|Last modified: 14-11-2012|
No, it does not stand for I Still Don't Need it :-) It actually stands for Integrated Services Digital Network (in French: Réseau Numérique à Intégration de Services.) It offers richer services on the regular local loop that links your premises to the telco's Central Office, except the signal is digitized over here (your home/office) instead of over there (the C.O.). Services that used to be available only on ISDN (but some of them are now available on analog lines as well) are Caller ID, call waiting, etc.
Although an ISDN box (PBX, phone, etc.) is physically connect to the C.O. via a pair of regular copper wires, it offers two logical connections (B channels, each usually running at 64Kbps) and a control channel (D channel, usally running at 16Kbps), allowing you to either have two simultaneous connections with the outside, or combine the two B channels into one (called multi-link, ie. a 128Kbps connection to your ISP.)
Virtually all PBX's are now ISDN-based.
There are two types of ISDN subscriptions proposed by telcos: BRI (Basic Rate Interface: Two 64Kbps B channels and one 16Kbps D channel) and PRI (Primary Rate Interface: Typically twenty-three 64Kbps B channels and one 64Kbps D channel in the US, thirty B channels and one D channel in Europe.)
To connect equipments (routers, "modems", etc.) to an ISDN line, an NT1 adapter (France: TNR/terminaison numérique de réseau)is required, if not already provided by the telco. In the US, the subscriber must provide the NT1, while it's usually provided by the telco in the rest of the world.
The interface that comes out of a BRI line is known as the U interface. The NT1 device turns the single-pair U interface into the two-pair S/T interface (thus allowing more than one device to be connected.) Note that most ISDN devices sold in the US today have a built-in NT1 interface. After the NT1 device comes... the NT2 device (France: TNA/terminaison numérique d'abonné) that turns the T interface into the S interface; Practically speaking the NT2 device is typically a PBX. ISDN devices usually expect either a U connection (in which case, they have a built-in NT-1 device) or an S/T connection.
The T interface only allow one equipment (TE or TA) to be connected. The S interface allows more than one, but is only available for BRI connections. Both interfaces are located at the same point, hence the S/T name.
ISDN devices are called Terminal Equipment 1 (TE1), while non-ISDN devices (ie. analog devices, which provide an R interface) are known as TE2 and require a Terminal Adapter (TA) to turn the analog signal into an ISDN signal.
Please note that some ISDN phone can connect either directly to an ISDN line or to an ISDN PBX, while others are manufactured for only one of those uses (ie. some can only be connected to an ISDN line, while others can only be connected to a PBX.)
Here's a diagram from Ralph Becker's tutorial:
In this case, each additional ISDN line translates into an extra U interface at your premises.