This standard describes two transmission methods for providing high-bit-rate digital subscriber line service over a single twisted pair (HDSL2) or two twisted pairs (HDSL4). The specifications of the standard provide for full-duplex transmission with a payload of 1.544 Mbps over the full Carrier Serving Area (CSA). HDSL2 transceivers that are compliant with the specifications are interoperable with another HDSL2 compliant transceiver independent of manufacturer. Two pair compliant transceivers (HDSL4) are not interoperable with single pair transceivers (HDSL2). The HDSL2 specification is not intended to provide for interoperability with prior generation HDSL (T1.TR.28) transceivers. Purpose The purpose of this standard is to assist manufacturers@ providers@ and users of products connected to the telephone network with an understanding of the technology associated with HDSL2 transceivers. In order to meet this purpose@ requirements and guidelines are provided. When the requirements contained in the specifications of the standard are met@ an HDSL2 transceiver is standard compliant. Guidelines@ as informative appendices@ provide technical details for assisting manufacturers and users but are not requirements for compliance. In order for HDSL2 transceivers and HDSL4 transceivers to be interoperable@ within their respective loop operation@ a frame structure@ a transmitter physical layer@ a start-up procedure@ electrical characteristics@ and other considerations are specified. The specifications ensuring transceiver interoperability and performance begin with a list of normative references followed by a list of definitions and acronyms. The next sections of the standard contain a reference model@ framing structure details@ physical layer details@ startup procedures@ maintenance requirements@ testing procedures@ and other requirements. The appendices@ containing data and additional information@ are provided as a supplement to the specifications. The transmit levels for HDSL2 and HDSL4 with the normative power backoff comply with the Spectrum Management Standard@ T1.417-2001.