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Understanding G.fast Technology: Features and Specifications

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This page explains the fundamental features of G.fast technology. It provides links comparing G.fast with VDSL and highlights the differences between the two. G.fast is a technology designed to coexist with VDSL2, adding enhanced capabilities for high-speed data communication over digital subscriber lines. It’s primarily intended for local loops shorter than 500 meters.

The specifications for G.fast are outlined in ITU-T G.9700 and ITU-T G.9701.

G.fast Technology Features

Here are some key features of G.fast technology:

  • Speed: Offers speeds ranging from 150 Mbps to 1 Gbps, depending on the loop length.
  • TDD Duplexing: Employs Time Division Duplexing (TDD), where both downstream (DS) and upstream (US) communications utilize the same frequency band but different time intervals for transmission. It supports low-power states, and the DS/US ratio can be easily adjusted.
  • Bandwidth: Supports a bandwidth of up to 100 MHz.
  • DMT Modulation: Uses Discrete Multi-Tone (DMT) modulation with 2048 subcarriers, a subcarrier spacing of 51.75 KHz, and allocates 12 bits per subcarrier.
  • PHY Layer Retransmission: The Physical (PHY) layer supports retransmission, improving resilience against impulse noise while maintaining low latency.
  • Vectoring: Supports vectoring, which helps to cancel far-end crosstalk.
  • Forward Error Correction (FEC): Supports FEC with trellis coding and Reed-Solomon (RS) coding, as defined in G.993.2 VDSL2, along with retransmission block interleaving as defined in G.998.4.
  • Network Time Transport: Provides transport of network time and time of the day.
  • Loop Length: Designed to operate over loop lengths of 250 meters using a 24 AWG (American Wire Gauge) pair of wires.
  • Broadband Forum Architectures: Supports TR-156 and TR-167 broadband forum architectures.