G.fast

G.fast is a digital subscriber line (DSL) standard for local loops shorter than 500 m, with performance targets between 150 Mbit/s and 1 Gbit/s, depending on loop length.[1] 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. [2]

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 5th 2014.[1][3][4][5] Development was coordinated with the Broadband Forum's FTTdp (fiber to the distribution point) project.[6][7] The name G.fast is a recursive acronym for fast access to subscriber terminals;[8] the letter G stands for the ITU-T G series of recommendations. Limited demonstration hardware was demonstrated in mid-2013.[9] The first chipsets were introduced in October 2014, with commercial hardware expected in 2015, and first deployments planned for 2016.[10][11][12]

Technology

Modulation

Frequency spectrum of the G.fast standard compared to VDSL2

In G.fast, data is modulated using discrete multi-tone (DMT) modulation, as in VDSL2 and most ADSL variants.[13] G.fast modulates up to 12 bit per DMT frequency carrier, reduced from 15 in VDSL2 for complexity reasons.[14]

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.[1] This spectrum overlaps with 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;[8] G.9701, codenamed G.fast-phy, is the G.fast physical layer specification.[6][15] 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.[1]

Duplex

G.fast uses time-division duplexing (TDD), as opposed to ADSL2 and VDSL2, which use frequency-division duplexing.[1] Support for symmetry ratios between 90/10 and 50/50 is mandatory, 50/50 to 10/90 is optional.[1] 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.[1]

Channel coding

The forward error correction (FEC) scheme using trellis coding and Reed-Solomon coding is similar to that of VDSL2.[1] 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.[1] To respond to abrupt changes in channel or noise conditions, fast rate adaptation (FRA) enables rapid (<1 ms) reconfiguration of the data rate.[1][16]

Vectoring

Performance in G.fast systems is limited to a large extent by crosstalk between multiple wire pairs in a single cable.[14][13] 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.[1][13] Testing by Huawei shows that non-linear precoding algorithms can provide an approximate data rate gain of 25% compared to linear precoding, however the increased complexity leads to implementation difficulties, high power consumption, and more costs.[13]

Performance

In tests performed in July 2013 by Alcatel-Lucent and Telekom Austria using prototype equipment, aggregate (sum of uplink and downlink) data rates of 1.1 Gbit/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.[14][17] On older, unshielded cable, aggregate data rates of 500 Mbit/s were achieved at 100 m.[14]

Service rate performance targets over 0.5 mm straight loops[A][1]
Distance Performance target[B]
<100 m, FTTB 500-1000 Mbit/s
100 m 500 Mbit/s
200 m 200 Mbit/s
250 m 150 Mbit/s
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.

Deployment scenarios

The Broadband Forum is investigating architectural aspects of G.fast and has, as of May 2014, identified 23 use cases.[1] Deployment scenarios involving G.fast bring fiber closer to the customer than traditional VDSL2 FTTN (fiber to the node), but not quite to the customer premises as in FTTH (fiber to the home).[12][18] The term FTTdp (fiber to the distribution point) is commonly associated with G.fast, similar to how FTTN is associated with VDSL2. In FTTdp deployments, a limited number of subscribers at a distance of up to 200–300 m are attached to one fiber node, which acts as DSL access multiplexer (DSLAM).[12][18] As a comparison, in ADSL2 deployments the DSLAM may be located in a central office (CO) at a distance of up to 5 km from the subscriber, while in some VDSL2 deployments the DSLAM is located in a street cabinet and serves hundreds of subscribers at distances up to 1 km.[14][12] VDSL2 is also widely used in fiber to the basement.[19]

A G.fast FTTdp fiber node has the approximate size of a large shoebox and can be mounted on a pole or underground.[20][12] In a FTTB (fiber to the basement) deployment, the fiber node is in the basement of a multi-dwelling unit (MDU) and G.fast is used on the in-building telephone cabling.[18] In a fiber to the front yard scenario, each fiber node serves a single home.[18] The fiber node may be reverse-powered by the subscriber modem.[18] For the backhaul of the FTTdp fiber node, the Broadband Forum's FTTdp architecture provides GPON, XG-PON1, EPON, 10G-EPON, point-to-point fiber Ethernet, and bonded VDSL2 as options.[7]

