G.Fast: rationale unclear for copper 2.0

Stephen Wilson/Informa Telecom & Media
20 Dec 2011

A fundamental problem for operators wishing to roll out FTTH is the problematic final few metres of laying the fiber, which account for a large majority of the overall costs of rollout. For example, Swisscom claims that 80% of the overall cost of an FTTH deployment is in the section between the plug in the subscriber’s home and the manhole in the street, which is perhaps 150m of drop cabling away from the subscriber’s home.

The promise of G.Fast is to enable operators to offer FTTH-like speeds of up to 1Gbps but without rolling out fiber in the troublesome final metres. The aim of G.Fast would be to leave a maximum of 200 meters of copper. At the same time as shortening the copper loop further than today’s FTTC deployments G.Fast promises a number of innovations that would enable operators to increase the maximum speeds they can offer to end customers.

Present day VDSL2 systems can work using a maximum of 30MHz of spectrum (although it is really only with systems supporting up to 17MHz that large volumes have been deployed). G.Fast promises to significantly increase this to perhaps somewhere between 70-140MHz. DSL systems work by dividing the data into various sub channels or tones which contain up to 15 bits, using so called discrete multi-tone modulation. G.Fast would aim to move beyond this maximum of 15 and therefore provide higher speeds. Further improvements in coding and modulation would also enable significant gains in speeds. The overall aim of these advancements would be to define a protocol that can support an aggregate upstream and downstream capacity of 1Gbps, although in practice the aggregate in the field would be somewhere above 500Mbps.

Significant amounts of work need to be done before G.Fast can become a commercial reality. A timeline of perhaps four years might be required before the technology becomes commercially available. The technology will need to be standardized for operators to accept it and reduce the risks of vendor lock in.

The ITU began a project on G.Fast in February 2011 and Alcatel Lucent states that it expects to demonstrate a proof of concept in its labs next year, having started research on the technology in February 2010. Another part of the work in the standardizing of G.Fast will be in ensuring the provision of power to the remote device in the field, say 50m away from the subscriber’s home, by using the residential gateway.

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