ADSL2/2+

ADSL2 and ADSL2+: Extending Broadband on the Local Loop

ADSL (Asymmetric Digital Subscriber Line), standardized in ITU-T G.992.1, carried data over a copper subscriber pair while leaving a voiceband service available. Its asymmetric design matched applications dominated by downstream traffic, but its rate depended on loop loss, crosstalk, and the available spectrum.

ITU-T G.992.3 defines ADSL2, G.992.4 defines splitterless ADSL2, and G.992.5 defines ADSL2+ as an extended-bandwidth delta to G.992.3. All three use DMT and negotiate the supported mode during initialization. Their actual rate is selected for the line rather than guaranteed by the product label. The recommendations improve the use of the existing copper local loop without changing the local-loop medium.

ADSL2 (G.992.3) and Splitterless ADSL2 (G.992.4)

G.992.3 keeps the ADSL2 POTS spectrum close to the first-generation profile. In Annex A, the downstream passband reaches 1.104 MHz. The recommendation requires support for net rates of at least 8 Mbit/s downstream and 800 kbit/s upstream; higher rates are optional. Annex C adds requirements for operating in the same cable as ISDN described in G.961 Appendix III. G.992.4 is not Annex L. It is the splitterless ADSL2 delta specification.

Key changes in ADSL2:

• Dynamic rate and framing: G.992.3 supports up to four frame bearers and four latency paths, with optional dynamic rate repartitioning and seamless rate adaptation. The modem selects parameters during initialization and can change the rate within configured limits.
• Diagnostics: New procedures cover successful and unsuccessful initialization, loop characterization, and troubleshooting. They expose more information about why a line trains at a given rate or fails to train.
• Power management: The L2 link state reduces transmit activity when it is enabled by management. Power cutback and the lower activity level can also reduce the disturbance presented to other lines in the cable.
• Splitterless and reach profiles: G.992.4 permits simultaneous POTS and V-series data without a customer-premises splitter and adds training and power-management features. Reach Extended ADSL2 is Annex L of G.992.3, with its own spectral masks. It should not be attributed to G.992.4.

ADSL2+ (G.992.5): Extended Downstream Spectrum

G.992.5 is a delta to G.992.3. It adds 512 downstream subcarriers up to 2.208 MHz, compared with 256 subcarriers and 1.104 MHz in common ADSL2 POTS profiles. The extra tones carry more data on short, clean loops, but they do not remove attenuation or crosstalk limits.

What changes in ADSL2+:

• Downstream spectrum: The 2.208 MHz limit applies to the POTS, ISDN, and all-digital operation modes defined by G.992.5. The 4.3125 kHz DMT spacing therefore provides 512 downstream subcarriers.
• Rates: G.992.5 requires net support of at least 16 Mbit/s downstream and 800 kbit/s upstream; higher values are optional. Implementations commonly advertise up to 24 Mbit/s downstream on short lines. In POTS deployments, upstream is commonly about 1 to 1.4 Mbit/s, while Annex M assigns more spectrum to upstream traffic.
• Interoperability: During initialization, the transceivers identify support for G.992.5 and other G.992.x modes. Multi-mode equipment can select the mode supported by the far end. Both endpoints must support G.992.5 to use the ADSL2+ mode, and otherwise the link can train in a supported lower mode. The DSLAM at the network end and the subscriber's modem must support ADSL2+ for that mode.

Shared Benefits Across ADSL2/ADSL2+ Variants

Both recommendations retain the same DMT-based line model and add mechanisms for adapting transmission to the copper pair:

• FEC and interleaving: Reed-Solomon forward error correction and interleaving protect the data path against a part of the errors caused by impulse noise. Interleaving increases delay, while a fast path provides lower delay with less protection.
• Per-tone adaptation: DMT measures the signal-to-noise ratio for individual subcarriers and assigns each one an appropriate bit count and gain. Noisy tones can be given fewer bits or removed from the usable set.
• Spectrum-aware fallback: High-frequency ADSL2+ tones suffer more loss on long loops. Initialization can therefore select ADSL2 or a reduced bit loading when the line cannot support the extended band. ADSL2+ should not be presented as a general reach extension.