 as follows: The ISDN terminates in the Network Termination 1 (NT1) and the socket in the NT1 is the regulatory boundary between the Public B channel. This is a 64kbit/s channel. It carries user information Telephone Operator's (PTO) network and the liberalised customers' such as voice encoded at 64kbit/s (with 8kHz sampling), or premises environment in Europe. (In the United States the local loop wideband voice, or data information. pair is the regulatory boundary and the NT1 is part of this customers D channel. This is either 16kbit/s or 64khit/s. It is primarily premises equipment). It provides the layer 1 interface between the intended to carry signalling information for circuit switching by local loop transmission system and the ISDN interface layer 1. Into the ISDN. In addition to signalling it may also be used to carry this socket is plugged the four wire bus often known as the S or T packet-switched data. It uses a layered protocol in accordance bus because it is at the S and T reference points in the CCITT with the CCITT Recommendations 1.440, 1.441, 1.450 and reference model. This Bus can operate in 2 modes, point to point or 1.451. point to multipoint. In the point to point mode one Terminal Equip- 44/8 The Integrated Services Digital Network user data which is switched by the network to provide an end to end BASIC ACCESS MULTILINE ACCESS transmission service. B channel paths are established by signalling GENERAL I.420 1.421 messages in the D channel. In a multi-terminal situation all termi- nals have access to the D channel by the use of an access procedure, but each B channel is allocated to a particular terminal during call set-up and is not capable of being shared between terminals. The line coding used is Alternate Mark Inversion (AMI), with the LAYER 1 1.430 I.431 binary ones encoded as high impedance and the binary zeros as full width pulses. Violations of the coding mark the beginning and end of the layer 1 frames. The physical connection across the interface consists of eight wires, 2 balanced