Introduction to Cable Modems

Introduction to Cable Modems

The idea behind cable modems is to allow data communications over existing cable TV systems, without requiring a wholesale replacement of the cable TV infrastructure. In reality, some modification of the infrastructure is required - such as for junction boxes - but normally operators can avoid any mass re-laying of cables.

Cable modems in fact can trace their history to the early days of Ethernet, when broadband Ethernets were being developed. These never became popular in the marketplace but the ideas have now been revamped. The original simple ideas about cable modems derived from Ethernet were not much good in reality because the electromagnetic environment within an existing cable TV system is very noisy and there is limited bandwidth available for upstream communications (set-top to head-end). But some expert developers have now got involved, many from backgrounds of experience in radio, cellular or military communications where noise is a fact of life.

The first operational generation of cable modems were of limited throughput - around the 1 megabit/s level - and there was much discussion of how useful they actually were in real cable networks. Since then the speed and the reliability has improved greatly - the current generation of operational modems, such as from Motorola, work at Ethernet speeds - 10 megabit/s. This sounds really fast, but that is ignoring some of the design features of Ethernet.

Ethernet does not give an actual 100% data throughput - the total throughput is only about 30% of 10 megabit/s, that is, 3 megabit/s. This throughput has to be shared between all the users of cable modems on a given cable TV segment, and there might be 100 of these with, say, 10 using it at a time, so one can expect a usable bandwidth downstream (where one needs high bandwidth) of around 300 kilobit/s. That is still pretty fast compared with ISDN, and likely to be a lot cheaper too - when the services are ready, which is not quite yet. But 300 kilobit/s does not allow broadcast-quality video, although it is more than adequate for Internet video-conferencing.

Types of Cable Modem

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