Computers store all information as binary numbers. The binary number system uses two binary digits, 0 and 1, which are called bits. The amount of data that a computer network can transfer in a certain amount of time is called the bandwidth of the network and is measured in kilobits per second (kbps) or megabits per second (mbps). A kilobit is 1 thousand bits; a megabit is 1 million bits. A dial-up telephone modem can transfer data at rates up to 56 kbps; DSL and cable modem connections are much faster and can transfer at a few mbps. The Internet connections used by businesses can operate at 45 mbps or more, and connections between routers in the heart of the Internet may operate at rates from 2,488 to 9,953 mbps (9.953 gigabits per second). The terms wideband or broadband are used to characterize networks with high capacity and to distinguish them from narrowband networks, which have low capacity.
Both cable modems and DSL connections are classified as broadband connections because they operate at higher speeds than dial-up connections. The chief difference between cable modems and DSL arises from their intended use. A DSL connection is dedicated to a single subscriber, and a cable modem connection is shared among many (possibly more than 100) subscribers. Sharing means that the amount of service a cable modem subscriber receives depends on how many of the subscribers who share the connection are using the Internet at the same time. If two subscribers use their modems at the same time, each modem operates at approximately half the maximum speed; if 100 subscribers use their modems at the same time, each operates at approximately 1/100 of the maximum speed. Therefore, to provide the same service as a dedicated DSL connection, a cable modem must have a much higher total capacity.