FEC

Command

FEC -- Control Forward Error Correction.

Synopsis

FEC <port> [0-255]

Description

The FEC command allows you to display and set the Reed Solomon Forward Error Correction properties for a port. At present, the only property which can be controlled is FEC on/off.

Forward error correction is useful on links with high BER (bit-error-rates). Up to 8 erroneous bytes per frame may be automatically corrected, at the expense slightly larger frames. FEC is not required on links with very low BER.

In order to make use of FEC, the port needs to be using a KISS TNC with the CRC check disabled, or an SCC or YAM card.

Options

The first argument is a port number, and is required. If not supplied, a syntax reminder is displayed.

The second argument is optional. If supplied, the FEC flags for the specified port are set to the numeric value of the argument. If not supplied, the current value is reported.

A zero value turns FEC off, and a non-zero value turns it on.

Examples

FEC 1   - Display current FEC setting for port 1.
FEC 1 0 - Turn off FEC on port 1

Notes

A suitable KISS ROM image for use with FEC is KISSFEC.BIN, which should be provided in the XRPi release package. Note that this ROM is for use only with XRPi / XRouter, and only for FEC.

The FEC flag may be changed in future to allow it to be enabled on TX, RX, both or neither. This would allow one way FEC, e.g. for use in the case where a link is bad in one direction but good in another. Other flags may be used to control parameters such as interleaving.

Files

FEC is normally disabled by default, and is enabled by including the FEC=1 directive in the appropriate PORT configuration block in XROUTER.CFG. It can then be turned on and off at will from the command line.

If FEC is used, the "allow L2 fragmentation" bit must be set in the port CFLAGS, to allow XRPi to fragment large L3 frames which might otherwise exceed the reduced L2 Paclen.

Caveats

Frames containing FEC look like garbage to "normal" AX25systems. Thus in order to use FEC on a link, BOTH ends ofthe link must be FEC-capable.

Stations who are not running FEC may interpret FEC-encoded frames incorrectly, leading either to a failure to decode or possibly to corrupt node broadcasts or inaccurate APRS positioning. Therefore FEC should not be used on a shared channel

Availability

Sysops-only.

See also

CFLAGS(7) -- Connection Control Flags.
XROUTER.CFG -- Main Configuration File.