Telephone
For example, when dialing a modern telephone, the telephone number is encoded and transmitted across the telephone line as Dual-Tone Multi-Frequency (DTMF) tones. The tones "control" the telephone system by instructing the telephone company's equipment where to route the call to. These control tones are sent over the same channel and in the frequency range (300Hz to 3.4kHz) as the voice and other sounds of the telephone call. In-band signaling is also used on older telephone carrier systems to provide inter-exchange information on how to route calls. Examples of this kind of in-band signaling system are SS5 and R2.
Separating the control signals, also referred to as the control plane, from the data (if a bit-transparent connection is desired) is usually done by escaping the control instructions. Occasionally, however, networks are designed so that data is (to a varying degree) garbled by the signaling. Allowing data to become garbled is usually acceptable when transmitting sounds between humans, since the users rarely notice the slight degradation, but this leads to problems when sending data that has very low error tolerance, such as information transmitted using a modem.
In-band signaling is insecure because it exposes control signals, protocols and management systems to the user(s), which may result in falsing. In the case of the blue boxes that were popular in the 1960s and 1970s, such falsing was deliberate. By using blue boxes to generate the appropriate tones, a caller could abuse functions intended for testing and administrative use to make free long-distance calls.
Modems may also interfere with in-band signaling, so in some countries, a guard tone is employed to prevent this.
Read more about this topic: In-band Signaling
Famous quotes containing the word telephone:
“The press and politicians. A delicate relationship. Too close, and danger ensues. Too far apart and democracy itself cannot function without the essential exchange of information. Creative leaks, a discreet lunch, interchange in the Lobby, the art of the unattributable telephone call, late at night.”
—Howard Brenton (b. 1942)
“It’s a hard feeling when everyone’s in a hurry to talk to somebody else, but not to talk to you. Sometimes you get a feeling of need to talk to somebody. Somebody who wants to listen to you other than “Why didn’t you get me the right number?””
—Heather Lamb, U.S. telephone operator. As quoted in Working, book 2, by Studs Terkel (1973)
“But even in a telephone booth
evil can seep out of the receiver
and we must cover it with a mattress,
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and bury it,
bury it.”
—Anne Sexton (1928–1974)