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:
“Be on guard so that your hearts are not weighed down with dissipation and drunkenness and the worries of this life, and that day catch you unexpectedly, like a trap.”
—Bible: New Testament, Luke 21:34.35.
“We often contradict an opinion when it is actually only the tone with which it was put forward that is uncongenial to us.”
—Friedrich Nietzsche (1844–1900)