CITS3002 Computer Networks  
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Simplified Satellite Broadcasting

As an introduction to Local Area Networking, and multiaccess communication, let's take a very simplified look at satellite broadcasting:

  • Many users share a single channel.

  • Propagation at the speed of light, 300 000 km/sec.

  • However, the distance travelled is large - 35,880km for conventional TV satellites, resulting in round trip times of ~270-700msec.
    We can contrast this with the emerging constellations of Low Earth Orbit (LEO) satellites between 200-2000km, with round trip times of ~35msec.

  • Bandwidth, typically 500Mbps, is currently 5x higher than typical LAN-based networks because it is less limited by the speed of local infrastructure.

  • Cost is the same whatever the distance between sender and receiver. Satellite costs have dropped dramatically in the last few years.

  • Satellite acts as a repeater of incoming signals, amplifying and re-broadcasting these signals.

  • If two stations broadcast simultaneously the satellite will receive and re-broadcast the sum of these two signals, resulting in garbage. Such simultaneous broadcasts are termed collisions.

  • A sender can listen to the re-broadcast of their own packets and determine whether a collision has occurred. Notice that there are no acknowledgements.

  • Users are uncoordinated and can only communicate via the channel.

CITS3002 Computer Networks, Lecture 4, Local Area Networks (LANs), p1, 18th March 2020.