If you have cable TV at home, then depending on where you live in the world it may be possible for you to enjoy the benefits of a cable modem (if your not doing so already). You are probably already aware that a cable modem is a device that is designed to provide a broadband Internet connection without the use of a phoneline. But, have you ever wondered exactly how cable modems work? If so, then read on! This article is designed to inform you exactly that, plus a little more on the side ;-) However, some of this article delves into technical depth so if you have trouble understanding parts of this article then please do consult the Talkbroadband Forums.

Cable Modem Types:

External Cable Modems:


External meaning outside of your computer, an external modem is a small box that can provide a computer with an Internet connection through either USB or Ethernet. As of recent times, most cable modems do come with USB ports installed as well as the usual Ethernet port. If you have an external cable modem and wish to use Ethernet to connect it to your computer, you will need to install a NIC (Network Interface Card) inside your computer.

Internal Cable Modems:


Internal cable modems come in the form of a PCI (Peripheral Component Interconnect) bus card for PC's. There are alternative standards (PCMCIA devices for example) for Apple Mac's and laptops. There are certainly disadvantages when using internal cable modems. For instance, if you want to power down just the cable modem itself, then it would mean completely shutting down the computer.

Set Top Box Integrated Cable Modems:


In some instances a cable TV operator will issue you a set-top box with an in-built cable modem. This type of cable modem is commonly used for services like interactive TV and Internet via your television. However, there are a number of companies that use this type of cable modem to supply Internet access to your computer. Connecting a computer to the set top box cable modem via Cat5 Ethernet cable will also provide Internet access. See diagram below:

Downstream / Upstream

Cable Modems Use CATV Infrastructures

CATV Networks
CATV (Cable Access Television, sometimes known as Community Antenna Television) is referred to as cable TV. Cable modems use CATV infrastructures that partly consist of HFC (Hybrid Fibre-Coax).

HFC (Hybrid Fibre Coax) Networks
A HFC network consists of both fibre optic cable and coaxial cable. They are used in different sections of a network to transport broadband data. On HFC networks, companies can use fibre optic cable from the networks head-end to all nodes. From these nodes a coax cable then feeds towards the cable modems in residential homes or business locations/offices.

• A Tuner
• A Modulator
• A Demodulator
• A MAC (media access control) device
• A CPU

The Tuner
The tuner connects to the CATV outlet. The tuner receives digital modulated signals and then forwards them towards the demodulator. There will be circumstances that the tuner can be attached to a diplexer, which enables the tuner to use a range of frequencies for both the downstream and upstream traffic.

Demodulator
The demodulator has four main functions, these are:


• QAM (Quadrature Amplitude Modulation)
  
• A/D converter (Analog to Digital)
  
• Error correction
  
• MPEG (Moving Picture Experts Group) synchronizer
  
  The QAM takes the radio-frequency signal and changes it so that the A/D converter can process it. The A/D converter then converts the signal into a digital signal of 0's and 1's. The error correct module then compares the data received to a known standard, fixing errors in transmission. The MPEG synchronizer is then used to sequence and organise data groups into their correct order.

Modulator or Burst Modulator
The modulator has three main functions, these are:


• To insert data that is ready for error correction
  
• QAM (Quadrature Amplitude Modulation)
• D/A conversion (digital to analog)
  
  A modulator, sometimes known as the burst modulator, converts digital data back into analog radio frequency signals so that they are ready for transmission.

MAC (Media Access Control)
The MAC acts as the interface between parts of various network protocols. Either the CPU (Central Processing Unit) of the cable modem or the CPU of the computer is used to take some pressure away from the MAC because of the complexity of its functions.

CPU (Central Processing Unit)
Most cable modems have a CPU inside them which handles processing in the same way as the CPU inside hardware dial-up analog modems. The CPU is the brain that controls all of the activity inside the cable modem itself.

As you can see a cable modem is essentially an extremely powerful and in some ways complex radio transmitter and receiver. The technology works using radio signals transmitted to and from the cable modem which serves to translate these radio waves into the digital signals used on a computer system. Many cable operators use fibre optic networks which although at the moment are limited in the bandwidth they provide to customers they do have the backbone in place to potentially provide extremely high bandwidth Internet far exceeding that which xDSL technology is capable of achieving.

Also cable modems literally provide an 'always on' connection due to the fact that they themselves maintain a bridge to the Internet regardless of the status of your computer hardware. xDSL however actually uses the computer hardware to establish and maintain its Internet connection, therefore making the technology only online as long as your hardware is operational.