SATELLINE-EASy / SATELLINE-EASy 869
User Guide, Version 3.0
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wait until the reception has stopped before transmitting the buffered data. This will result in
timing slacks to the system, but decreases the number of collisions on the air; this is particularly
useful in systems based on multiple random accesses.
5.4
Forward Error Correction (FEC)
FEC improves the reliability of data transfer over the radio by adding additional correction
information to the radio messages. Based on that information, the receiving radio modem will
be able to correct erroneous bits provided the ratio of erroneous and correct bits is reasonable.
However, the use of FEC decreases the data throughput, because the amount of transmitted
data increases about 30 % (see Appendix B). FEC should be used on long distance links and/or
if the radio channel is “noisy” in other words suffering from interfering signals.
5.5
Error checking
When the error checking is switched on, the radio modem will add a checksum to the
transmitted data. When the data is received, the checksum is verified before data is forwarded to
the serial port. There are two different options for error checking that can be accessed in the
Additional setup menu in the Programming Mode:
Error check
checks data only partially while data is received.
Full CRC16 check
adds two checksum characters at the end of the user data message. At the
reception end the receiver receives first the whole package and if the checksum matches the
data message is forwarded to the serial port. If Full CRC16 check is selected it must be set ON
for all radio modems in the same network. Otherwise the checksum characters appear at the
end of user message on the serial port.
5.6
TX delay
The radio modem can be configured to delay the beginning of a radio transmission by
1...65000 ms. The function can be used to prevent packet contention in a system, where all
substations would otherwise answer a poll of a base-station simultaneously. During this delay
data sent to the radio modem is buffered. Even when the priority setting is "RX", the radio
modem is prevented to change over to the receiving mode during the period of the TX delay. If
TX delay is not needed, its value should be set to 0 ms.
NOTE!
All radio modems, which are to communicate with each other, must have the same
setting for FEC (ON or OFF). If the transmitting radio modem and the receiving radio modem
has different settings, data will not be received.
