Guard tone is a feature of wireline modems.
The guard tone is sent by the answering modem after it has sent the answer tone. It is a single continuous tone with a frequency of either 1800 Hz or 550 Hz, sent at a level of -6dB or -3dB, respectively, below the level of the transmitted data signal power.
It is used with V.22 and V.22bis modulation modes to prevent the high-band data signal from interfering with the operation of billing apparatus in certain countries where in-band signalling was commonplace in the 20th century.
1800 Hz is used in the UK and some Commonwealth countries, while 550 Hz is used in some Scandinavian countries. Guard tones are not used in North America.
The guard tone is controlled by the &G AT command of the Hayes command set.
Famous quotes containing the words guard and/or tone:
“Most of us are aware of and pretend to detest the barefaced instances of that hypocrisy by which men deceive others, but few of us are upon our guard or see that more fatal hypocrisy by which we deceive and over-reach our own hearts.”
—Laurence Sterne (1713–1768)
“If the oarsmen of a fast-moving ship suddenly cease to row, the suspension of the driving force of the oars doesn’t prevent the vessel from continuing to move on its course. And with a speech it is much the same. After he has finished reciting the document, the speaker will still be able to maintain the same tone without a break, borrowing its momentum and impulse from the passage he has just read out.”
—Marcus Tullius Cicero (106–43 B.C)