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A cable modem acts like a LAN interface by
connecting a computer to the Internet. The cable modem connects a
computer to the cable company network through the same coaxial
cabling that feeds cable TV (CATV) signals to a television set.
Generally, cable modems are designed to provide Internet access only,
whereas analog modems or ISDN adapters allow dial-in to any service
provider or service in a remote access server. With a cable modem, the
cable company must be used.
The cable modem service is also an "always-on" technology, similar to DSL. A standard cable modem has two connections. One port is connected to the TV outlet and the other is connected to the subscriber's PC. The cable modem will then communicate over the cable network to a device called a Cable Modem Termination System (CMTS). The speed of the cable modem depends on traffic levels and how the overall network is set up. Although the server being contacted is at a remote location, cable modem access is more like a direct LAN connection than remote access. A dial-up connection may be required for data upload using the local phone line. This is because the cable company infrastructure is still one-way. In such a case, a phone jack will be built into the cable modem. Cable modems are capable of receiving and processing multimedia content at 30 Mbps, which is literally hundreds of times faster than a normal telephone connection to the Internet. In reality, though, subscribers can expect to download information at speeds of between 0.5 and 1.5 Mbps because the bandwidth is shared by a number of other users in the neighborhood. The modem receives digitally altered signals. A demodulator is built into the modem and, if it is a two-way modem, a burst modulator is used for transmitting data upstream. Cable modems are available as internal and external units. Most internal cable modems are in the form of PCI cards. An external cable modem is a small box with a coaxial CATV cable connection. Usually, a splitter is used to divide the signal between the TV and the cable modem. The box is connected to an Ethernet card in the computer through UTP Ethernet. External USB devices may also be available to connect the modem to the computer USB port without requiring an Ethernet card. There is currently no standard for cable modems in the cable access industry. As a result, there are many competing proprietary products. Cable service, speed, reliability, setup, and configurations may vary significantly from one cable company to another. Currently, the most common cable modem brands are from Cisco Systems, 3Com, Com21, Bay Networks, Motorola, RCA, Toshiba, and Terayon. The following sites offer more information:
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