3012—Overview and specifications
294 NGT Transceiver System Technical Service Manual
Selective transmissions
Selective transmissions involve a two-way communication process between two data
modem stations. Data is transferred in information packets. Every packet contains error-
detection information to let the receiving station know if the data was correctly received.
Notification of any corrupt transmissions is sent back to the sending station and re-
transmitted in the next information packet.
Group and broadcast transmissions
Group and broadcast transmissions involve a one-way communication process used to
send data to multiple data modem stations. During a transmission, a receiving station is
unable to respond to the transmitting station.
The transmitting station sends information packets in the same way as in a selective
transmission, but all packets are sent four times. After the transmission, a receiving
station will have four copies of each packet of information from which to extract correct
data. This is sufficient for most conditions. However, errors may occur under poor
channel conditions, resulting in blocks of missing data.
For this reason, it is recommended that only text messages are sent in group and
broadcast transmissions, as one error would be sufficient to make an encrypted or
compressed message indecipherable.
Indicators
The data modem has a LED indicator and a Throughput indicator on the front of the
unit. The LED indicator is a multicolour LED. The Throughput indicator comprises
10 red LEDs that form a bar graph of the potential data throughput.
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