Cable Modem Theory

by: Rolf V. Østergaard

Cable Modem Installation - Page 5

When installing a Cable Modem, a power splitter and a new cable is usually required. The splitter divides the signal for the "old" installations and the new segment that connects the Cable Modem. No TV-sets are accepted on the new string that goes to the Cable Modem.

The transmitted signal from the Cable Modem can be so strong, that any TV sets connected on the same string might be disturbed. The isolation of the splitter is also normally not sufficient, so an extra high-pass filter is often needed in the string that goes to the TV-sets. The high-pass filter allows only the TV-channel frequencies to pass, and blocks the upstream frequency band.

Data-interface

On any kind of external cable modem (the majority of what is in use today), you obviously need some kind of data-interface to connect the computer and the cable modem.

Ethernet

On most external modems, the data-port interface is 10 Mbps Ethernet. Some might argue that you need 100 Mbps Ethernet to keep up with the max. 27-56 Mbps downstream capability of a cable modem, but this is not true. Even in a very good installation, a cable modem can not keep up with a 10 Mbps Ethernet, as the downstream is shared by many users.

The 1st version of the MCNS standard, that dominates the US market, specified 10 Mbps Ethernet as the only allowable data-interface. The DVB/DAVIC standard is totally open, allowing any type of interface. Other types of interfaces are being incorporated in the MCSN standard to allow for a wider range of cable modem configurations.

USB (Universal Serial Bus)

Among others, Intel recently announced that they are working with Broadcom on cable modems with USB interface. This is expected to bring down the installation hassle for the many users with less computer skills. Obviously you do not need to open the box to install an Ethernet card, if the computer has an USB interface. If the computer does not have an USB interface, you will need to install that (and you are back to about the same hassle-level as with the Ethernet interface).

Cost

The installation cost is a significant issue, as this is something that needs to be done in the house of every subscriber. The CATV operators and equipment manufactures needs to try really hard to push down the installation cost, to keep the whole operation profitable.