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.
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.