3 High-speed subscriber lines according to G.fast standard
3.1 Standard G.fast – basic features

We can found in literature that the standard G.fast is called as the 4th generation xDSL system. The beginnings of the development of this standard are dated back to the beginning of February 2012. The standardization took place in 2014. The objectives of the new standard of xDSL are several:

- higher transfer rates in comparison with the existing type of xDSL connections - up to 500 Mbit/s to circa 100 m, respectively 150 Mbit/s to 250 m (it is also considering about transfer rates up to 1 Gbit/s)
- utilization of short lengths of subscriber lines, called ad FTTdb (Fiber To The distribution point) - the location of distribution points will be very close to subscribers (up to 250 meters) - equivalent to the concept of FTTB (Fiber To The Building)
- inverse power
- the possibility of self-installation service with parameters of optical connections and simplicity of xDSL connections

All the above mentioned objectives are closely related into two key areas. The first area is the question how to increase throughput, the second is an area of increasing reliability. Throughput of connections can be increased in two ways:

- extension of the frequency band - this option was used in the previous generation of xDSL with the demand for spectral compatibility with VDSL2 systems
  - Mask of PSD – beginning at 138 kHz; 2,5 MHz; 18 MHz a 30 MHz and end at 106 MHz; 212 MHz a 300 MHz
  - Respect for additional frequency bands, such as VHF broadcast (usually called as "Western Standard" in Europe), DAB (Digital Audio Broadcast) - so called as III. TV band 174-240 MHz.
- cancellation of crosstalks - a process of elimination of crosstalk type FEXT (Far End Cross Talk) using the appropriate modulation type called as VDMT (Vectored Discrete Multi Tone)

Increasing the reliability of the entire system provides by the following two mechanisms:

- inverse multiplex
- phantom circuits

In contrast to ADSL and VDSL, which use FDD, the connections with the extreme bandwidth uses TDD. This allows for much more efficient and flexible allocation of bandwidth and eliminates the dilemma of allocating bands for each direction of transmission. Time synchronization is not a problem, because they must be dealt already with regard to the modulation VDMT.

Introduction of TDD also provides simple support for saving modes. In the context of TDD duplex mode timing are introduced mandatory divider ratios of 90/10 and 50/50, an optional divider ratios are then from 50/50 to 10/90.

Among the last-mentioned, but key features of a standard G.fast include the implementation of adaptive modulation DMT with required support VDMT.