gfastWhen gigabit accesses are mentioned, the first thing that comes to mind for most of us (especially in Spain) is a fibre-optic access network for speeds beyond 50Mb. Carriers take the optical fibre to your home, install an ONT and that’s it. Customers can choose between 100Mbps, 300Mbps, 600Mbps and even 1Gbps plans, depending on what they’re willing to pay. If there is no fibre coverage at your place, the best you can hope for is a DSL access.

However, the term “gigabit access” refers to those that are capable of offering 1 Gbps speeds: optical fibre, UTP cable, coaxial cabling (DOCSIS 3.1) and, now that G.fast exists, even traditional telephone wiring. A gigabit access is a goal, not a technology.

Despite all gigabit networks being born equal, “some are more equal than others”: regulations, coverage, costs, level of intrusiveness, etc. are different in each case. When it comes to the latter, deploying new cabling in a private domain is considered highly intrusive and may require the agreement of the homeowners’ association (multi-dwelling units). These approvals can be hard to obtain, since deployment may require a common investment, works and inconveniences some owners may not be willing to bear. This can hinder the adoption of technologies such as optical fibre.

G.Fast: advantages

G.fast (ITU G.9700 and G.9701) solves some of these issues thanks to the use of the ever-present telephone wiring. It is the new addition to the ADSL, ADSL2, ADSL2+ and VDSL2 family. Its main characteristics are as follows:

•  DMT modulation, with a maximum load of 12 bits per dial tone (compared to the 15 offered by VDSL2)
•  Frequencies that exceed 200 MHz (compared to the 30-35 MHz offered by VDSL2)
• Upstream and downstream Time Division Duplexing (TDD), instead of the Frequency Division Duplexing (FDD) used by the other access networks belonging to the family. The discontinuous nature of TDD offers an additional advantage, as it can provide support to low-power scenarios.

Fast Rate Adaptation (FRA) helps manage abrupt changes in channel conditions (the broader the channel being used, the more probable it is for some conditions in a sub-band to vary)

Does this solution reach gigabit speeds?

Yes, but… (“nothing someone says before the word “but” really counts”) only in very short distances. The term “very short” must be taken literally, since we mean less than 100m. So, when is this useful? By simply deploying a mini-DSLAM (the size of a shoe box) in a building. G.fast can provide DSLAM coverage to private homes using the pre-existent wiring, significantly reducing the investment, nuisances and work needed to obtain gigabit access via other types of cabling as mentioned above.


About the author

Santiago FontSantiago Font
Telecommunications Engineer in the R & D department. Within that department he is specialized in xDSL.

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