G.fast can provide ultra-high speeds over copper twisted pairs, up to and sometimes even exceeding speeds of 1 Gbps. The loop lengths for G.fast are from 50 to 250 meters (150 to 750 feet). G.fast is standardized as ITU-T Recommendation G.9701. Similar to vectored VDSL, G.fast supports vectoring, which reduces crosstalk that is found in multi-pair cables and at higher frequencies. The first versions of G.fast operate over frequencies of up to 106 or 212 MHz, and use linear vector pre-coding to eliminate crosstalk in the downstream direction. Future versions of G.fast and multi-gigabit G.fast (G.mgfast) may operate over frequencies of up to 848 MHz, and may support higher-performance non-linear pre-coding to allow for even higher speeds.
Time-Division Duplexing (TDD) instead of Frequency-Division Duplexing (FDD)
Unlike prior DSL technologies which used Frequency-Division Duplexing (FDD), G.fast uses Time-Division Duplexing (TDD). With TDD, the system transmits only downstream signals for a fraction of time, and transmits only upstream signals for the remaining time. TDD allows the speed asymmetry to be varied at will among all the lines emanating from the same Distribution Point Unit (DPU) —the name for a G.fast DSLAM. This allows some areas to be served with business-class symmetric service, while other areas can be served with asymmetric service that best addresses consumer needs. Transceivers operating in the same bundle must use the same TDD ratio, and TDD frames must be synchronized to a common clock to eliminate Near-End Crosstalk (NEXT). Dynamic Time Assignment (DTA) can vary the TDD ratio in real time in response to traffic variations.
G.fast Deployment Considerations
Low-cost, Self-install, OLR and Reverse Powering
G.fast is amenable to low-cost self-install deployment, similar to ADSL. G.fast supports new On-Line Reconfiguration (OLR) techniques, including Fast bit-Rate Adaptation (FRA), to adapt to changes in the transmission environment and overcome the harsh home wiring environment. G.fast supports reverse-powering, which sends power from the customers’ CPE to the DPU, thus eliminating the need for network power. Reverse powering needs to provide approximately 10 Watts, which is sufficient because the G.fast transceivers are very close to the subscribers and require little transmitter power. Reverse powering, and low-power modes planned for G.fast, are expected to lower G.fast deployment costs by eliminating the need for costly network powering and battery back-up of remote DPUs.
Fiber-To-The-distribution point (FTTdp)
G.fast is expected to be deployed in a Fiber-To-The-distribution point (FTTdp) architecture as shown in the Figure above. The “dp” may also be called the “terminal” or “drop-wire terminal,” and is where the Distribution-Point-Unit (DPU) is located. Fiber is fed to the terminal, and from there, very short copper cables and drops, up to about 400 meters (1200 feet) long, serve subscribers.
Compatibility and Co-existence
Some service providers will require G.fast DPU equipment to be backwards compatible with vectored VDSL. Such G.fast transceivers can fallback to support VDSL, albeit with lower performance compared to a dedicated VDSL transceiver. This is useful for initial installation of G.fast as an existing VDSL customer can be cut-over to the new G.fast DPU. At a later time, a customer can receive a G.fast-capable modem, which will automatically sync up in G.fast mode.
G.fast can coexist with ADSL and VDSL on adjacent pairs of copper wires by using frequencies above these technologies. Operation above typical VDSL frequencies implies a minimum G.fast frequency of about 19 MHz, and this incurs a significant performance penalty on longer G.fast loops, beyond about 100m. Dynamic Spectrum Management (DSM) can enable G.fast to use some high frequencies in the VDSL band, increasing G.fast speeds by about 100 Mbps.
G.fast and G.mgfast are candidates for Fiber-To-The-Building/Basement (FTTB) deployments and for feeding 5G small cells as they proliferate. G.fast is now widely available and the first version of G.mgfast should be standardized in 2019.
Read blog post: G.fast, MGfast, and Beyond