Broadband

These days everyone is moving to broadband. It's getting to the stage where some people get it just because it's there. But there's no doubt about it: for those of us who actually spend any significant amount of time on the net (whether it's for peer-to-peer, driver downloads, gaming, or just browsing and email), there's simply no going back to dial-up.

But just what is broadband? And what's the difference between cable and ADSL? There are so many options, but I hope to put it all into some sort of perspective here...

Cable modem

A cable modem allows a computer to receive and send data over a cable television network. The incoming signal is broadband because a wide range of frequencies is used for transferring data. Hence cable TV channels and incoming data can be shared in the same cable (using a technology known as frequency division multiplexing or FDM). Typically, a 6MHz frequency band is dedicated to downstream data. Because the data travels through the TV cable, you no longer need to sacrifice your phone line.

The cable modem is a true modem, because it takes the incoming analog signal and converts this to digital output. However, a cable modem also has some extra components. First, it will have a tuner to receive the cable signal. In addition, it will have separate modulator and demodulator, a MAC mechanism similar to (but more complicated than) those typically found in an Ethernet LAN card, and a CPU.

In order to use an external cable modem, you will need to connect the unit to your PC. Typically, this is done by attaching it to an Ethernet LAN card. However, many solutions allow you to attach to a USB port. While this may (sometimes) be an easier implementation for the user, USB communication inevitably uses some PC processing power. An Ethernet card will handle much of this processing itself, and is therefore a more efficient option.

One problem with the cable modem approach is that a cable modem user shares bandwidth with any neighbours on the same exchange. If you happen to live in an area where there are many cable modem users, you may well find that your bandwidth is severely compromised! This will become more and more of a problem as cable modem connection becomes ever more popular.

Which provider? Well, I live in the UK and used to be reasonably happy with NTL. That was until they outsourced their broadband technical support to people that don't speak English and who understand nothing about broadband, routers, computers, or even toasters. (Probably.)

So I switched to ADSL, which brings us nicely on to the next part...

Digital Subscriber Line

The conventional twisted pair wiring of telephone lines can actually carry much more data than just voice communication. This is where DSL comes in. It splits the line into one low and one high frequency band. The low frequency band is used for voice communication (i.e. telephone calls) while the high frequency band is used to carry digital information. Typically this band extends from about 25kHz all the way up to 1MHz. It is generally accepted that the frequency range audible to human ears is from about 20Hz up to 20kHz. That said, phone lines do not offer the same quality of sound reproduction that you would get from your average hifi, so in fact the frequency range used by voice communication by telephone is more like 100Hz to 4kHz. As you can see, there is plenty of room for data to sit on the top without risk of interference.

DSL is also adaptive. Modems actually probe the frequencies available on the line to determine wich frequencies will be optimal. Because DSL is adaptive, it does not guarantee a particular data rate.

ADSL is simply a variant of DSL: Asymmetrical DSL. This means that the upstream bandwidth is much lower than the downstream bandwidth. Practically speaking, this is usually okay because most people download much more data than they upload. Most people only upload data when, for example, sending emails or requesting webpages.

One limitation of DSL is that you must be within 3 to 4 miles of your nearest digital switching station. If not, then tough. However, switching stations often appear where demand is high, so if you want ADSL in your area and you can't yet get it, make sure you register your interest!

(Alternatively, you can always use ISDN: Integrated Services Digital Network. Typically, this comes in the form of two 64kbps dedicated phone lines. However beware... These lines are very expensive to rent and very expensive to use. When ISDN first arrived, it was significantly faster than dialup. But modems have improved over the years and these days, there's not much benefit in using ISDN over dial-up.)

What's next

The next section takes a look at TCP/IP.