To understand what the NBN is, it is helpful to first understand the current, copper based, telephone network and it’s history.
The telephone network was designed 50-100 years ago to simply carry telephone calls. While the central core (the bit that carries your calls over a long distance) of the telephone network has been upgraded repeatedly over it’s history, the end bits – between you and the nearest exchange is still a simple pair of copper wires carrying your voice signal – the so called “last mile”.
The internet came along in the mid 90’s and soon we all wanted to connect to it. At the time the only way to do it (short of laying a new cables) was to use dial-up modems. These devices converted the data signals into sound (do you remember the connection noises they made?) and transmitted them across the telephone network to your “dialup provider”. The speed that they could send data was very slow. The telephone network was only designed to transmit audible sound, anything of a higher frequency (higher speeds need a higher frequency) just didn’t make it through (and in fact were filtered out by the exchange).
Interestingly these dialup modems are still being used today in our faxes, EFTPOS and alarm systems. They still transmit their small amount of data by converting the data into sound.
A technology called ADSL came along in the early 2000’s which improved speeds massively (by about 100 times). Even though the copper cable was not designed to carry high frequencies, it was found that a very small amount of the signal would make it through. With advanced signal processing techniques this small signal could be extracted as long as the copper cable was not too long and the connections were fairly good (i.e. not too much water in the joints). So the carriers delivered the internet to the exchange and then used ADSL to get it out to the subscribers.
However this technology was at best a retrofit and as the demand for speed increased we have reached the limit of ADSL too. There is now a new technology called VDSL, which is basically a faster version of ADSL, however it works over a much shorter cable distance (a few 100m) and generally requires even better quality cables.
Many people believe (and I am one of them) that the copper cable has simply reached the end of it’s useful life. The only option left for faster speeds is to change the technology completely: Fibre optics.
Fibre optic cables are designed to carry extremely high frequencies, they can therefore be used to carry data at incredibly high speeds. Indeed the speed limitation is not in the fibre itself, but the devices at each end that are used to process the signal. The current speed record for fibre is 1.05 petabits/s – that is around 100 million times faster than ADSL speeds (typically around 10 megabits/s).