Conventional Channel Allocation

We can employ two common schemes to share the medium:

Polling

Either the satellite or a ground station offers the channel to an individual user for a specified amount of time.

Delays of 270msec make this impractical.

Who should be polled? Should priorities be given to the 100s or 1000s of potential customers?

Frequency and Time Division Multiplexing

There are two significant forms - frequency division multiplexing allocation (FDMA) and time division multiplexing allocation (TDMA).

Using FDMA the channel is divided into N frequency bands (slots) for a maximum of N users. Guard bands are placed between these to limit interference.

Using TDMA the channel is divided into slots based on time intervals, typically 125usec. Each potential user may then use the whole channel for their time quantum (in a manner similar to operating system timesharing).

Both FDMA and TDMA are very inefficient since the actual number of users, M, is generally <<N or >>N, and traffic is often 'bursty'.