Publié le 20 décembre 2013
The dawn of G.Fast
Beyond vectoring and bonding, G.Fast is a future technology that makes it possible to deliver gigabit speeds. G.Fast gained formal consent at the December 2013 ITU-T meeting in Geneva, Switzerland. This standard will be ratified in 2014, which means that product availability and deployment may start as early as 2015. In the meantime, this blog will hopefully give you a heads-up about what is coming up.
G.Fast is designed for cases that bring fiber closer to the end user, meaning that there are less users per node (but still more than one, so vectoring is required), and the copper loop length is reduced. The various scenarios include FTT Curb, FTT Manhole, FTT Pole and FTT Building, all of which fall under the common “FTTdp” nomenclature, which is short for Fiber-to-the-distribution-point, the technical specifications of which are currently being established by the Broadband Forum (BBF, WT-301). If fiber goes even deeper, i.e., to the home without actually entering it, the result will be a single user per each very short loop, in which case G.Fast will be fully applicable (but will not require vectoring).
G.Fast succeeds in achieving such increased performance over VDSL2 thanks to two main factors, the first being extra bandwidth. VDSL2 is limited to 17 MHz bandwidth (technically VDSL2 can use up to 30MHz, but in practical terms, this bandwidth is only seen in MDUs, as it lacks reach), whereas G.Fast has pushed beyond that limitation to 106 MHz during a first phase, and with plans to eventually reach up to 212 MHz. The second factor involves a shortened copper loop applicable to the various above-mentioned applications, thereby reducing attenuation at high frequencies. Both of these factors, i.e., high bandwidth and shorter loops, lead to higher bit rates, from 150 Mbit/s on 250 m loops all the way up to 1 Gbit/s on 100 m loops at 106 MHz, which would be doubled with the prospective 212 MHz.
Although this increased performance is one of G.Fast’s great advantages, this new technology brings much more to the table:
Ease of Installation
Simply put, there is no need to drill any new holes into the home (in a brownfield scenario, of course) to allow entry of fiber. The existing copper-wired infrastructure within the home is all that’s needed. G.Fast is “self-installing,” meaning that a customer can simply take any modem and connect it to his or her existing home network. This of course leads to reduced cost and time versus a typical technician installation. This ease of installation also applies to the distribution point: wherever it is (manhole, pole, etc.), the DSLAM deployment can be referred to as “install and forget.” A final aspect of this “ease of installation” category is powering. To speed up deployment, a few powering alternatives are defined in G.Fast. The traditional approach of power being generated from within the network (e.g., from a nearby cabinet) is still valid. However, another option that can be employed is powering from the CPE equipment itself, which has been coined “reverse powering.” Because the amount of power needing to be grabbed from the customer is quite limited, G.Fast is extremely energy efficient.
G.Fast uses a bandwidth from 2.2 to 106 MHz (and eventually, 212 MHz). Although there are already multiple other services within this spectrum, G.Fast will have low power consumption (below -70 dBm/Hz), and in turn, interference with nearby services will be very unlikely. G.Fast also has a flexible notching capability that makes it possible to notch out entire bands. So, as an example, you could start above 17 MHz for co-existence with VDSL2, and then use the entire band (starting from 2.2 MHz) once those VDSL2 customers have migrated to G.Fast.
Being highly versatile, G.Fast can be configured to support many use cases. For example, because G.Fast is time-division multiplexed, the time allowed for upstream and downstream transmission is flexible, meaning that the upstream-downstream ratio can be easily adjusted from 10:90 to 90:10. With the anticipated 212 MHz and a 50:50 split on a short loop, it will be possible to achieve 1 Gbit/s symmetrical. This also improves energy efficiency, because power consumption can be scaled down in proportion to the traffic. For instance, ultra-low power can be used for limited-bandwidth applications, such as VoIP.
Vectoring 2.0 can be applied on top of G.Fast, enabling a high bit rate over multiple users.
This significant improvement in DSL speed will in turn speed up fiber deployments, because fiber does not have to “enter” the home in order for customers to benefit from very high speed. Most of the deployment cost is associated with civil works costs for the installation of the fiber: the closer the fiber is to the user, the higher the cost. The typical cost difference between an FTTC and FTTH deployment is a factor of three. To this is added the cost savings resulting from not having to make appointments with homeowners or drill at every home, thereby eliminating the need to make several repetitive truck rolls in a given sector/neighborhood. That’s because G.Fast is designed to be self-installing and very robust on any type of wiring.