In-band signaling
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In telecommunications, in-band signaling is the sending of metadata and control information in the same band, on the same channel, as used for data.
For example, when dialing a modern telephone, the telephone number is encoded and transmitted across the telephone line as Dual-Tone Multi-Frequency (DTMF) dial 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 same band (300Hz to 3.4kHz) as the voice and other sounds of the telephone call. In-band signalling 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 signalling 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), as in the case of the blue boxes that were popular in the 1960s and 1970s. 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.
In-band signalling applies only to Channel Associated Signalling (CAS). In Common Channel Signaling (CCS) separate channels are used for control and data, as opposed to the shared channel in CAS, so all control is out-of-band by definition.
In computer programming, magic numbers are used for in-band signaling of file formats.
When out-of-band communication is unavailable, one of two techniques may be used to preserve network transparency.
- Encapsulation: The bundling the control data in the a packet's header and then removing the header (and/or footer) of the packet at the far end, restoring the data to be the same as the original.
- Bit Stuffing: The insertion of noninformation or escape characters to modify, synchronize and justify the data so it never looks like signaling information (and remove the stuffed bits and escape codes at the far end, restoring the data to be the same as the original).
[edit] See also
[edit] External links
- InBandSignal from the Portland Pattern Repository
