How Cable Internet Works

Historically Internet access was made possible using Telephone line as the medium of transfer, using a dial-up modem. As the demand for faster Internet access grew, newer technologies evolved such as Internet access via the Cable Television network
For Cable Internet access on PC, a Cable Modem is required at user's end. A cable modem is an external device that connects to the computer to provide high-speed data access via cable TV networks. A Cable Modem sends and receives data to and from the Internet by using the existing coaxial cable network.

The modem translates cable signals the same way a telephone modem translates signals from a telephone line. Cable modems translate radio frequency (RF) signals to and from the cable plant into Internet Protocol (IP), the communications protocol spoken by all computers connected to the Internet.

A cable modem connects to a computer through a device called an Ethernet card (Network Interface Card). This card provides connectivity between the cable and the computer and interprets the signaling from the cable modem so that the computer's software can display it correctly.

Cable TV Networks are high band-with networks i.e. 550 to 750 MHz by their very nature of design. These networks were traditionally built as one way networks carrying 60-100 Cable TV channels downstream i.e. from Headend to the Subscriber. Internet delivery over Cable TV however requires a two-way network since information of selected website, etc. has to be transmitted to the Headend, from where it would be directed to the website of interest. Higher frequencies flow toward the subscriber and the lower frequencies go in the other direction.

Hence setting up of a robust two-way Cable TV network is the first requisite before deploying Cable Modems on a Cable TV network. This is done by upgrades to the amplifiers in the cable distribution network etc.

The Internet signals are in the digital domain and these need to be interfaced to the Analog CATV world. This interface is termed Cable Modem Termination System (CMTS) and typically serves 2000 - 3000 Cable Modems and is connected to a high-speed data link. A typical CMTS consists of an Input interface, Router, Cable Modem card and a powerful Microprocessor.

The CMTS resides at the ISP POP, which receives high bandwidth data from the ISP network center as well as Cable TV signal from the Cable TV Head-end.