Very high speed digital subscriber line transceivers 2 (VDSL2)
|Publication Date:||1 February 2019|
This Recommendation is an enhancement to [ITU-T G.993.1] that supports transmission at a bidirectional net data rate (the sum of upstream and downstream rates) up to 200 Mbit/s on twisted pairs. This Recommendation is an access technology that exploits the existing infrastructure of copper wires that were originally deployed for plain old telephone service (POTS).
This Recommendation specifies only discrete multi-tone (DMT) modulation and incorporates components from [ITU-T G.993.1] (VDSL), [ITU-T G.992.3] (ADSL2), and [ITU-T G.992.5] (ADSL2 plus).
Whilst POTS uses approximately the lowest 4 kHz and asymmetric digital subscriber line (ADSL) uses approximately 2 MHz of the copper wire spectrum, this Recommendation is defined to allow the use of up to 30 MHz of the spectrum. This Recommendation can be deployed from central offices, from fibre-fed cabinets located near the customer premises, or within buildings.
The availability of bandwidth up to 30 MHz allows ITU-T G.993.2 transceivers to provide reliable high data rate operation on short loops. Without the use of the US0 band, this Recommendation should operate reliably over loop lengths that are similar to those of [ITU-T G.993.1], or slightly longer lengths due to the mandatory support of trellis coding. The addition of the US0 band and means to train echo cancellers and time-domain equalizers (TEQs) also allows this Recommendation to provide reliable operation on loops up to approximately 2 500 metres of 26 American wire gauge (AWG) (0.4 mm).
This Recommendation defines a wide range of settings for various parameters (such as bandwidth and transmitter power) that could potentially be supported by a transceiver. Therefore, this Recommendation specifies profiles to allow transceivers to support a subset of the allowed settings and still be compliant with the Recommendation. The specification of multiple profiles allows vendors to limit implementation complexity and develop implementations that target specific service requirements. Some profiles are better suited for asymmetric data rate services, whereas other profiles are better for symmetric data rate services.
The annexes of this Recommendation include band plans and power spectral density (PSD) masks that address region-specific requirements.
Like [ITU-T G.993.1], this Recommendation defines upstream power back-off (UPBO) to mitigate far-end crosstalk (FEXT) caused by upstream transmissions on shorter loops to longer loops. The mechanism is the same as in [ITU-T G.993.1].
As with other ITU-T Recommendations in the ITU-T G.99x series, this Recommendation uses [ITU-T G.994.1] to initiate the transceiver training sequence.
Changes in this Recommendation relative to [ITU-T G.993.1] include:
• The definition of profiles to support a wide range of deployment scenarios (e.g., central offices, fibre-fed cabinets located near the customer premises, and within buildings).
For TPS-TC sublayer specifically, changes in this Recommendation relative to [ITU-T G.993.1] include:
• Support for STM interfaces;
• Support for PTM interfaces based on IEEE 802.3 64/65 octet encapsulation;
• Support for ToD-TC functionality for transport of time-of-day over VDSL2 links.
For PMS-TC sublayer specifically, changes in this Recommendation relative to [ITU-T G.993.1] include:
• Improved framing (based on [ITU-T G.992.3]);
• The definition of two latency paths and two bearer channels;
• Improved on-line reconfiguration (OLR) mechanisms (based on [ITU-T G.992.3]), including optional SRA, special operations channel (SOS), and dynamic interleaver change;
• Control of delay variation;
• Improved overhead channel;
• Improved forward error correction (FEC) capabilities, including a wider range of settings for the Reed-Solomon (RS) encoder and the interleaver;
• Improved interleaving with controlled memory-split between downstream and upstream;
• Support of all integer values of impulse noise protection (INP) up to 16 symbols;
• Optional impulse noise monitor (INM) function.
For PMD sublayer specifically, changes in this Recommendation relative to [ITU-T G.993.1] include:
• The definition, in annexes, of band plans up to 30 MHz to support a bidirectional net data rate up to 200 Mbit/s;
• Support for extension of the upper band edge of the US0 band to as high as 276 kHz (based on Annex M of [ITU-T G.992.3]);
• The definition of means to improve the performance of US0 (specifically, support in initialization for training of time domain equalizers and echo cancellers);
• A requirement to support the US0 band in the upstream direction for some profiles;
• Support for spectrum shaping of the US0 band in the upstream direction;
• Support for a management information base (MIB)-controlled PSD mask mechanism to enable in-band spectrum shaping (based on [ITU-T G.992.5]);
• Alternative electrical length estimation method (AELE-M1);
• Optional equalized FEXT UPBO method;
• A requirement for downstream and upstream transmitters to notch, simultaneously, 16 arbitrary operator-defined radio frequency interference (RFI) bands;
• Support for downstream maximum transmit power (profile dependent) of up to 20.5 dBm;
• Mandatory support of trellis coding (based on [ITU-T G.992.3]);
• The definition of receiver-determined tone ordering (based on [ITU-T G.992.3]);
• Mandatory support of all integer-bit constellations from 1 bit to 15 bits;
• Support for optional cyclic extension (CE) lengths as large as ¼ of a symbol period;
• Optional virtual noise (VN) mechanism;
• The definition of VTU-R receiver-selected pilot tone(s), including the option not to select a pilot tone;
• Insertion of a sync symbol after every 256 data symbols to signal OLR transitions;
• Improvements to initialization, including the definition of a channel discovery phase, a training phase, and a channel analysis and exchange phase;
• Definition of re-initialization policy parameters;
• Definition of link activation methods and procedures;
• Support for a VTU-R lineprobe stage during initialization;
• Support for a wide range of test parameters and accuracy requirements (based on [ITU-T G.992.3]);
• The definition of a loop diagnostic mode;
• Support for an optional all-digital mode;
• Definition of downstream and full vectoring-friendly operating modes.