Out-of-band

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Out-of-band is a technical term with different meanings in data communication, telephony, computer science and operating systems.

1. In communications, out-of-band communication is the exchange of call control information on a dedicated channel, separate from that used by the telephone call or data transmission. See out-of-band signaling. (Also related is the use of the different system files stdout and stderr in the Unix pipes and filters-architecture.)
2. In computer networking, out-of-band data (called "urgent data" in TCP) looks to the application like a separate stream of data from the main data stream. This can be useful for separating two different kinds of data. Note that just because it is called "urgent data" does not mean that it will be delivered any faster, or with higher priority than data in the in-band data stream. Also beware that unlike the main data stream, the out-of-band data may be lost if the application cannot keep up with it.
3. The term is sometimes used to describe what people in the communications industry call "shift characters", such as the ESC that leads control sequences for many terminals, or the level shift indicators in the old 5-bit Baudot codes.
4. In personal communications, methods other than electronic mail (such as telephone or snail-mail) are called "out-of-band".

The term "out-of-band" is sometimes confused with the term "out-of-bounds", which in computer programming refers to values returned by a function that are not in its "natural" range of return values, but rather signal some kind of exception. Many C programming language functions that normally return a non-negative integer will return -1 to indicate failure. (See also end-of-file.) It is actually a clear example of in-band signalling since it uses the same "channel" for control and data.

In-band signalling is the opposite of out-of-band signaling.

See also

• Out-of-band agreement