Interface standard

In telecommunications, an interface standard is a standard that describes one or more functional characteristics or physical characteristics necessary to allow the exchange of information between two or more systems or pieces of equipment. Communications protocols are an example.
An interface standard may include operational characteristics and acceptable levels of performance.
In the military community, interface standards permit command and control functions to be performed using communication and computer systems.

Telephones

There are many interface standards between analog telephone central office equipment and customer-premises equipment. Single voice paths generally include analog audio connections, either a two-wire circuit or four-wire circuit plus signaling paths to indicate call progress and status information, such as ringing, answer supervision, etc. Some of the known interface types are: