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1.4 FEC Mode (B-Mode)
Description
The FEC mode is for one-way, uninterrupted transmission of mes-
sages, for example, weather forecasts and emergency bulletins, to
no one particular station or stations. The sending station is known
as the BSS (B-Mode Sending Station), the receiving station the
BRS (B-Mode Receiving Station).
This mode uses a simple forward-error correcting (FEC) technique
of sending each character twice at a 280 ms interval. The first trans-
mission is termed DX (direct transmission), the second RX (re-
peated transmission).
Message: " a b c d e f g h i j "
First transmission: a b c d e f g h i j
Second transmission: a b c d e f g h i j
Output code ex.: a b c a d b e c f d g e h f i g j h i j
280 ms
Figure 1-3 FEC mode transmission technique
The receiving station tests the DX and RX characters for adher-
ence to the 4-mark/3-space constant ratio, and prints only
unmutilated DX or RX characters, or prints a space if both are
mutilated.
Another version of the FEC mode is the FEC-selective mode. This
mode uses a call code for selective calling to one or more stations.
Only those stations with the correct code will receive the data cor-
rectly.
Initiating a Call
When a BSS initiates a broadcast call it transmits synchronizing
signals to align phasing of the BRS. Upon detection of this signal
the BRS’s are switched to the receiving condition and will remain
in this condition until the completion of the message. If the muti-
lated character error rate exceeds a certain percentage, the BRS
reverts to standby condition.
Termination of Communication
The sending station sends three consecutive idle signals α imme-
diately after the last transmitted information signal in the DX posi-
tion.