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

  1. In communications, the exchange of call control information on a dedicated channel, separate from that used by the telephone call or data transmission. (Also related is the use of the different system files stdout and stderr in the Unix pipes and filters-architecture.)
  2. Sometimes used to describe what communications people 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.
  3. In personal communication, using methods other than electronic mail, such as telephone or snail-mail.
  4. In computer programming: values returned by a function that are not in its "natural" range of return values, but rather signal some kind of exception. Many C functions that normally return a non-negative integer return -1 to indicate failure. (See also end-of-file.) This use confuses "out-of-band" with "out-of-range". It is actually a clear example of in-band signalling since it uses the same "channel" for control and data.