Answer tone

The answer tone is the first signal sent by the answering modem after the billing delay. In its most basic form, it is a single continuous tone with a frequency of 2100 Hz (or 2225 Hz for Bell modes). It is the tone heard by the caller after dialing the number.

Answer tone is a feature of wireline modems.

The plain 2100 Hz tone is meant to disable echo suppressors on international trunk connections. It may include 180° phase reversals at intervals of 450 ms to disable network echo cancellers. It may also be amplitude modulated by a 15 Hz signal to indicate ITU-T V.8 capability.

References

Mueller, Scott (September 1996). "Communications and Networking". Upgrading and Repairing PCs (6th ed.). QUE. pp. 482–483. ISBN 0-7897-0825-6.
Bigelow, Stephen J. (October 13, 2000). "Modem Troubleshooting". PC Technician's Troubleshooting Pocket Reference (2nd ed.). McGraw-Hill Professional. pp. 287 – 289. ISBN 0-07-212945-X.

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