G.fast is a digital subscriber line (DSL) protocol standard for local loops shorter than 500 meters, with performance targets between 100 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 Mbit/s at nearly 500 meters and the EU announced a research project. Formal specifications have been published as ITU-T G.997.2, 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 letter G in G.fast stands for the ITU-T G series of recommendations; fast is a recursive acronym for fast access to subscriber terminals. 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.
Technology G.fast service is provided to users by DPUs (Distribution Point Units) which are installed near the customer often at a distance of up to 100 meters and connected via optical fiber to an internet service provider. DPUs can be installed in several locations such as multi-dwelling unit basements, utility poles, curb boxes, or manholes, and can be powered by customer premises equipment called NTUs or network termination units, in what is called reverse powering or reverse power feeding.
Modulation 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 specifies 106 MHz profiles and the second version specifies 212 MHz profiles, 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.
Duplex 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.
GigaDSL GigaDSL is a frequency-division-duplex (FDD) version of G.fast. Qualcomm believes GigaDSL offers a faster upgrade from VDSL in some regions like Korea and Japan. To date, however, it's the only chip supplier backing ITU standardization of GigaDSL. GigaDSL remains a transitional technology, and traditional TDD-based G.fast is expected to dominate larger post-VDSL growth.
Channel coding The forward error correction (FEC) scheme using trellis coding and Reed–Solomon coding is similar to that of VDSL2. FEC does not provide good protection against impulse noise. To that end, the impulse noise protection (INP) data unit retransmission scheme specified for ADSL2, ADSL2+, and VDSL2 in G.998.4 is also present in G.fast. To respond to abrupt changes in channel or noise conditions, fast rate adaptation (FRA) enables rapid (<1 ms) reconfiguration of the data rate.
Vectoring Performance in G.fast systems is limited to a large extent by crosstalk between multiple wire pairs in a single cable. Self-FEXT (far-end crosstalk) cancellation, also called vectoring, is mandatory in G.fast. Vectoring technology for VDSL2 was previously specified by the ITU-T in G.993.5, also called G.vector. The first version of G.fast will support an improved version of the linear precoding scheme found in G.vector, with non-linear precoding planned for a future amendment. Testing by Huawei and Alcatel shows that non-linear precoding algorithms can provide an approximate data rate gain of 25% compared to linear precoding in very high frequencies; however, the increased complexity leads to implementation difficulties, higher power consumption, and greater costs. Since all current G.fast implementations are limited to 106 MHz, non-linear precoding yields little performance gain. Instead, current efforts to deliver a gigabit are focusing on bonding, power and more bits per hertz.
Performance In tests performed in July 2013 by Alcatel-Lucent (now Nokia) and Telekom Austria using prototype equipment, aggregate (sum of uplink and downlink) data rates of 1100 Mbit/s were achieved at a distance of 70 m and 800 Mbit/s at a distance of 100 m, in laboratory conditions with a single line. On older, unshielded cable, aggregate data rates of 500 Mbit/s were achieved at 100 m.
A A straight loop is a subscriber line (local loop) without bridge taps. B The listed values are aggregate (sum of uplink and download) data rates.
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