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The phrase “out of band” in IT refers to activity that occurs outside of a specific telecommunications frequency band. This phrase is used in different ways to talk about different kinds of multichannel messaging or multichannel signaling.
In telecommunications, out-of-band signaling provides more sophisticated models for sending signals over the public switched telephone network or another network. IT professionals can also use the term "out of band" generally to talk about a message that is not included in a particular channel, for instance, where there may be an implicit understanding between two parties that is not deliberately broadcasted over a particular data transfer line.
There is also something called “out-of-band authentication” that illustrates the security principle of using out-of-band data. Here, out-of-band authentication means that a signal is sent over some other medium to more fully authenticate a message and its sender. One instance of this is how modern banks now use mobile phone authentication methods to back up Internet access to online banking (also called two-factor authentication). This is an excellent and clear example of out-of-band authentication and the use of the term "out of band" — here, the trajectory of data associated with the user signing on to online banking on the Internet is the "band," and the cell phone communication is "out of band." That is why it delivers better security.