Broadband through the sewers? Obvious jokes aside, does Brisbane’s plan to roll out its own fibre rather than wait for the National Broadband Network prove Australia’s need for speed? Or does the cheaper cost per home of the UK-based i3 Group’s patented “FS System” technique prove that the NBN is over-priced? The devil is in the detail.

Running optical fibre down the sewers isn’t a new idea, despite Fairfax’s easily shocked Ellen Lutton calling it a “shock new broadband scheme“. i3 was pimping its Fibrecity projects in Bournemouth, Dundee and Northampton in January 2008, when the company was still called H2O Networks. Even then, technology news site The Register acknowledged that the idea had been “sloshing around the networking business for years”.

At first glance it makes sense to go down the sewers to the end of the street, and from there lay fibre to individual premises using “microtrenching”, a slot only a few centimetres wide. According to the  NBN Implementation Study, about 70% of the network build cost is “civil works”. Using existing “passive infrastructure” such as sewers rather than building your own ducts represents a massive cost saving. Indeed that’s why NBN Co’s deal to use Telstra’s ducts knocks billions off the NBN’s price tag. And unlike copper cables, which must be kept pressurised with gas to keep them dry, fibre can cope with the, erm, moist environment.

i3 Group CEO Elfed Thomas, speaking on this week’s  A Series of Tubes podcast, claims they’ve been doing this for five years, and they can lay 400 metres of fibre a day compared with a mere 30-40 metres per day if you have to construct your own ducts.

Thomas also says that in Brisbane they’ll be providing not one fibre to each home and business but two — for redundancy, for double the speed, or so premises can be served by two competing retailers — all for a cost of $600 per home compared with the NBN’s thousands.

But those details …

Doubling up the fibre is no big deal. The NBN Implementation Study reckons that the physical fibre represents just 3% of the total cost.

The key difference is that, unlike the NBN, i3 is only building a  dark fibre network, merely running cable to the outside of the premises. It’ll be up to the retail telcos to install the customer premises equipment at $400 or $500 a pop, plus the equipment at the exchange, plus  backhaul links.

Building in the city and suburbs will always be cheaper than outer metropolitan and rural areas. The NBN is all about the higher-value metropolitan markets cross-subsidising the rural areas so there’s equal pricing for everyone.

And, of course, the project still has to come together. Despite Thomas’ claims to have been doing this for five years, according to the  Fibrecity website Dundee is still rolling out but Northhampton seems to have vanished. In Bournemouth, as reported two months ago, i3’s sewer deal with Wessex Water has fallen through, citing contractual and technical issues. Bournemouth will use more traditional methods, presumably at higher cost. Everything else — including roll-outs in Derby, Halton, Nottingham, Plymouth, York, Ireland and even Iceland as well as Brisbane — is still just talk.

Should Brisbane’s network go ahead, the question will be whether NBN Co still builds its own fibre there. Communications Minister Senator Stephen Conroy has indicated the government might force private network operators to meet NBN technical standards, effectively letting them build sections of the NBN and prevent duplication.

We shall see. The Telecommunications Legislation Amendment (Competition and Consumer Safeguards) Bill 2009 is expected to hit Parliament this week.