Cable Modem

A communication device that Modulates and demodulates (MoDem) data signals to and from a cable television system. Cable modems select and decode high data-rate signals on the cable television system (CATV) into digital signals that are designated for a specific user.

There are two generations of cable modems; First Generation one-way cable modems transmit high speed data to all the users into a portion of a cable network and return low speed data through telephone lines or via a shared channel on the CATV system. First generation cable modems used asymmetrical data transmission where the data transfer rate in the downstream direction was typically much higher than the data transfer in the upstream direction. The typical gross (system) downstream data rates ranged up to 30 Mbps and gross upstream data rates typically range up to 2 Mbps. Because 500 to 2000 users typically share the gross data transfer rate on a cable system, cable modems also have the requirement to divide the high-speed digital signals into low speed connections for each user. The average data rates for a first generation cable modem user rage up to 720 kbps. Until the late 1990's, most cable modems used first generation technology.

Second generation cable modems offered much data transmission rates in both downstream and upstream directions. Second generation cable television systems use two-way fiber optic cable for the head end and feeder distribution systems. This allows a much higher data transmission rate and many more channels available for each cable modem. As of the year 2000, approximately 35% of the total cable lines in the United States had already been converted to HFC technology.

This figure shows a block diagram of a cable modem. This diagram shows that a cable modem has a tuner to convert an incoming 6 MHz RF channel to a low frequency baseband signal. This signal is demodulated to a digital format, demultiplexed (separated) from other digital channels, and is decompressed to a single data signal. This data signal is connected to a computer typically in Ethernet (e.g. 10 Base T Ethernet) or universal serial bus (USB) data format. Data that is sent to the modem is converted to either audio signals for transfer via a telephone line (hybrid system) or converted to an RF signal for transmission back through the cable network.

Cable Modem Diagram

Related Cable Television - CATV Terms

Contribution Network
Headend
Off Air Receivers
Rate Shaper
Channel Signal Combiners
Metadata Management
Playout Scheduling
Hybrid Fiber Coax - HFC
Cable Ready Televisions
Cable Modems
Cable Telephone Adapters
Cable Telephony
Wireless Cable
Video on Demand VOD
National Television Standards Committee NTSC
Phase Alternating Line PAL
Data Over Cable Service Interface Specifications DOCSIS
Pay per View PPV

Cable Television - CATV Books

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Introduction to Cable Television (CATV)

This book provides an overview of cable television system technology including cable modems, digital television, HDTV, along with how cable systems are converting from analog systems to digital networks so they can offer advanced services such as ultra broadband, video on demand (VOD) and addressable advertising.

$19.99 Printed, $16.99 eBook

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