A Cable Modem is a device that mediates data transfer between individual computers and a larger network, such as that used by a Cable Internet service provider. Much like the now-antiquated dial-up modem technology, they convert signals between digital and analog formats. The term 'Modem' is a portmanteau of the words 'Modulate' and 'Demodulate'. Modulation is the process of impressing information on an electrical wave at a set frequency and voltage, so that it can travel across long distances via media such as copper wire, fiber optic cable, or radio signals. Demodulation is the process of reading the information out of a received wave so it can be processed by a computer or other electronic device.
Modern modems of this variety are composed primarily of three parts: a System-on-Chip (SoC), analog/digital conversion hardware, and a power adapter. The SoC has all the components of a very basic computer running a stripped-down operating system, typically based on Linux or Unix. This is the component in charge of handling all the traffic to and from the computers that the modem attaches to the larger network. Analog/Digital conversion hardware changes digital DC signals to AC ones that are modulated onto the network's carrier signal.
These modems typically have three ports. These are the power adapter input, coaxial connector, and Ethernet connector. The power connector converts 120 or 230 volt household current into about 12 volts of DC current. The coaxial connector needs to be attached to a cable outlet in the same way a TV would. The Ethernet connector can be attached either to a single computer, or to a router.
Connecting a Cable Modem to a router allows more than one computer to be attached at a time. A wireless router allows laptops, cell phones and other devices to connect to the modem without Ethernet cables. The supported wireless protocols are 802.11a, 802.11b, 802.11g, and 802.11n. Although 802.11n is not yet fully standardized, many devices support the draft specification regardless. 802.11a is mostly antiquated. 802.11b, though now less common than 802.11g, is not common to see nowadays. Typically, one or more wired Ethernet devices can also be connected to a wireless router.
A modem is usually provided by a cable Internet provider. You can typically provide your own, however, a provider typically only supports one or two specific models. The MAC address, which is usually printed on a sticker somewhere on the unit, needs to be given to the service provider in order to link it to your service account. It is very important to get a modem that supports the latest standards used by the Cable Internet network. The latest standard is DOCSIS 3.0 (EuroDOCSIS 3.0 in Europe.)