How A Cable Modem Works

All across the UK, TV companies have been ripping up the streets, laying fibre optic and coaxial cables to the homes in their franchise areas.

While an extra 50 or so channels of TV programs or cheaper phone calls may be a good reason to have a cable connection, the design of the cable TV network hides one of the secrets of high speed Internet access.

The UK's cable TV systems aren't just one way pipes, pouring video into homes. The network equipment in street-side cabinets and buried under the pavements is designed to provide a return path to the cable companies network centres. Originally intended as part of any future teleputer network, these paths are the key to high-speed Internet access.

Calling a cable modem a modem is something of a misnomer. Like a telephone line modem, a cable modem modulates and demodulates signals, but that's where the similarities end because the cable modem is much more complex than the traditional device.

It's probably safer to compare a cable modem to a network card or a small router because a cable modem can be used to provide shared access to a small network of PCs, while handling encrypted data streams and authentication.

Cable modems have actually been around a while, with one of the first systems developed by First Pacific Networks initially tested in the UK in the early 1990s - though it was originally intended to be used as a means of delivering telephone calls over a cable TV network.

In the end, it proved cheaper for cable TV networks to use traditional technologies for their telephone services, but the prospect of high bandwidth Ethernet-style connections over a cable TV remained.

LANcity has produced a whole family of cable modem products. As well as a personal cable modem, designed for connection to a single PC, LANcity produces a workgroup system for a small home or business network of up to four PCs, and a multi-user system suitable for use in a large business or school. Its unique finned design makes the LANcity cable modems look like a Pentium processor's heat sink.

Like the Motorola system (see heading below) a LANcity cable connects to your PC via a standard network card and a 10base Ethernet cable.

Based around the proprietary UniLINK protocol, the LANcity system offers a full duplex 10Mbps system. If you connect your PC to a LANcity cable modem, and if no one else is on line, you'll get as good a performance as your office's Ethernet LAN.

You'll also be able to take advantage of a network that has been designed to prevent unauthorized access to your PC's data, as the LANcity modem will only forward IP packets to specific machines. This means that no one will be able to listen into your link to the cable TV head-end.

A look at Motorola's CyberSURFR, the most popular cable modem in the US

Motorola's CyberSURFR is the most popular cable in the US. Together with their matching head-end connection equipment, the three largest cable TV networks in the US use CyberSURFRs. Looking like a large external modem, this is, in fact, a very different device.

You can connect it your PC with a standard 10baseT Ethernet cable, or plug it into an Ethernet hub to provide an Internet connection for a small PC network. As the CyberSURFR doesn't use any IP addresses, it works with the rest of the Motorola Cable Data System to forward IP packets from your PC to your cable TV service provider's network and the rest of the Internet.

The CyberSURFR is an asynchronous system, sending data to you much faster than you can transmit it. As most home Internet connections are used for file downloads, Web surfing and e-mail, this isn't a problem. Even so, the CyverSURFR promises to be a lot faster than a traditional modem. Data is transmitted down the cable TV system on a shared 30Mbps channel, but this is shared with all the other users in your area. Even if you're the only person on-line you won't get all 30Mbps though because you're limited to a 10Mbps connection.

Your upstream connection is a lot slower because you're sharing a single 768Kbps channel with your neighbours. Despite this, you'll find it faster than your old modem.

There are of course plenty of limitations on performance; ranging from the specification of your PC, to the number of people accessing the connection equipment at the cable TV companies network centre!

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