Former FCC chief of staff Blair Levin has expressed skepticism that US ISPs have enough incentives to adopt G.fast technology.[21]

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 Van der Putten, Frank (2014-05-20). "Overview of G.fast: Summary overview and timeline". G.fast Summit 2014. Retrieved 2014-10-09.
  2. ↑ "100+ megabits 400 meters.". G.fast News. Fast Net News. February 4, 2015.
  3. ↑ "G.9700 : Fast access to subscriber terminals (G.fast) - Power spectral density specification". ITU-T. 2014-12-19. Retrieved 2015-02-03.
  4. ↑ "G.9701 : Fast access to subscriber terminals (G.fast) - Physical layer specification". ITU-T. 2014-12-18. Retrieved 2015-02-03.
  5. ↑ "G.fast broadband standard approved and on the market". ITU-T. 2014-12-05. Retrieved 2015-02-03.
  6. ↑ 6.0 6.1 "New ITU broadband standard fast-tracks route to 1Gbit/s". ITU-T. 2013-12-11. Retrieved 2014-02-13.
  7. ↑ 7.0 7.1 Starr, Tom (2014-05-20). "Accelerating copper up to a Gigabit in the Broadband Forum". G.fast Summit 2014. Broadband Forum. Retrieved 2015-03-13.
  8. ↑ 8.0 8.1 "ITU-T work programme - G.9700 (ex G.fast-psd) - Fast access to subscriber terminals (FAST) - Power spectral density specification". ITU-T. 2014-01-29. Retrieved 2014-02-14.
  9. ↑ Ricknäs, Mikael (2013-07-02). "Alcatel-Lucent gives DSL networks a gigabit boost". PCWorld. Retrieved 2014-02-13.
  10. ↑ "Sckipio Unveils G.fast Chipsets". lightreading.com. 2014-10-07. Retrieved 2014-10-09.
  11. ↑ Hardy, Stephen (2014-10-22). "G.fast ONT available early next year says Alcatel-Lucent". lightwaveonline.com. Retrieved 2014-10-23.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 Verry, Tim (2013-08-05). "G.fast Delivers Gigabit Broadband Speeds To Customers Over Copper (FTTdp)". PC Perspective. Retrieved 2014-02-13.
  13. ↑ 13.0 13.1 13.2 13.3 "G.fast: Moving Copper Access into the Gigabit Era". Huawei. Retrieved 2014-02-13.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 Spruyt, Paul; Vanhastel, Stefaan (2013-07-04). "The Numbers are in: Vectoring 2.0 Makes G.fast Faster". TechZine. Alcatel Lucent. Retrieved 2014-02-13.
  15. ↑ "ITU-T work programme - G.9701 (ex G.fast-phy) - Fast Access to Subscriber Terminals (G.fast) - Physical layer specification". ITU-T. 2014-01-07. Retrieved 2014-02-14.
  16. ↑ Brown, Les (2014-05-20). "Overview of G.fast: Key functionalities and technical overview of draft Recommendations G.9700 and G.9701". G.fast Summit 2014. Retrieved 2015-03-13.
  17. ↑ Ricknäs, Mikael (2013-12-12). "ITU standardizes 1Gbps over copper, but services won't come until 2015". IDG News Service. Retrieved 2014-02-13.
  18. ↑ 18.0 18.1 18.2 18.3 18.4 Wilson, Steve (2012-08-14). "G.fast: a question of commercial radio, manholes, prison sentences and indoor vs outdoor engineers". telecoms.com. Retrieved 2014-02-13.
  19. ↑ "Planning of Fibre to the Curb Using G. Fast in Multiple Roll-Out Scenarios - Volume 2, No.1, March 2014 - Lecture Notes on Information Theory". LNIT. Retrieved 2014-07-19.
  20. ↑ Maes, Jochen (2014-05-20). "The Future of Copper". G.fast Summit 2014. Alcatel-Lucent. Retrieved 2015-03-13.
  21. ↑ Talbot, David (2013-07-30). "Adapting Old-Style Phone Wires for Superfast Internet: Alcatel-Lucent has demonstrated fiber-like data-transfer speeds over telephone wiring—but will ISPs adopt it?". MIT Technology Review. Retrieved 2014-02-13.