Summary

This Recommendation 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 ISDN (see Appendix I or II/G.961 [1]) services;

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

ADSL transmission on the same pair with voiceband services and operating in an environment with TCM‑ISDN (see Appendix III/G.961 [1]) services in an adjacent pair, is for further study.

ADSL transmission without underlying service, optimized for deployment with ADSL over ISDN service in the same binder cable, is for further study.

ADSL transmission with extended upstream bandwidth, simultaneously on the same pair with voiceband service, is for further study.

This Recommendation specifies the physical layer characteristics of the extended bandwidth Asymmetrical Digital Subscriber Line (ADSL) interface to metallic loops. As compared to the ADSL2 transceiver defined in ITU‑T Rec. G.992.3 [5], the operating modes use double the downstream bandwidth. When operating on the same pair with voice band services, an additional operating mode is defined, also 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 (ADSL2+ transceiver) can simultaneously convey all of the following: a number of downstream frame bearers, a number of upstream frame bearers, a baseband 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 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 ITU‑T Rec. 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.

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.

History

This Recommendation describes extended bandwidth ADSL2 (ADSL2+) transceivers, as a delta to the second generation ADSL (ADSL2 ITU‑T Rec. G.992.3).

This Recommendation has been written to provide additional features, relative to ITU‑T Rec. G.992.3. ITU‑T Rec. G.992.3 was approved on the 29 of July 2002. Several potential improvements have been identified to better address higher data rates for shorter loops and longer reach for high data rates. This Recommendation provides a new ADSL U‑interface specification, including the identified improvements, which the ITU‑T believes will be most helpful to the ADSL industry.

Relative to ITU-T Rec. G.992.3, the following application-related feature has been added:

Improved support for services requiring high downstream data rates (e.g., broadband entertainment services).

Relative to ITU-T Rec. G.992.3, the following PMS‑TC-related feature has been added:

Support of up to 3 Reed‑Solomon codewords per symbol;

Relative to ITU-T Rec. G.992.3, the following PMD‑related features have been added:

Extended downstream bandwidth to 2.208 MHz (512 subcarriers) for all operation modes (POTS/ISDN/All Digital Mode);

Downstream spectrum control with individual maximum transmit PSD at U‑C reference point per subcarrier, under operator control through CO‑MIB, allows configuration per the regional requirements (e.g., North America, Europe or Japan) and deployment scenarios (e.g., CO or Remote).

Downstream spectrum shaping during Showtime (shaped transmit PSD in the passband, i.e., non-flat) for improved downstream transmit PSD flexibility.

Through negotiation during initialization, the capability of equipment to support the G.992.5 and other G.992.x ADSL Recommendations is identified. For reasons of interoperability, equipment may choose to support multiple Recommendations, such that it is able to adapt to the operating mode supported by the far‑end equipment.