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TekSpek Modems / Routers
Broadband ADSL vs Cable

Broadband ADSL vs Cable

Date issued:

The purpose of this TekSpek is to delineate the differences in broadband Internet connections and to explain the hardware required in each case.

On the surface we're asking a relatively simple question here: "What do I need to get my broadband Internet working?" Of course, it's not quite as simple as that.

Worry not, however, for this TekSpek covers both the differences between the two popular kinds of broadband and what you'll need to get your home computer(s) connected up.

ADSL and cable broadband
ADSL and cable broadband Broadband Internet is a term used to describe a high-speed Internet connection; a connection that's generally a lot faster than the dial-up modems of old. However, the word broadband doesn't define how that high-speed connection is obtained. It could be from a leased line, satellite link, ISDN, ADSL, cable or others. However, only two of the aforementioned technologies are used by the majority of residential users; ADSL and cable.

So what are they? We'll start with ADSL.

ADSL An Assymetric Digital Subscriber Line utilizes the copper phone cable that is present in a vast number of houses. Regular voice conversations are transmitted over these copper cables in the frequency range from 0 to 4KHz. While this is sufficient for voice (and what was used by dial-up modems) copper can carry signals at higher frequencies than this. Digital Subscriber Lines exploit this fact and use frequencies beyond the 4 KHz used for regular phone calls.

ADSL uses the range 26KHz to 138KHz for its data upstream, and 138 KHz to 1.1MHz for the downstream. Notice that the downstream has a greater frequency range… this is why downloads over ADSL are quicker than uploads, and also why it's referred to as Assymetric.

To use ADSL, you need a few things, not all of which are in your control. Firstly, you need to be within a reasonable distance of your telephone exchange. The further you are away from the exchange, the weaker the transmitted and received signals are, slowing down the maximum speed of the connection. There comes a point where the line is just too long to run a stable, fast ADSL connection. ADSL providers will check the line to see if it's suitable.

Second, the telephone exchange must have the correct hardware in it. Your broadband connection needs to connect to what's known as a DSLAM, located within the exchange. A small number of rural exchanges in the UK remain without ADSL capabilities.

Now for the bit you can control. Much like with dial-up, a modem is required, but this is a special ADSL modem. An ADSL modem will establish a link with the DSLAM in the exchange, and check the quality of the line in 4KHz frequency blocks across the range of frequencies we mentioned earlier. It'll only use blocks that aren't too noisy. ADSL can operate at up to 8Mbps downstream (1Mbps upstream), providing your ISP supports it. However, the modem will ultimately decide how good the line is and thus what speed it will operate at.

Numerous ADSL modems are available and they're so cheap that ISPs tend to bundle them for free upon subscription. However, if you want to connect multiple machines to the ADSL line, you'll need an ADSL router. This is a router with an ADSL modem built into it, and crucially, is where people often fall foul of the difference between ADSL and cable. ADSL routers don't work with cable and vice-versa. There are (sometimes) solutions to get around this, but for simplicity's sake, it's best to ensure that you get an ADSL router.

One final thing you'll need for ADSL is filtering. While an ADSL modem won't create anything particularly audible in a phone conversation, the modems themselves can be interfered with by telephones and other devices that use voice frequencies down the line. By plugging all telephones, etc., into ADSL micro-filters (also known as low-pass filters), they can only transmit and receive at the frequencies they're designed for, meaning they don't add any high frequency interference to the line.

Poor or long internal house telephone wiring can also hinder ADSL speeds. It's often best to connect the ADSL modem or router to the master socket, keeping it as close to the exchange as possible and keeping noise to a minimum.

Onto cable, then. This is a technology that makes use of available bandwidth on cable television networks. In a cable network, a residential area will have a cable line running through it, which all subscribing properties are connected to. As such, the more people in one area using cable Internet, the slower it'll be for individual users, and providers don't usually stipulate a minimum speed.

Cable Internet users connect to the cable network by means of a 'Cable Modem', which is essentially a bridge between the cable network and the user's computer or home network. Often, a cable modem will bridge between an Ethernet network and the cable network. This means the user can connect up a router via the Ethernet port on the modem. The router itself doesn't need a modem. Routers with built-in cable modems exist, and some providers may sell/loan them to subscribers. However, an ADSL modem will not work with a cable Internet system.

To get cable Internet, there needs to be a cable service provider operating in your area. Here in the U.K., quite a few places don't have cable coverage, leaving them with only ADSL or dial-up as choices - unless users fancy more exotic connection types. As we've already mentioned, cable networks are shared between users, but so are ADSL Internet connections… just further upstream than cable. Generally, cable Internet is more stable, because it doesn't depend on the quality of the user's telephone line and its distance from the exchange.

Which is best ?
This is really a matter of opinion. For some it doesn't matter, because they either don't have a cable service in their area, or can't get a decent ADSL service. For those with the luxury of choice, well, see what ISPs are offering and find something that suits you.

Still, regardless of what's available to you and which type of broadband suits your needs, you should now be aware of the basic differences between the two technologies, and be able to choose the right router/extra hardware to go with it. If you're still in doubt, investigate before you buy to avoid confusion and inconvenience.