Telstra technicians work on a Fibre-to-the-Node cabinet. Tens of thousands of these would line the streets of Australia if FttN was the basis of an NBN.

Richard Chirgwin describes the conditions under which FttN - the Coalition's preferred NBN-alternative technology - would be implemented. All descriptions assume that a copper network is both available and is in good condition. Among the questions it raises are the following: how can it be implemented without creating a wholesale monopoly, a hardware-supplier monopoly and without decimating existing competition in the ADSL market? Furthermore, the question is raised regarding which VDSL technology would be used when the fastest methods (which have been part of the public debate) are still years away from becoming standards.

VDSL2 with vectoring is an important and exciting technology. For one thing, it's going to be popular for distributing high-speed services within apartment complexes, delivering fibre-like service speeds without demanding the building to be rewired. But is it a replacement for the NBN?

If you take up designing communications technologies, one of the first things you'll learn is that there are only three issues you need to deal with: capacity, latency and noise.

The same problems have confronted everyone who ever wanted to communicate over a long distance. When the Romans lit signal fires, the medium had limited capacity (enough for a single, pre-agreed message), long latency (a message might take hours to reach its destination) - and would fail if there was noise (such as heavy cloud between two points).

It took humanity thousands of years to set down a universal mathematical rule describing the relationship between capacity and noise - it didn't happen until the middle of the twentieth century, when one of the fathers of "information theory", Claude Shannon (working with others and building on the work of others) set down a straightforward equation that still holds true.

You don't want or need the equation, but it's so simple I've put the Excel formula at the end of the article for anyone who's interested. It results in the table below:



(Please note: these numbers represent a thought experiment. I know they're not "real world" data. It's clear, however, that this writer lives on a high-noise line out of the Petersham exchange, since a connection of 5.1 Mbps is a little higher than I usually achieve.)

In a perfect world, it's clear that VDSL2 (the technology proposed in most copper-based fibre-to-the-node models) offers a path to a high-bandwidth network with performance competitive to the NBN's proposed fibre - if you ignore the existing path to 1Gb/s offered by the NBN.

So what's wrong with this picture?

The factor I didn't include is distance. High frequencies "attenuate" - that is, lose their signal strength - more quickly on copper than low frequencies. As you can see, VDSL2 achieves its high speeds by using an analogue bandwidth of 30 MHz, compared to the more conservative 2.2 MHz of ADSL2+. The impact on reach is dramatic, as illustrated below.



I've selected this particular image to avoid accusations that it was chosen from interested parties: it comes from a vendor, Asotel, whose interest is in promoting VDSL2.

As the signal becomes "quieter" (that is, more attenuated), the signal-to-noise ratio deteriorates, and VDSL2's performance advantage is lost. This is why it's put forward as a fibre-to-the-node technology: because you have to put the DSLAM near consumers to get decent performance.

This creates a number of challenges - technical challenges, certainly, but also regulatory and in the structure of the industry.

The technical challenges

As Malcolm Turnbull has rightly pointed out, 100 Mb/s isn't the boundary of DSL-based technologies. Even Australia has had a role in extending what DSL technologies can deliver - for example, there's the excellent work by Dr. John Papandriopoulos that made headlines back in 2007.

Alcatel-Lucent is one of the leaders in supercharging VDSL technology with a technology called "vectoring".

I'm not going to embark on an explanation of vectoring, but what the technology does is this: it continually watches the lines the equipment's connected to, and adjusts the equipment accordingly - rather like a human on an old-style wireless set, continually adjusting the set to stay tuned to a distant, faint signal.

Why not deploy VDSL2 with vectoring, instead of rolling out fibre?

One reason is that the technology isn't ready to put in live networks yet.

VDSL2 is well-established. However, vectoring is so young that it was only last year that equipment suppliers began testing their ability to connect their boxes to each other (it took place in October 2012 - there's a press release here).

Supply competition is vital to carriers. The battle between Alcatel-Lucent, Ericsson and Huawei to supply ISPs with their network equipment helps keep our broadband prices down. As it now stands, any carrier trying to deploy vectoring would have a very limited choice of vendors - and right now, would be constrained to using only one supplier in its network.

