NBN to trial fibre to the pit technology

Three-month FttDP trial for National Broadband Network

NBN principal technology officer, Tony Cross, with a DPU and a plug-in power pack.

NBN principal technology officer, Tony Cross, with a DPU and a plug-in power pack.

NBN has announced plans for a three-month trial, starting next month, of fibre to the distribution point (FttDP) technology, a cheaper alternative to fibre to the premises, but one that it does not expect to be commercially available until at least 2017.

With FttDP a small box, a distribution point unit (DPU), is located in the street pits where at present the copper pairs serving a few homes are joined to larger multi-pair copper cables.

The DPU takes the optical signal from the fibre, converts it to VDSL and transmits it over the last few metres of copper into the customer’s home. It is powered over that copper from a power point in the customer’s home using a plug-in power pack.

FttDP requires the use of a different fibre network topology to that presently deployed for fibre to the premises (FTTP), the so-called ‘skinny fibre’. This was the subject of heated debate in a Senate Estimates after a leaked document revealed that NBN had been trialling the skinny fibre.

In NBN’s current FTTP network the signal is carried on a single fibre from the exchange and split optically into signals on up to 32 separate fibres each of which serves one customer. This splitter is housed in a box on the street.

In the skinny fibre version the single fibre signal is split into signals on eight or 16 fibres and second splitters, located in the pits on the street, split the signal again onto single fibres, one per customer.

This technology is substantially cheaper that the current FTTN technology because the first splitter is much smaller and can be located underground, and because the fibre cable coming from it has fewer fibres it is thinner and easier and cheaper to install in the existing ducts – hence the term skinny fibre.

This technology is now mature and NBN CEO Bill Morrow said that there was “no question” that it would be used going forward. However he said NBN’s FTTN rollouts to existing premises, about 300,000 in total, were too advanced to deploy it, but it would be used for future, greenfield FTTN rollouts.

In an FttDP network the fibre coming out of the DPU is simply plugged into a port on the spitter in the pit – no fibre splicing is required.

Morrow said the trial would be undertaken to about 20 or 30 homes of NBN staff in Sydney and Melbourne. However he said the technology was not expected to be commercially available until about 2017, by which time NBN is hoping the price of DPUs will be about $300.

For the trial, NBN is using DPUs from Corning and from Nokia (formerly Alcatel-Lucent), its incumbent supplier of FTTN and FTTP technology.

NBN principal technology officer, Tony Cross, said that one of the biggest barriers to the development of FttDP had been the difficulty of getting the VDSL technology into a small, waterproof unit that could operate reliably underground without overheating. The DPUs being used in the trial are about the size of a thick paperback novel.

He said another attractive feature of the technology was its ability to be activated simply by the customer plugging in the power adaptor. Without power the unit routes the existing ADSL service over the copper from the exchange. When powered it disconnects the copper and switches to optical and VDSL delivery.

FttDP technology also has the advantage that it can be easily upgraded as technology developments increase the bandwidth that can be delivered over copper. G.fast technology is already in trials around the world and Nokia (Alcatel-Lucent) claims it can deliver 500Mbps over 250 metres of copper.

Nokia’s Bell Labs also claims to have conducted lab trials of an XG-Fast technology and to have achieved 1Gbps symmetrical over a distance of 70 metres on a single copper pair.

NBN’s cost vs technology balancing act

NBN announced its FttDP trials to journalists on a tour of NBN installations around Brisbane where it sought to cool the heated debate about FTTP vs FTTN vs FttDP by demonstrating how its technology choice is governed by its remit to deliver the required bandwidth at the lowest cost.

The example given was a handful of scattered rural properties outside Brisbane, which will be served by FTTP.

Morrow said FTTN, the preferred option, would have been too costly because it would have cost $200,000 to get power to the node. Satellite was ruled out because there are properties in the beam covering that area that need the capacity and that are too remote for FTTN or FTTP.

Fixed wireless was also deemed unsuitable because of the terrain, and too costly for the few premises to be served.

Morrow also sought to justify NBN’s purchase of large quantities of copper cable for its FTTN rollout. His admission to Senate Estimates last October that NBN was spending about $14 million on 1800km of copper cable caused something of a stir.

This cable is used to connect the nodes of the FTTN network to the existing street pillars in the Telstra network. Morrow said the location of the node was determined in part by access to power, and it was not always possible to locate these close to the pillars.

Stuart Corner travelled to Brisbane as a guest of NBN.

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