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Thursday, 12 August 2010 16:06

NBN's 1Gbps not news, just 'clarification'


With exquisite timing prime minister Julia Gillard has 'announced' an 'upgrade' to the capabilities of the NBN saying it will deliver not 100Mbps but 1Gbps to subscribers. The reality is it has been able to do that all along. NBN Co has simply clarified its rules.

The timing is particularly exquisite coming as it does in the wake of the Coalition's policy announcement, which focussed on the how rather than the why of its NBN proposal, which contained unrealistic and misleading claims for the capabilities of HFC and wireless technologies and which lead to an embarrassing technically-focussed interrogation of Liberal Party leader, Tony Abbott by the ABC's Kerry O'Brien.

Announcement of the 'upgrade' was made by Gillard at the official opening of the Tasmanian NBN. It has resulted in Abbott making an even bigger fool of himself than he did on the ABC, with a knee-jerk dismissal of Gillard's statement as "utterly implausible."

In a press conference in the outer Sydney suburb of Penrith, Abbott said: "It's very hard to take seriously a government which suddenly pulls yet another technological rabbit out of a hat just because it's under enormous pressure in the closing stages of an election campaign."

Sorry Tony, but there are no rabbits, no hats and no trickery, at least not technical trickery. Here's the reality.


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The technology being used in the FTTH portion of the NBN is GPON, gigabit passive optical networking. In GPON a single fibre pair carries 2.4Gbps to an optical splitter, the node that splits this signal into a number of individual fibres, one pair per home. If there are no more than 24 homes served by that node you can absolutely guarantee that they could all get 100Mbps simultaneously, if there were sufficient capacity in the backhaul.

Once you go above 24 your theoretical maximum drops below 100Mbps but also you have to share the optical power among ever more fibres and that limits the distance over which you serve homes from that node.

However, and this is the key point, the nature of the technology is such that the full 2.4Gbps of bandwidth is available on each and every one of those fibre pairs. No matter how many of them there are. The just cannot all get 1Gbps at the same time.

The other limiting factor is the technology in the customer equipment that sits on the end of the fibre: the optical network terminator (ONT). This is planned to have four ethernet ports and if those were only 100Mbps ethernet ports you could not get one service at 1Gbps and even running all four at 100Mbps, you'd get only 400Mbps total.

However, according Paul Brooks, CEO of Layer 10 Advisory, the cost differential between 1Gbps ethernet and 100Mbps is insignificant so in all likelihood the NBN's ONTs will come with 1Gbps ethernet ports. "You can't even go into Harvey Norman these days and buy a small ethernet switch with 100meg ethernet, they are all 1gig," Brooks said.


With only 2.4Gbps in the shared fibre only two users on the node would be able to get the full 1Gpbs simultaneously. However in practice it would be very unlikely that every user with a one gig service would demand the full bandwidth. "the chances of this happening by the millisecond are very small," Brooks said.. "And by that stage you'd be limited by backhaul capacity."

He added that if demand warranted it would be a simple matter to install another splitter in the node and connect another fibre pair into the backhaul network "They don't make cables with one one fibre pair."

The Coalition in its policy claimed that HFC could deliver 100Mps, but that is over the shared HFC portion of the network. The equivalent statement for the GPON NBN is that it can deliver 2.4Gbps. Both statements need to be qualified by saying how many users have to share this bandwidth.

So what has changed to warrant Gillard's announcement? "NBN has simply clarified the rules," Brooks said. "They have been underselling the network."

He explained that the 100Mbps limit was essentially an administrative decision that would restrict a customer from signing up to a retail service offering in excess of 100Mbps or to multiple retail services whose aggregate bandwidth offerings exceeded this 100Mbps limit, and NBN Co as now said that limit wil not apply.

Utterly implausible? I think not.


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