The Integrated Services Digital Network (ISDN) is the oldest attempt of extending the digital infrastructure to the subscriber. ISDN comes in two flavors, the Basic Rate Interface (BRI) delivering 2 x 64 kbps channels + 16 kbps channeling totaling 144 kbps and Primary Rate Interface delivering 30 voice channels + 2 signaling channels equaling an E1 of 2.048 Mbps.
At the subscriber premises a BRI is terminated in a Network Termination box which converts the line signal from the line T interface to a local synchronous interface, the S four-wire interface supporting up to eight devices of which two may be active at the same time. The S bus carries 40 V. Analogue equipment connects to the S bus via a Terminal Adapter (TA).
The local loop (not the S bus) is a normal balanced two-wire pair carrying the T interface line signals, at a voltage at between 25 and 96 VDC.