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The term out-of-band has different uses in communications and telecommunication. In case of out-of-band control signaling, signaling bits are sent in special order in a dedicated signaling frame. D-channel in ISDN is an example of out-of-band and CCS signaling methods.
The word band originates from the term band as used in radio and other electronic communications.
General usage 
In general language, out-of-band refers to communications which occur outside of a previously established communication method or channel.
Here are two very simple, but illustrative, 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 or sending an electronic text message – 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 as a whole.
- 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, 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 method.
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. Just because it is called "urgent data" does not mean that it will be delivered any faster or with higher priority than in-band data. Also, unlike the main data stream, the out-of-band data may be lost if the application cannot keep up with it. "Urgent data" notifies the receiving connection that the separate stream is more important than the main stream. Therefore it must first check the separate stream in order to process the main stream normally.
On Unix-like computers, out-of-band data can be read with the recv() system call. A process or process group can be configured to receive SIGURG signals when out-of-band data is available for reading on a socket, by using the F_SETOWN command of the fcntl() system call. This is a form of asynchronous I/O.
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.
When issuing unscheduled patches, e.g., major patches that aren't released on Patch Tuesday, Microsoft refers to these patches as "out of band." However, these patches are still delivered via the same channels through which scheduled patches are delivered, not via a separate channel (or "band") as their use of the phrase might suggest.
Out-of-band can also be referred to add-on features that are released separately from a product's main release cycle. For example, an internal software application is released by a company's IT department, but some custom reports might then be released as a follow-on outside this release cycle.
In authentication, out-of-band refers to utilizing two separate networks or channels, one of which being different from the primary network or channel, simultaneously used to communicate between two parties or devices for identifying a user. A cellular network is commonly used for out-of-band authentication. An example of out-of-band authentication is when an online banking user is accessing their online bank account with a login and a one time password is sent to their mobile phone via SMS to identify them. The primary channel would be the online login screen where the user enters their login information and the second separate channel would be the cellular network. This added layer of security prevents the likelihood of hackers and malware from compromising access to the complete authentication process, however, this method of authenticating a user is known to be vulnerable to man-in-the-middle (MITM) attacks.
See also 
- The Jargon File "Out-of-band", def. 3