NBN must limit downloads to 12 Mbps downloads until copper handover

'Transition period' between ADSL2+ and FTTN to force sensible hand brake

Network deployment rules and the Shannon-Hartley Theorem are crimping national broadband network (NBN) speeds, and will do for some time.

As noted in new nbnTM documents turned up by Kenneth Tsang, who created the rollout tracking site myNBN.info, the wholesale deployment rules put the brakes on fibre-to-the-node service speeds for as long as they have to live alongside existing ADSL2+ services using the Telstra copper.

The May 27-dated document, linked by Tsang, imposes a 12 Mbps downstream / 1 Mbps upstream limit on a line's Peak Information Rate (PIR) during the “transition period”.

During that period, the company is mandating a downstream power back-off for DSLAMs on the fibre-to-the-node part of the network.

From a technical point of view, that's an unsurprising decision. Crosstalk is noise, and noise is the enemy of bandwidth: one of the reasons the Communications Alliance has put so much effort into network deployment rules over the years is to make sure retailers using Telstra copper don't get into an escalating war turning up the power to maintain speeds.

The NBN is confronted with the same technical problem, with an added twist: ADSL and its successors do not like a mid-stream signal injection.

Having left the exchange at (say) 15 dBm, the ADSL2+ signal to Alice will be tiny by the time it reaches the distribution point where nbnTM wants to install an FTTN node.

If nbnTM blasts away with a 15 dBm signal at that location, Bob will get his 25 Mbps (or better) happily, but Alice won't get anything at all.

The Communications Alliance is well aware of the problem, and is currently working on its network specifications to allow for VDSL2 services (its working group is here).

The current deployment rules also limit the frequency spectrum services can use to ADSL2+ profiles – so the higher frequencies needed for faster VDSL2 services are also waiting on the Comms Alliance work.

Obeying the Shannon-Hartley theorem does, however, create a potential political discomfort for the government, because unless nbnTM can move fast, there'll be tens of thousands of services operating under the 12 Mbps “downstream back-off” cap in 2016, when the next election happens.

“The government not only failed to deliver the NBN, it cut the speed!” is an irresistible political slogan even it it's based on a misunderstanding of technology (when did technical clue get the voters excited anyway?).

Communications minister Malcolm Turnbull will be much happier if he can make an equally inaccurate claim – that under his watch, he's “sped up” the NBN.

Vulture South will be watching the election campaign with interest … ®

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