Satellite Networks

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 Satellite Networks  





Satellite technology is a wide area network technology that is inherently broadcast in nature (i.e., every host on the network can see every packet that is sent, regardless of the source or destination). Satellite network services can be purchased from several major providers to deploy a Private Virtual Network for your enterprise. Typical speeds range from 9600 bps to 24 Mbps in the forward direction with lower-speed back channels. Round trip latencies of over 500 ms are common with geostationary satellites, although planned future services using Low Earth Orbit Satellites, such as Teledesic, are supposed to offer lower latencies and higher speeds.

In order to support multicasting on a satellite network, which is inherently a broadcast medium, each site’s LAN should be connected to a satellite receiver via a multicast router. Since each site will receive the signal broadcast from the satellite, the multicast receiver either forwards or blocks the received data depending on whether hosts on the LAN belong to the host group.

In general, satellite networks work best for implementing multicast delivery trees that have a single source and do not require a large amount of feedback. That’s because, even though some satellite networks (such as VSAT) can be bidirectional, the return channel, or back channel, operates at a lower speed than the main broadcast channel. To cope with this, systems like StarBurst MultiCast (see Chapters 6 and 9) uses a negative acknowledgment system, which keeps return traffic to a minimum, to promote reliable multicasting over satellite networks.

Two types of satellite networks can be used for multicasting. There are unidirectional systems, such as those used for broadcasting television signals, and there are bidirectional systems, of which VSAT (Very Small Aperture Terminal) is the most common.

For a unidirectional satellite network to operate with TCP/IP protocols and multicasting, a separate back channel is needed. Some satellite service providers offer unidirectional satellite services with a terrestrial back channel to the central uplink site. This land-based back channel can take many forms, such as the Internet, frame relay, ISDN, or modems.

A number of carriers offer VSAT data services for local as well as global coverage. In a VSAT network, the central site (the uplink) sends data to the satellite which in turn beams the signal to the earth where any VSAT terminal within the satellite beam may pick up the signal. In the reverse direction, the VSAT terminal sends a lower-speed signal to the satellite which is then directed to the central uplink site.

Many-to-many multicast applications such as conferencing are not easily accommodated on satellite networks—your remote sites are only tied to the central uplink site rather than to each other. In the case of a VSAT network, conferencing would require that transmissions from one VSAT site to another VSAT site be routed through the central uplink site. Satellite networks are thus most appropriate for such multicasting applications as broadcasting of software upgrades and large files, such as inventory reports.





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