SATELLINE-2ASxE
User Guide, Version 2.0
35
9 Multi Modem Data Systems
9.1 Transmission sequences
The data should be transferred in continuous sequences or in sequences divided into blocks.
Sequences that are too short (e.g. 1 character) should not be sent because the "overhead-
information" (synchronization plus terminal address) takes a great deal of processing time in relation
to the actual data and thus slows down the data transmission. The recommended length of a data
packet is 50 - 500 characters.
The size of a system using one radio channel can be increased when the timeframe for transmission
of one terminal is short. This must be taken into account when planning a system. The operating
range of a system can be extended by increasing both the number of base stations and the number
of radio channels.
It is important that only one transmitter can be active on the same channel at the same
time.
9.2 Polling
The system can be configured as a "Master-Slave" network which allows one radio modem to
control the others in the system. Slave units can communicate with the Master during the time
allocated to a particular Slave by the Master unit.
The advantage of the polling mode is that collisions (i.e. simultaneous transmissions) do not occur.
The disadvantage of this method is that the transmitter of the Master is switched on half of the time
in situations when there is no data transmission from a Slave to the Master.
A polling protocol is not included in the transparent radio modem. It is a protocol of higher hierarchy
and supplied by the system.
9.3 Multi Master
The system can also be configured as a "Multi-Master" network. In this situation any radio data
modem can start transmission after first testing either the CTS line or the CD line to make sure that
the radio channel is free.
The advantage of this system is that the transmitter is ON only during data transmission. The
disadvantage is that if all the mobile stations do not "hear" each other collisions may occur.
A multi master protocol is not included in the transparent radio modem. It is a protocol of higher
hierarchy and supplied by the system.