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:
“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)
“The telephone gives us the happiness of being together yet safely apart.”
—Mason Cooley (b. 1927)
“In the United States all business not transacted over the telephone is accomplished in conjunction with alcohol or food, often under conditions of advanced intoxication. This is a fact of the utmost importance for the visitor of limited funds ... for it means that the most expensive restaurants are, with rare exceptions, the worst.”
—John Kenneth Galbraith (b. 1908)