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:
“Irony, forsooth! Guard yourself, Engineer, from the sort of irony that thrives up here; guard yourself altogether from taking on their mental attitude! Where irony is not a direct and classic device of oratory, not for a moment equivocal to a healthy mind, it makes for depravity, it becomes a drawback to civilization, an unclean traffic with the forces of reaction, vice and materialism.”
—Thomas Mann (1875–1955)
“Whose are the truly labored sentences? From the weak and flimsy periods of the politician and literary man, we are glad to turn even to the description of work, the simple record of the month’s labor in the farmer’s almanac, to restore our tone and spirits.”
—Henry David Thoreau (1817–1862)