Expert: Mark C J Posen - 9/7/2009

Question
Dear Sir,
I know why we are using FEC.Plz tell me why we are using 3/4 and 5/6 etc.what the means of 3 and 4 or 5 and 6?

Sanat

Answer
Hello,

Many thanks for your question and I apologise for the short delay in answering.

In any digital transmission system there is the risk that noise or interference on the transmission path may corrupt the signal, resulting in transmission errors in the received data.   Error correction techniques are used to identify and correct transmission errors.

The most simple approach is an ARQ system, where a checksum is transmitted with the data packets to allow the identification of errors.   If an errored packet is received, a retransmission request is sent back down the line and the errored packet is retransmitted.   Very high data reliability can be obtained in this way.

However, for satellite links the round trip time is of the order of 1 second, which means that this kind of ARQ error correction is not really feasible, since the delay in getting the corrected packet would make the received data stream difficult to use in real time.

Therefore the approach of Forward Error Correction or FEC has been developed.   FEC transmits in the forward link the original data, a means of identifying when there has been a transmission error *and* additional data that allows the receiver to correct the error *without* having to request the data to be sent again (i.e. the return link is not used for error correction, hence it is called *forward* error correction).

The subject is too complex to go into detail as to how this works (but see here for more detail: http://en.wikipedia.org/wiki/Forward_error_correction), but essentially the schemes all work by adding some redundancy in the data stream so that the original data can be reconstructed.   

A simple representative example would be to transmit each packet 3 times and to compare the received results.  If any differences in any data bit are identified then the majority decision is taken (e.g. if the 3 packets have in one bit position "1", "0" and "1" then the "0" would be taken as an error and the "1" to be the correct data).   In reality is is normally more complex than this, and sophisticated codes are developed to allow maximum data correction for minimum additional data transmitted.

When we describe an FEC scheme we are indicating the ratio between the number of original data bits and the transmitted data+coding bits.   So if we add 1 coding bit to every 3 data bits (thus transmitting 4 bits on the channel) this is rate 3/4 coding.  Likewise, if we add 1 coding bit to every 5 data bits (thus transmitting 6 bits on the channel) this is rate 5/6 coding.

Clearly, the more coding bits that are transmitted the better is the scheme at correcting errors, *but* this is at the cost of requiring more bits to be transmitted, thus requiring more bandwidth (thus more cost) in the satellite channel.

I hope this helps.   Please remember to rate this answer.

Regards,

Mark C J Posen C.Eng. MIET
RPC Telecommunications Ltd.
http://www.rpctelecom.com