SATELLINE-3AS NMS / Epic NMS / VHF
User Guide, Version 3.0
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6.3
Error correction
SATELLINE-3AS(d) NMS has the error correction feature called the FEC method (FForward EError
Correction). When FEC is enabled (ON), the radio modem automatically adds additional error
correction information, which increases the amount of transmitted data by 30 %. It is used by the
receiving radio modem to correct erroneous bits - as long as the ratio of correct and erroneous
bits is reasonable. FEC improves the reliability of data transfer via the radio interface especially
in unfavourable conditions. FEC function should be used when link distances are long and/or if
there are lot of interferences in the radio channels used. Using the FEC function will decreases
the data transfer throughput by app. 30 %. See Appendix B for the exact transfer delays
introduced by using FEC function.
6.4
Error checking
The purpose of the error checking is to detect possible errors happened in the data transfer. The
radio modem transfers user data over the radio link by fitting it to sub-frames and adds
checksums to the transmitted data. In case the error checking is switched on, the receiving
modem calculates the checksums and verifies the received data before it forwards the data to
the serial port. SATELLINE-3AS NMS supports three different methods of error checking in order
to verify that the received data is proper:
No Error Check
In this mode the received data is NOT verified at all. This is the fastest mode because received
data is given out to the terminal device immediately after it has been received. This is the default
mode and it is recommended in case the user protocol includes the error-checking feature of its
own.
Partial Error Check
This mode means that the radio modem checks the received data in small data blocks and
sends the validated sub-messages to DTE. Once an error is detected, the rest of the message is
ignored. That is, if an error takes place in the middle of a message then the first part of the
message is forwarded to the terminal device, but the data inside the erroneous sub-frame and
the rest of the data will be ignored.
Full Error Check
In this mode the radio modem waits until the end of the reception and checks the whole
received message before forwarding it to DTE. The additional time delay compared to No Error
Check is roughly the time needed to transfer the user data message over the serial line.
NOTE!
All radio modems that 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 correctly.
