G.fast

G.fast is a digital subscriber line (DSL) protocol standard for local loops shorter than 500 m, with performance targets between 150 Mbit/s and 1 Gbit/s, depending on loop length. High speeds are only achieved over very short loops. Although G.fast was initially designed for loops shorter than 250 meters, Sckipio in early 2015 demonstrated G.fast delivering speeds over 100 megabits nearly 500 meters and the EU announced a research project that is Gigabits Over the Legacy Drop (GOLD) project.

Formal specifications have been drafted as ITU-T G.9700 and G.9701, with approval of G.9700 granted in April 2014 and approval of G.9701 granted on December 5, 2014. Development was coordinated with the Broadband Forum's FTTdp (fiber to the distribution point) project. The name G.fast is an acronym for fast access to subscriber terminals; the letter G stands for the ITU-T G series of recommendations. Limited demonstration hardware was demonstrated in mid-2013. The first chipsets were introduced in October 2014, with commercial hardware introduced in 2015, and first deployments started in 2016.

In G.fast, data is modulated using discrete multi-tone (DMT) modulation, as in VDSL2 and most ADSL variants. G.fast modulates up to 12 bit per DMT frequency carrier, reduced from 15 in VDSL2 for complexity reasons.

The first version of G.fast will specify 106 MHz profiles, with 212 MHz profiles planned for future amendments, compared to 8.5, 17.664, or 30 MHz profiles in VDSL2. This spectrum overlaps the FM broadcast band between 87.5 and 108 MHz, as well as various military and government radio services. To limit interference to those radio services, the ITU-T G.9700 recommendation, also called G.fast-psd, specifies a set of tools to shape the power spectral density of the transmit signal; G.9701, codenamed G.fast-phy, is the G.fast physical layer specification. To enable co-existence with ADSL2 and the various VDSL2 profiles, the start frequency can be set to 2.2, 8.5, 17.664, or 30 MHz, respectively.

G.fast uses time-division duplexing (TDD), as opposed to ADSL2 and VDSL2, which use frequency-division duplexing. Support for symmetry ratios between 90/10 and 50/50 is mandatory, 50/50 to 10/90 is optional. The discontinuous nature of TDD can be exploited to support low-power states, in which the transmitter and receiver remain disabled for longer intervals than would be required for alternating upstream and downstream operation. This optional discontinuous operation allows a trade-off between throughput and power consumption.

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