A data cap, often erroneously referred to as a bandwidth cap, is an artificial restriction imposed on the transfer of data over a network. ... Implementation of a data cap is sometimes termed a fair access policy, fair usage policy, or usage-based billing by ISPs.
Generally, each user of a network is expected to use high speed transmission for a short time, for example to download a megabyte web page in less than a second. Continuous usage, such as when sharing files or streaming videos can seriously impair service for others. In DSL, where the core network is shared but the access network is not, this concept is less relevant. However, it becomes more relevant in cable internet, where both the core network and the access network are shared, and on wireless networks, where the total network bandwidth is also relatively narrow.