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 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.

Extra Space

You might think that a television channel would take up quite a bit of electrical "space," or bandwidth, on a cable. In reality, each television signal is given a 6-megahertz (MHz, millions of cycles per second) channel on the cable. The coaxial cable used to carry cable television can carry hundreds of megahertz of signals -- all the channels you could want to watch and more.

In a cable TV system, signals from the various channels are each given a 6-MHz slice of the cable's available bandwidth and then sent down the cable to your house. In some systems, coaxial cable is the only medium used for distributing signals. In other systems, fiber-optic cable goes from the cable company to different neighborhoods or areas. Then the fiber is terminated and the signals move onto coaxial cable for distribution to individual houses.

Streams

When a cable company offers Internet access over the cable, Internet information can use the same cables because the cable modem system puts downstream data - data

sent from the Internet to an individual computer -- into a 6-MHz channel. On the cable, the data looks just like a TV channel. So Internet downstream data takes up the same amount of cable space as any single channel of programming. Upstream data -- information sent from an individual back to the Internet -- requires even less of the cable's bandwidth, just 2 MHz, since the assumption is that most people download far more information than they upload.

Putting both upstream and downstream data on the cable television system requires two types of equipment: a cable modem on the customer end and a cable modem termination system (CMTS) at the cable provider's end. Between these two types of equipment, all the computer networking, security and management of Internet access over cable television is put into place.

Inside the Cable Modem

Cable modems can be either internal or external to the computer. In some cases, the cable modem can be part of a set-top cable box, requiring that only a keyboard and mouse be added for Internet access. In fact, if your cable system has upgraded to digital cable, the new set-top