Cable modems have the primary function of bringing high-speed Internet access to consumers. They can take advantage of the existing high bandwidth available on the cable-television network that providers have rolled out to the majority of urban residences in many developed countries. Prior to the availability of such cabling systems, standard Internet access involved relatively slow dial-up access via the public switched telephone network.
Users in a single locality share the available bandwidth provided by the single coaxial cable in the neighborhood. Therefore, depending on the usage of the service (known as contention) Internet connection speeds can vary from blisteringly fast to dead slow. Note however that uncertainty affects any measurements of internet speed, as bottlenecks can occur anywhere outside a user's or an ISP's control.
Cable modems most commonly use DOCSIS standard (Data Over Cable Service Interface Specification).
Cable modem manufacturers include: