The term out-of-band has different uses in communication

Communication

Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...

s and telecommunication

Telecommunication

Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

. 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 whisper
  Whisper
  -In fiction:* Whisper , fictitious character in the popular Fantasy RPG Fable* Whisper, a character created by Ian Fleming in the James Bond novel and film Live and Let Die* Whisper , a 2007 horror film...
  
  ing or sending an electronic text message -- whether for privacy
  Privacy
  Privacy is the ability of an individual or group to seclude themselves or information about themselves and thereby reveal themselves selectively...
  
   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
  Internet
  The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
  
   or telephone
  Telephone
  The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
  
  , using his employer's computer
  Personal computer
  A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
  
   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
  Computer network
  A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
  
  . In this situation, his personal cell phone communications are considered out-of-band, relative to the main conversation method.

Telecommunications

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

Common Channel Signaling

In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...

).

Computing

In computer network

Computer network

A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....

ing, out-of-band data (called "urgent data" in TCP

Transmission Control Protocol

The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...

) 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. "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.

In computer administration, out-of-band management refers to system console

System console

The system console, root console or simply console is the text entry and display device for system administration messages, particularly those from the BIOS or boot loader, the kernel, from the init system and from the system logger...

 access provided, even in the event of primary network subsystem (hard and/or software) failure. This can be done via a console server

Console server

A console server is a device or service that provides access to the system console of a computing device via networking technologies....

 or with a remote access card (RAC) which has its own processor, memory, battery, network connection, and access to the system bus.

In personal communications, methods other than e-mail (such as telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 or snail-mail) are sometimes called "out-of-band".

See also

• In-band signaling
  In-band signaling
  In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...
  
  
• Out-of-band agreement
  Out-of-band agreement
  In the exchange of information over a communications channel, anout-of-band agreement is an agreement or understanding betweenthe communicating parties that is not included in any message sent...
  
  
• Out-of-band management
  Out-of-band management
  In computing, out-of-band management involves the use of a dedicated management channel for device maintenance...
  
  
• Ident
  Ident
  The Ident Protocol, specified in RFC 1413, is an Internet protocol that helps identify the user of a particular TCP connection. One popular daemon program for providing the ident service is identd.-How ident works:...