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Chapter 2Overview of current CATV network p2.4 Cable modem technologyDigital data signals are carried over Radio Frequency (RF) carrier signals on a cablesystem. Digital data utilises cable modems, devices that convert digital information intoa modulated RF signal and convert RF signals back to digital information. Theconversion is performed at the subscriber’s premises, and again by the headendequipment handling multiple subscribers.Cable modems have some advantages over other competing technologies, such as dialupmodem and the digital subscriber line (DSL). Although cable modems can operate atspeeds up to 50 Mbps downstream and 12 Mbps upstream [22] and [35], they arenormally programmed to operate at about 500 kbps - 2 Mbps upstream and ≈ 500 kbps -10 Mbps downstream. Subscribers are always connected, eliminating the call set-uptimes. Some of the disadvantages are that cable modems are less secure than DSLmodems because the line is shared with others in the neighbourhood and data speedsvary according to the number of active users.2.4.1 Cable modem configurationsCurrently there are three configurations for cable modems: external cable modems,internal cable modems, and interactive set-top boxes (STB), as illustrated in Figure 2.4.• External cable modem configurations are common and there are severalcombinations. Four examples are given here. The first combination is to have a PC,Cable Modem Termination System/Interactive Network AdaptorCableModemCable ModemRouter/Bridge/SwitchNIC CardNIC CardNIC Card(Cable Modem)Set-top-box10/100Base HubInteractive -TVExternal Cable ModemInternal Cable ModemSet-top-boxFigure 2.4 – Cable modem configurations.2-7