3012—Overview and specifications
NGT Transceiver System Technical Service Manual 293
Types of data transmission
There are three types of data transmission:
• selective call
• group call
• broadcast call
Selective calls guarantee error-free transmission, while group and broadcast calls may be
subject to errors under poor channel conditions.
Selective call
A selective call allows transmission of data to a single, specific station.
Once a link has been established using a selective call, data may be sent in both
directions between the two stations.
The data modem has been optimised for data transfer, however interactive chat mode is
possible between the two stations.
Group call
A group call allows transmission of data to as many as 99 stations at one time. Therefore,
it is useful for sending a message to a number of stations without having to establish a
separate link with each station.
Broadcast call
A broadcast call allows transmission of data to all stations that are ready and able to
communicate on the channel selected.
Transmission protocol
Data link establishment
Before data can be transmitted to a receiving station, the data modem must establish a
link. This is done using a robust signalling protocol that determines the address of the
receiving station and the frequency offset between the sending and receiving stations.
Once the link has been established, control is transferred to a high-speed data
transmission protocol to send the data.
The link establishment protocol is less sensitive to channel conditions than the high-
speed signalling protocol. This means that under some channel conditions, link
establishment may succeed, but data transmission may fail.
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