ANSI T1.413 is a technical standard that defines the requirements for the single asymmetric digital subscriber line (ADSL) for the interface between the telecommunications network and the customer installation in terms of their interaction and electrical characteristics.
They were published as ANSI T1.413-1995 and ANSI T1.413-1998 (sometimes called "issue 2") titled Network and Customer Installation Interfaces — Asymmetric Digital Subscriber Line (ADSL) Metallic Interface.
It defines the minimum requirements for satisfactory performance of ADSL systems using the Discrete Multi-Tone (DMT) line code.
DMT divides the useful bandwidth of the standard two wire copper medium used in the public switched telephone network (PSTN), which is 0 to 1104 kHz, into 256 separate 4.3125 kHz wide frequency bands called sub-carriers.
In order to combine voice telephone service with downstream and upstream ADSL signals the bandwidth is split into discrete parts using frequency-division multiplexing (FDM).