How does a cable modem work?
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 six MHz wide at frequencies between 88 to 750 MHz. Using QAM 64 technology, downstream speeds can reach up to 27 MBs on the six MHz channel through which the data is transferred. This signal usually runs adjacent to other television signals. Cable modems transmit upstream signals at frequencies between five and 42 MHz. This frequency range carries more noise than the downstream signal, such as noise from the home, loose connectors on the cable line, etc. All of this noise makes the upstream signal more difficult to successfully transmit. Although QPSK modulation technology makes the upstream signal more robust at this noise level, it sends data at slower speeds than the QAM 64 technology.
Charter designates one cable channel to receive Internet and one cable channel to send to the Internet. A cable modem recognizes and communicates with these two channels, providing access to the Internet. Because our network uses fiber technology, Charter is able to provide more bandwidth than DSL or dial-up. And more bandwidth means faster service!
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 practically 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 network interface 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 channel, somewhere between 50 MHz and 750 MHz. Currently, 64 QAM is the preferred downstream modulation technique, offering up to 27 Mbps per 6 MHz channel. This signal can be placed in a 6 MHz channel adjacent to TV signals on either side with
Simple. Your cable modem talks to a Cable Modem Termination System (CMTS), the component that exchanges digital signals with cable modems on a cable network. The CMTS is located at the local office of your cable television company. The downstream information flows to all connected users, just like an Ethernet network. It’s up to the individual network connection to decide whether a particular block of data is intended for it or not. On the upstream side, information is sent from the user to the CMTS. Other users don’t see that data at all. The narrower upstream bandwidth is divided into slices of time, measured in milliseconds, in which users can transmit one “burst” at a time to the Internet. The division by time works well for the very short commands, queries and addresses that form the bulk of most traffic from the consumer to the Internet. A CMTS will enable as many as 1,000 users to connect to the Internet through a single 6 MHz channel. Since a single channel is capable of 30-40
Cable Internet access operates as its own cable channel and, therefore, co-exists with the video/audio channels offered by the cable company. Un-modulated digital signals are represented as being high (1) or low (0). How fast these levels can switch during transmission is determined by the “bandwidth” of the system. A cable modem modulates and demodulates electrical signals in the same sense that the telephone modem does. However, since coaxial cable can carry much higher wave frequencies, cable modems are far more sophisticated.
