CATV for data transfer

The CATV system uses a single TV channel (8MHz or 6MHz in Europe and North America respectively) for downstream transfer to transfer data to a group of subscribers (up to about 1000) over a substantial range (typically 100Km). This uses 64 or 256 level QAM giving actual data rates of 31.2Mbps to 51Mbps. Note that the actual data rate that you get depends on how many people are online simultaneously.

The same group of subscribers also shares the same upstream channel. This is always in the lower range of frequencies that CATV allows (5-42MHz in North America or 5-65MHz in Europe) which is usually noisier than the higher frequencies. This did not matter when the system was devised since it would only be used for limited interactivity. However, this noisier channel causes problems when sending data, and this restricts the data rates that are possible. Rather than the 64/256 level QAM that is possible for the downstream channel, 16 level QAM is used. Practically, this means that the downstream data rate is about an order of magnitude (factor of 10) faster than the upstream rate. However, as with ADSL, due to the way that the Internet is usually used, this is not overly important.

To allow data to be transferred along the CATV system, special equipment (called a Cable Modem Termination System – CMTS) has to be attached to the headend.

For more information on how data is transferred using CATV, click here.


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