ITU-T Recommendations

Recommendation ITU‑T G.992.5 describes asymmetrical digital subscriber line (ADSL) transceivers on a metallic twisted pair that allows high‑speed data transmission between the network operator end (ATU‑C) and the customer end (ATU‑R), using extended bandwidth. This Recommendation defines a variety of frame bearers in conjunction with one of two other services, or without underlying service, dependent on the environment:

1) ADSL transmission simultaneously on the same pair with voiceband service.

2) ADSL transmission simultaneously on the same pair with integrated services digital network (ISDN) (see Appendix I or II or Recommendation ITU‑T G.961) services.

3) ADSL transmission without underlying service, optimized for deployment with ADSL over voiceband service in the same binder cable.

4) ADSL transmission without underlying service, optimized for deployment with ADSL over ISDN service in the same binder cable.

5) ADSL transmission with extended upstream bandwidth, simultaneously on the same pair with voice band service.

ADSL transmission on the same pair with voiceband services and operating in an environment with TCM‑ISDN (see Appendix III of Recommendation ITU‑T G.961) services in an adjacent pair is for further study.

This Recommendation specifies the physical layer characteristics of the extended bandwidth asymmetric digital subscriber line (ADSL) interface to metallic loops. As compared to the ADSL2 transceiver defined in Recommendation ITU‑T G.992.3, the operating modes use double the downstream bandwidth. When operating on the same pair with voiceband services, an additional operating mode is defined, using the double upstream bandwidth.

This Recommendation has been written to help ensure the proper interfacing and interworking of ADSL transmission units at the customer end (ATU‑R) and at the network operator end (ATU‑C) and also to define the transport capability of the units. Proper operation shall be ensured when these two units are manufactured and provided independently. A single twisted pair of telephone wires is used to connect the ATU‑C to the ATU‑R. The ADSL transmission units must deal with a variety of wire pair characteristics and typical impairments (e.g., crosstalk and noise).

An extended bandwidth ADSL transmission unit (ADSL2plus transceiver) can simultaneously convey all of the following: a number of downstream frame bearers; a number of upstream frame bearers; a baseband plain old telephone service (POTS)/ISDN duplex channel; and ADSL line overhead for framing, error control, operations and maintenance. Systems support a net data rate ranging up to a minimum of 16 Mbit/s downstream and 800 kbit/s upstream. Support of net data rates above 16 Mbit/s downstream and support of net data rates above 800 kbit/s upstream are optional.

This Recommendation includes mandatory requirements, recommendations and options; these are designated by the words shall", "should" and "may", respectively. The word "will" is used only to designate events that take place under some defined set of circumstances. This Recommendation is written as a delta Recommendation relative to Recommendation ITU‑T Rec. G.992.3. For the clauses which have been changed, this Recommendation contains complete replacement text (unless explicitly indicated). For the clauses which have not been changed, this Recommendation contains only the clause heading, with reference to Recommendation ITU‑T G.992.3.

This Recommendation defines several optional capabilities and features:

– transport of STM and/or ATM and/or Packets;

– transport of a network timing reference;

– multiple latency paths;

– multiple frame bearers;

– short initialization procedure;

– dynamic rate repartitioning;

– seamless rate adaptation;

‑ extended impulse noise protection;

‑ erasure decoding;

‑ virtual noise;

‑ impulse noise monitor.

It is the intention of this Recommendation to provide, by negotiation during initialization, for U‑interface compatibility and interoperability between transceivers complying with this Recommendation, and between transceivers that include different combinations of options.

Annex C to this Recommendation has been published independently due to its size and its specific structure.