It's reasonable to say that VDSL2 using the vectoring standard (look up G.vector if you want to know more) won't be ready for real-world deployment for three to five years - somewhere between 2016 and 2018, just to start the project.

There's a second challenge: vectoring would create a wholesale-level monopoly in the network.

Today, your broadband is delivered on copper lines owned by Telstra - but the equipment is owned by iiNet, Optus, iPrimus, Telstra, TPG and others.

Vectoring, however, works best if all the lines are connected to one box. If there are two different boxes - in the absence of full, mature interoperability - they will get into what amounts to an "arms race" trying to get the best performance on the lines they serve.

Hence, to use VDSL2 plus vectoring to deliver high-speed broadband would require a regulated wholesale monopoly. Whether this is a good thing or a bad thing depends on your point of view, but it's inconsistent to oppose the NBN's industry structure, while proposing a technology that demands a similar structure.

The final question is one of reach: how much of Australia might miss out on a fibre-to-the-node network?

It's difficult to predict, but the footprint of ADSL in Australia is instructive. There are roughly 5000 exchanges in the Telstra network - and the carrier also publishes a full list of exchanges where ADSL and ADSL2+ have been deployed.

After more than ten years of ADSL rollout in Australia, ADSL has reached 58 per cent of Telstra exchanges, and ADSL2 has reached 48 per cent of them.

It's a reasonable bet that somewhere between 42 per cent and 52 per cent of exchanges will be outside the FTTN rollout, if ADSL hasn't reached them ten years into the rollout. In other words - the best we could expect from FTTN is that is gets to around half of Telstra's exchanges.

Of course, that half of the network covers more than 90 per cent of Australia's population - there are an awful lot of exchanges that serve very small communities - but I haven't the raw data for the final analysis: how many of that 90 per cent is within reach of a node site?

Let's look at two electorates - Macquarie, and Wentworth. As you can see below, Wentworth could be very well-served by FTTN. The exchanges are quite close together, which means there aren't that many street cabinets required to cover the whole electorate. It only needs decent copper for things to work.



Wentworth and its exchanges.

The urban fringe, however - like Macquarie, which takes in Penrith and the Blue Mountains - is a different matter.



Macquarie and its exchanges.

It's not just that lots of people live at a distance from the exchange: I know from personal experience that the distance from home to the nearest cabinet in Macquarie can be beyond VDSL2's reach.

VDSL2, vectoring and noise

I'd like to close by returning to the noise discussion that I began with, because in general, fibre-to-the-node has one final undesirable characteristic: just like the NBN, it will "strand" ISP's existing investment in DSL equipment now installed in Tesltra exchanges.

This diagram might help explain why



The FttN cabinet makes so much noise that it overwhelms any traditional ADSL signal from the exchange.

You could rip out the copper from the exchange to the node. This leaves the "stranded" equipment no path to the homes. Or you could leave the copper in place, add fibre, and offer new services from the node (as in this image).

Unfortunately, the second option is likely, at the very least, to degrade services now deployed in the exchange.

You see, the node is a big, loud noise-maker - right in the middle of the copper loop. At the point where the ADSL2+ signal will have lost half its power - or more - the VDSL2 node will be shouting at users.

This may be solvable in theory, but it at least depends on a very co-operative relationship between the provider with the exchange-based ADSL2+ service and its probable competitor with the fibre-to-the-node network.

Either that, or a heavy regulatory hand to deal with the monopolistic behaviour that will arise.

Footnote: I promised that you can run Shannon's law calculations in Excel. First, give three cells the names "bandwidth", "signal" and "noise". The formula is:

=bandwidth*LOG((1+signal/noise), 2)

"Bandwidth" is the analogue bandwidth of the channel in Megahertz (for example, 2.2 MHz for ADSL2+). "Signal" and "noise" are just numbers - if you want to see what happens if you have 100 times as much signal as noise, put 100 in the signal cell and 1 in the noise cell. Have fun!