Communication: How Cable Modems Work

Cable is the broadband choice for many Internet users, but have you ever wondered how it works? Read on!

A cable modem is used to transfer information over the television's cable lines. Data that is transferred from the Internet to your computer is converted onto a 6 MHz channel, so that on the television cable, that data appears to be a TV channel, and downstream data takes up no more space than any other channel of programming. Information coming from a computer to the Internet (also known as "upstream") takes up considerably less space - just 2 MHz on the cable. Fortunately, most cable lines hold several hundreds of MHz of information, making it an optimal form of transfer, but for it to work the computer user requires a cable modem and provider with a cable modem termination system. One of these systems can allow 1,000 people to be connected to the Internet using just one 6 MHz channel. Between 30 and 40 megabits per second can be sent on the single channel, providing higher speed and better performance than dial-up telephone line modems.

A cable modem can either be internal to a computer or external. No matter which type, all cable modems are made up of the same main components - a tuner, demodulator, modulator, media access control device (MAC), and a microprocessor. The tuner is connected to the cable outlet (or to a splitter which separates the television programming channel and the Internet data channel). Because the Internet data comes in via a cable channel that is not being used, the tuner has only to receive this modulated signal and pass it to the demodulator. However, in some cases, a diplexer may be used. This allows the tuner to focus on a set of frequencies for the downstream Internet data. Another set is used for upstream data, or alternatively, a telephone modem may suffice for this purpose. This is because upstream tends to be significantly smaller than downstream data. In either case, the tuner merely passes the signal to the demodulator.

Generally, demodulators fulfill four tasks. First, a device called a quadrature amplitude modulation demodulator changes the radio frequency signal with the information on it into a simpler signal. This simple signal can then be processed by an analog-to-digital converter (A/D converter). The signal is changed into a digital series of zeros and ones by the A/D converter. Then, it is transferred to the error correction module, which checks the information it receives against a standard, fixing any abnormalities in the transmission. The groups of data are most commonly in MPEG format, which means an MPEG synchronizer is required to be sure these groups of data remain in their proper order.



The modulator converts digital computer data into radio frequency signals, as noted earlier. Cable modems that send upstream data via the cable lines use this type of modulator. Due to irregularities in the information traffic between a computer and the Internet, it is sometimes referred to as a burst modulator.

The media access control device (MAC) is used as an interface between a network's hardware and software. It is situated between the downstream and upstream section of the cable modem and is found in all computer network devices. In some cases, where the tasks carried out by the modem are very complex, a few of the MAC's functions may be performed by a central processing unit, either in the modem itself, or in the computer of the Internet user. A microprocessor often performs many of the MAC's functions as well.

The cable modem termination system (CMTS) on the cable provider's end directs incoming traffic from customers on one channel to an Internet service provider or ISP, which then connects it to the Internet. The cable provider will have servers which assign IP addresses to the users via Dynamic Host Configuration Protocols as well as servers used for accounting and logging. Downstream information is sent to all of the connected users and the upstream information moves from a user to the CMTS. The upstream is never seen by other users on the cable provider's system. It is then split into millisecond sections of time, and is transferred to its final destination.

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