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APPENDIX B: Loopback Tests
Appendix B. Loopback Tests
B.1 Introduction
Each time you turn on your modem, it performs an automatic self-test to ensure
proper operation. Your modem also has three diagnostic tests: local analog
loopback, remote digital loopback, and local digital loopback. These ITU-T V.54
loopback tests isolate telephone circuit and transmission problems.
In a loopback test, data from your computer loops through the circuits of your
modem and/or a remote modem before it appears on your monitor. When the
loop has been completed, the data on your PC’s monitor should match the
original data.
The local analog loopback test allows you to verify that the modem’s transmitter
and receiver circuits are functioning properly.
The local digital loopback allows you to verify that the local computer or terminal,
the two modems, and the transmission line between them are functioning
properly.
The remote digital loopback test allows you to verify that the remote computer or
terminal, the remote modem, the serial ports, the telephone line, and the local
modem are functioning properly.
NOTE
All loopback tests operate at all speeds except 300 bps.