Out-of-Band Signaling is any transmission technology in which signaling is separate from the data being transmitted. Out-of-band signaling uses one or more channels for transmitting data or voice information and one special out-of-band channel for performing signaling functions such as establishing and terminating the communication link, controlling flow, or transmitting error information.
The out-of-band channel can be:
- A physically separate set of wires (such as pins 4 and 5 of an RS-232 cable, which perform flow control functions and do not carry data)
- A multiplexed system in which bandwidth is divided into two or more channels within the same set of wires (such as Integrated Services Digital Network, in which the two B channels and one D channel are multiplexed onto the same set of wires). The D channel is a dedicated channel for control signals, and the B channels carry the data.
- The traditional SS7 telephone system uses an entirely separate network for control signals.
The opposite of out-of-band is in-band, in which signaling information is sent over the same channel as the data transmission. Out-of-band transmission is usually considered a better choice than in-band transmission for the following reasons:
- None of the valuable data bandwidth is used for signaling.
- The data stream is not interrupted with signaling information.
- The signaling information cannot be disrupted by the noise created by the data transmission.
- Data transmission characters cannot accidentally (or purposefully) initiate control actions.