How Cable Internet Works
Khalid
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.