Specifications

Page 2-2 RM2151 - RADIO TELEX COMMUNICATION
The characteristics of ARQ communication can be summarized:
It practically guarantees error-free data, assuming reception is possible at all.
Automatic exchange of identities guarantees connection with the requested station, also after rephasing conditions.
Communication is possible between only two stations at a time.
This gives some limited protection against unauthorised eavesdropping.
The receiving station must have an operating transmitter.
2.4 FEC operation
The second method is designated FEC (Forward Error Correction). This method of error correction is used if there
is more than one receiving station, and no replies are requested by the transmitting station. It consists of sending
the message in time diversity, that is, each message is sent twice. The receiving station thus has two chances to
receive each character correctly. If both characters are in error, a special character ( _ ) is printed.
The primary characteristics of FEC are:
A message may be transmitted to several receiving stations simultaneously.
It does not require transmitters at the receiving stations.
There is no acknowledgement of transmissions.
There is no active error correction.
There is no protection against eavesdropping.
A sub-class of FEC, designated SEL/FEC (Selective Forward Error Correction) allows selective calling of one or
more stations by means of call codes. The message is transmitted in inverted format with the Mark/Space signals
interchanged. Only the receiving stations with the correct call codes will receive the data correctly.