CRC4 checking is a means of determining if an error has occurred in received data.
Certain ISDN signalling protocols utilise Cyclic Redundancy Checking (CRC4) by default. Some exchanges may not support CRC4 checking. In such cases it will be necessary to either get the exchange to alter their configuration (turn their CRC4 checking option on) - or configure the Aculab firmware so the CRC4 checking is off (this mainly applies to the Euro ISDN protocol).
For more information about turning CRC4 off when running Euro ISDN, see the latest ETS300 release notes regarding the -s50,255 switch.
When running SYSDIAG, if Layer 1 'USR' field keeps changing from 'f1' to 'f9' this is a good indication that:
1) The far end is configured for CRC4 and you are not.
2) You are configured for CRC4 and the exchange is not.