSTs)] communicate via a GEO bent-pipe (or regenerating) satellite to a central earth station -hub- (or directly each other). An RCST can even represent the aggregation point of multiple users. The DVB-RCS system envisages a multi-frequency time-division multiple access (MF-TDMA) transmission in the return link and employs the DVB-S standard (using a form of time division duplexing transmission) for the forward link traffic. The ground segment is composed of the following elements: