Most ISDN services are provided through a device known as an  NT1. This stands for Network Termination type 1. The device is provided by your network operator and is part of the ISDN. This is the point at which you access the network The upper picture on the right shows the NT1 supplied by British Telecom.

In North America, the ISDN service provider does not provide the NT1 for you. You are provided with a simple socket to which you may attach a single ISDN device. If you wish to attach more devices, you must purchase an NT1 and connect it to this socket.

In most of the rest of the world, the NT1 is provided by the ISDN service provider and normally has two sockets into which you can plug ISDN devices.

The NT1 is an active device, with quite complex electronics that handle the transmission of the 144kbps data stream to and from the ISDN service provider's switch or exchange.

An NT1 cannot make calls by itself. You need to plug in Terminal Equipment (TE) or a Terminal Adapter (TA) to do anything useful.

Some NT1s (sometimes called NT1+ or Super-NT1) also have analogue ports on them so you can plug in an ordinary analogue phone. The lower picture on the right is the British Telecom Highway Super-NT1. The blue sockets provide ISDN; the white sockets are for connecting analogue devices.