Cable Modem Theory

by: Rolf V. Østergaard

Cable Modem Types - Page 4

A number of different Cable Modem configurations are possible. These three configurations are the main products we see now. Over time more systems will arrive.

External Cable Modem

The external Cable Modem is the small external box that connect to your computer normally through an ordinary Ethernet connection. The downside is that you need to add a (cheap) Ethernet card to your computer before you can connect the Cable Modem. A plus is that you can connect more computers to the Ethernet. Also the Cable Modem works with most operating systems and hardware platforms, including Mac, UNIX, laptop computers etc.

Another interface for external Cable Modems is USB, which has the advantage of installing much faster (something that matters, because the cable operators are normally sending technicians out to install each and every Cable Modem). The downside is that you can only connect one PC to a USB based Cable Modem.

Internal Cable Modem

The internal Cable Modem is typically a PCI bus add-in card for a PC. That might be the cheapest implementation possible, but it has a number of drawbacks. First problem is that it can only be used in desktop PC's. Mac's and laptops are possible, but require a different design. Second problem is that the cable connector is not galvanic isolated from AC mains. This may pose a problem in some CATV networks, requiring a more expensive upgrade of the network installations. Some countries and/or CATV networks may not be able to use internal cable modems at all for technical and/or regulatory reasons.

Interactive Set-Top Box

The interactive set-top box is really a cable modem in disguise. The primary function of the set-top box is to provide more TV channels on the same limited number of frequencies. This is possible with the use of digital television encoding (DVB). An interactive set-top box provides a return channel - often through the ordinary plain old telephone system (POTS) - that allows the user access to web-browsing, email etc. directly on the TV screen.