ISDN technology offers various interfaces for the subscriber and the network operator. The ISDN interfaces are based on the ISDN reference points. A distinction is made between the S interfaces and the U interfaces. S-interfaces are 4- wire interfaces that are located behind the network termination( NT) and are bus-capable. U-interfaces are 2-wire interfaces. They are located in front of the network termination, i.e. they are directly connected to the DIVO, and extend much further than the S interfaces, which are limited in their range
The best-known subscriber interface is the basic access with the S0 interface for connecting ISDN-capable terminals
The S0 interface provides each terminal device with two user channels, the B channels, each with a 64 kbit/s data rate. Furthermore, the S0 interface includes the control channel, D channel, with 16 kbit/s. The S0 interface is a 4-wire interface. It has a net bit rate of 144 kbit/s; this results from the addition of the data rates of the B channels and the D channel (64 + 64 + 16 kbit/s). Synchronization information is added to the net bit rate, bringing the total transmission rate to 192 kbit/s.
In addition to the basic connection, there is also the primary multiplex connection with the S2M interface for extensive data traffic. The primary multiplex connection is used to connect medium to large digitalprivate branch exchanges. It allows subscriber equipment to be connected to the ISDN local exchange, with a maximum information capacity of 30 x 64 kbit/s (i.e. 30 B channels). In addition, a D channel with a capacity of 64 kbit/s (D2) is available, which is used to transmit all the necessary signaling information between the subscriber area and the local exchange (DIVO). The standard for the S2M interface is described in ITU-T Recommendation I.431. The protocol of the D channel is identical to the signaling protocol for the ISDN basic access in the D0 channel.
The UK0 interface is terminated at the subscriber by the network termination (NT). The four-wire S0 interface is provided after the NT for the connection of ISDN terminals
The UK2 interface is an interface for transmission paths in primary multiplex connections with conventional copper twisted pairs. The same interface with fiber optics has the designation UG2 interface. Inaddition to these interfaces, there is also the UP0 interface, which works with the ping-pong method and is predestined for use in the area of private branch exchanges, because it allows existing in-house telephone cabling to be used for ISDN communication. The UP0 interface is a two-wire interface.