out-of-band

out-of-band adj.  [from telecommunications and network
   theory] 1. In software, describes values of a function which are
   not in its `natural' range of return values, but are rather
   signals that some kind of exception has occurred.  Many C
   functions, for example, return a nonnegative integral value, but
   indicate failure with an out-of-band return value of -1.
   Compare {hidden flag}, {green bytes}, {fence}.  2. Also
   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 email, such as telephones or {snail-mail}.