ADSL
ADSL (Asymmetric Digital Subscriber Line) is a technology for transmitting digital information at a high bandwidth over existing phone lines to homes and businesses. Unlike your regular dialup phone service, ADSL is always on.
The asymmetrical nature of ADSL stems from the fact that the downstream channel steals the major part of the bandwidth. For example, the downstream channel may be 256Kbps while the upstream channel is only 64Kbps. In common with cable, the asymmetric nature of ADSL exploits the fact that when surfing the Web, for instance, most of the multimedia information flows downstream to the user.
ADSL simultaneously accommodates an analogue voice channel on the line by dividing up the copper wire's 1.1MHz frequency spectrum. There are a couple of caveats: the bottom 4KHz is reserved for POTS and the amplification on the POTS segment is not the same over the various frequency ranges. There is a large guard band or separator between the POTS and data bands with the data consisting of 256 4KHz frequency domains from 64KHz to 1.1MHz. This separation of frequency domains is known as Discrete MultiTone (DMT).
Even though ADSL uses your existing POTS copper pair, it is not simply a matter of plugging in an ADSL modem and off you go. At your local exchange, which must be less than 3.5km away for ADSL to work, your copper pair is unplugged from the POTS and plugged into a DSLAM (Digital Subscriber Line Access Multiplexer).
Obviously in many rural areas you will be very lucky to be this close to an exchange, so in many cases ADSL may not be available (presuming of course that the local exchange does in fact have a DSLAM).
While we're on the subject of DSLAMs, many ADSL providers actually resell access to Telstra's infrastructure. However, this is changing as more and more providers, such as Pacific Internet, are renting space at Telstra's exchanges and installing their own DSLAMs. (Pacific Internet advised ZDNet Australia subsequent to publication of this article that they were not undertaking this activity.) Other providers that are also rolling out their own DSLAMs include XYZed (which has over 100), RequestDSL (over 30), iPrimus (around 20), and AAPT.
Basically, ADSL speeds range from 256Kbps to 6Mbps downstream and 64Kbps to 640Kbps upstream. As you effectively have your own dedicated line, this performance is pretty much assured. At some of the higher data rates, though, the price can rise quite dramatically.
I was sure you couldn't get T1 and T3 services in Australia, that it's a US standard, instead we have 2Mb links instead of their 1.54 ?