This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: Dec 2014

The frame efficiency is a measure of the function of the frame time used for transmission of traffic.

Frame efficiency may be defined as:

$Frame efficiency= Burst traffic bits+ total bits – overhead bits$

In the above equation, bits per frame are implied.

The overhead bits consist of sum of

• The preamble
• The postamble
• The guard intervals
• The reference burst bits per frame
• The DSI channel bits assignment per traffic burst.

Let the number of overhead bits per frame can be calculated as follows:

Referring to equation above the following conclusion may be drawn:

• A longer frame or a greater number of total bits, a result is higher efficiency if the overhead is kept fixed. However longer frames require larger buffer memories and also add to the propagation delay. Synchronization also may be made more difficult keeping in mind that the satellite position is varying with time.
• A lower overhead leads to higher efficiency, but again reduction in synchronisation and guard time may result in more complex equipment being required.

Therefore tradeoffs between TDMA efficiency and system implementation must be carefully considered in any TDMA design.

The frame period is usually chosen to be 125 msec (corresponding to 8 KHz sampling frequency for voice channels), which is the standard sampling period used in pulse code modulation (PCM) telephony system. This ensures that the PCM samples can be distributed across successive frames at the PCM sampling rate.