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APPENDIX A: Loopback Tests
Appendix A: Loopback Tests
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 remote digital loopback test allows you to verify that the local computer or
terminal, the serial ports, the local and remote modems, and the transmission line
between them are functioning properly.
The local digital loopback test allows you to verify that the remote computer or
terminal, the serial ports, the local and remote modems, and the transmission line
between them are functioning properly.
A.1 Local Analog Loopback Test (V.54 Loop 3)
In this test, data from your computer or terminal is sent to your modem’s
transmitter, converted into analog form, looped back to the modem’s receiver,
converted into digital form, and then sent to your monitor for verification. No
connection to the phone line is required.
Figure A-1. Local analog loopback test.
Computer or Termnal
Local Mini-Modem V.92
Digital
Analog
