How does a cable modem work?
The fact that the word “modem” is used to describe this device can be a little misleading only in that it conjures up images of a typical telephone dial-up modem. Yes, it is a modem in the true sense of the word – it MOdulates and DEModulates signals. But the similarity ends there because cable modems are pratically an order of magnitude more complicated than their telephone counterparts. Cable modems can be part modem, part tuner, part encryption/decryption device, part bridge, part router, part NIC card, part SNMP agent, and part Ethernet hub. Typically, a cable modem sends and receives data in two slightly different fashions. In the downstream direction, the digital data is modulated and then placed on a typical 6 MHz television carrier, somewhere between 42MHz and 750 MHz. There are several modulation schemes, but the two most popular are QPSK (up to ~10-Mbps) and 64QAM (up to ~36-Mbps). This signal can be placed in a 6MHz channel adjacent to TV signals on either side without distu
Cable modems modulate and demodulate RF signals. The cable modem receives data in what is known as the downstream signal. This signal is modulated on television channels that are 6 MHz wide at frequencies between 88 and 750MHz. Using QAM 64 technology, downstream speeds can reach up to 27Mb/s on the 6 MHz channel, through which the data is transferred. This signal usually runs adjacent to other television signals.
A cable television system typically has 60 or more channels, and most of them are used for programming services like CNN, ESPN and HBO. These channels also can be used to offer high-speed Internet access service. Cable modems connect to the cable network with a standard coaxial cable, just like the wiring that’s plugged into your television set to receive cable TV service. The cable modem then connects to your PC through a standard Ethernet interface.
