WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V9.1.0 Transmission Modes
241
SI-FEC
The SI-FEC sound is similar to the sound of SITOR-FEC. However, SI-FEC is used very seldom and stations
mostly switch to this mode only under extremely poor propagation conditions.
SI-FEC operates at symbol rates of 96 and 192 Bd on the radio link.
SI-ARQ stations can switch to FEC broadcasting operation in similarity to SITOR-ARQ (Mode A) and SI-
TOR-FEC (Mode B).
The SI-FEC mode uses the ITA-3 alphabet for data protection. Each character is sent twice, but with a
time delay. The repeated character is transmitted with inverted polarity.
For a pair of characters which have been decoded and recognized as error-free one character is displayed.
If both characters are received in error the underscore (_) character is displayed.
SITOR-ARQ
SITOR-ARQ is a simplex system where both stations transmit alternately on the same frequency.
The CCIR recommendation 476-3 defines a cycle of 450 ms and the two stations are designated as fol-
lows:
 ISS - information sending station
 IRS - information receiving station
The WAVECOM software always decodes the information of the ISS station.
SITOR systems almost exclusively operate at a symbol rate of 100 Baud and with a shift of 170 Hz.
After the receiver has been correctly tuned, the SITOR mode may be started by selecting a baud rate.
Fully automatic tuning to the signal center and shift may be achieved by selecting the Auto button.
The SITOR 7 bit alphabet employs a 3:4 mark-space ratio. The ISS transmits blocks of 210 ms duration
each containing 21 bits. In turn the IRS transmits acknowledgement bursts with a duration of 70 ms.
If the ISS does not receive an acknowledgement, a block containing the RQ character is sent. In the case
of blocks being received in error by the IRS, the last data block sent is repeated.
To Next Page
Loading...
Need help?
General
