ADSL is a kind of broadband Internet connection that you can get over your existing phone line. When people hear about this, they often wonder how on earth it can be true. Doesn’t the phone line need to be upgraded? How can all that extra data fit through an ordinary phone line? Why weren’t they doing this years ago, when I was still dialling up with my old modem? To understand the answers to these questions, it’s necessary to learn a little about how ADSL works.
ADSL stands for Asymmetric Digital Subscriber Line. The ‘asymmetric’ part simply means that you can download over the line faster than you can upload – good for home users, who generally download (get things from the Internet) far more than they upload (send things to it).
It’s the ‘digital’ part that is important, however. If you think back to old dial-up modems, remember the screeching noises they made when they were connecting? That’s because they were encoding the data they sent over the phone line as sound – all those ones and zeros were becoming a series of sounds. It was an analogue to digital to analogue conversion at each end, and was wildly inefficient.
What DSL does is use the unused frequencies of the phone line (that is, the parts that aren’t needed for voice signals) to carry digital signals, in the form of electrical pulses instead of sound – it’s a successor to ISDN, the expensive business precursor of the technology. This method is much, much faster than the old analogue way. The unused part of the line isn’t that big, but DSL splits it up into many smaller pieces and uses each one separately, allowing multiple ‘channels’ of data to be sent and received at once.
