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
− 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
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.
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