is a technical term with different uses in communications
. It refers to communications which occur outside of a previously established communications method or channel.
The use of the word "band" in "out-of-band" originates from the term as used in radio and other electronic communications.
In general language, "out-of-band" refers to communications which occur outside of a previously established communication method or channel.
Here are some very basic examples:
- A person is involved in a discussion with a group of people. During the discussion, he decides that some information is best kept between himself and one other member of the group, exclusively, such as by whispering -- whether for privacy reasons, or merely the need to avoid saturating the entire group with information they do not need. His communication with that individual may be considered out-of-band, relative to the group discussion.
- A person is at his place of work, and he is communicating with someone outside of his work place (for example, by Internet or telephone, using his employer's computer or telephone equipment). He decides that key parts of his conversation are best kept private from his employer, such as if he were discussing plans to change jobs. He would use his personal cell phone to transmit sensitive messages to, and receive related messages from, his external contact, in order to avoid use of his employer's phone or computer network. In this situation, his personal cell phone communications are considered out-of-band, relative to the main conversation.
Out-of-band communication is the exchange of call control information in a separate band from the data or voice stream, or on an entirely separate, dedicated channel (as in Common Channel Signaling
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.
In computer administration, out-of-band management refers to system console access provided, even in the event of primary network subsystem (hard and/or software) failure. This can be done via a console server or with a remote access card (RAC) which has its own processor, memory, battery, network connection, and access to the system bus.
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 signaling is the opposite of out-of-band signaling and out-of-band signaling is one method of achieving network transparency.)
In personal communications, methods other than e-mail (such as telephone or snail-mail) are called "out-of-band.