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ATIS 0600601 1999 Edition, June 1, 1999
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Integrated Services Digital Network (ISDN) – Basic Access Interface for Use on Metallic Loops for Application on the Network Side of the NT (Layer 1 Specification)
Includes all amendments and changes through Stabilization Notice (No longer revised / updated) , 1999
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The requirements of this standard apply to a single digital subscriber line (DSL) consisting of a line termination (LT), a two-wire metallic cable pair, and a network termination (NT) (see clause 3). The transmission system is designed to operate on two-wire twisted metallic cable pairs with mixed gauges.

This standard is based on the use of cables without loading coils, but bridged taps are acceptable with the exception of unusual situations. See 5.4 for a definition of cable plant over which the system should operate.

Specifically, the scope of this standard is as follows:

− it describes the transmission technique used to support full-duplex service on a single twisted-wire pair;

− it specifies both the input signal with which the NT must operate and the output signal that the NT must produce across the U reference point (see figure 2);

− it defines the line code to be used, and the spectral composition of the transmitted signal at the U reference point;

− it describes the electrical and mechanical specifications of the network interface at the U reference point;

− it describes the organization of transmitted data into frames and superframes;

− it defines the functions of the operations channel;

− it describes the maintenance modes of the NT.

Although this standard does not include, except for some aspects of the frame structure, any direct requirements concerning the network side of the interface, such requirements are implied. It shall be understood that the network side conforms to the standard if it interfaces appropriately with any conforming NT or equivalent. Appropriate interfacing shall be understood to mean that the aspects of the network service related to physical characteristics associated with the interface can be provided.


This interface standard was written to provide the minimal set of requirements to provide for satisfactory transmission between the network and the NT, while conforming, wherever possible, with the I-Series of the International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) Recommendations, and while not compromising the principles of evolution expressed therein. Equipment may be implemented with additional functions and procedures.

This standard presents the electrical characteristics of the integrated services digital network (ISDN) basic access signal appearing at the network side of the NT. It also describes the physical interface between the network and the NT. The transport medium of the signal is a single twistedwire pair that supports full-duplex (i.e., simultaneous two-way) service.

While this standard is written explicitly to describe the physical interface of the network and the NT, its use in other circumstances, such as equivalent use of NT1 functionalities behind an NT2 (e.g., a PBX) is not prohibited.