How CATV transfers data

On the downstream channel, each packet of data is broadcast to each subscriber in a similar manner to Ethernet. Each cable modem has its own 48-bit address which is unique to that modem and the address of the destination modem is included in each packet that is sent. Only the modem that it is addressed to picks up the packet, and it is ignored by all the others.

Since the upstream channel is shared between subscribers, when the modem needs to send data (e.g. to request a webpage), it requests transmission time from the headend (along a seperate channel) before being allowed to transmit. This can cause problems when two different signals try and send at the same time. In this case, the stronger (closer) signal swamps the weaker and so measures have to be taken to ensure fairness. Usually, this involves the cable modem itself detecting how far that it is from the headend and adjusting its signal strength accordingly to ensure fairness.

Return to previous page]

Introduction - Modems - ISDN - DSL - cable modems - Mobile comms - Conclusion - Glossary

The original text and layout of this page are copyright © Rajnish Bhaskar, 1997 – 2008. All images are copyrighted to their respective owners. All copyrights and trademarks are acknowledged on the copyrights page. If you have any comments or anything else to say, email me at and tell me. Read the privacy policy at LordOfTheMoon.com