ITU-T - International Telecommunication Union/ITU Telcommunication Sector
适用范围
This Recommendation defines signals@ messages and procedures for exchanging these between digital subscriber line (DSL) equipment@ when the modes of operation of the equipment need to be automatically established and selected@ but before signals are exchanged which are specific to a particular DSL Recommendation. For interrelationships of this Recommendation with other ITU-T G.99x-series Recommendations@ see Recommendation ITU-T G.995.1 (informative). The principal characteristics of this Recommendation are as follows: a) use over metallic local loops; b) provisions to exchange capabilities information between DSL equipment to identify common modes of operation; c) provisions for DSL equipment at either end of the loop to select a common mode of operation or to request the other end to select the mode; d) provisions for exchanging non-standard information between DSL equipment; e) provisions to exchange and request service and application-related information; f) support for both duplex and half-duplex transmission modes; g) support for multi-pair operation; h) provisions for DSL equipment at the remote end of the loop (xTU-R) to propose a common mode of operation (new in version 2 of the Recommendation through the use of new message type MP and its associated transactions); i) support for a re-transmission mechanism (new in version 3 of the Recommendation through the use of new message type REQ-RTX). j) support for exchanging the relative transmit and receive level of individual carriers in order to estimate loop characteristics.