Bandwidth cap 公平使用原则
（重定向自Download Limit）
A bandwidth cap, also known as a band cap or a data cap, limits the transfer of a specified amount of data over a period of time. Internet service providers commonly apply a cap when a channel intended to be shared by many users becomes overloaded, when the provider sees an opportunity to exploit their customers lack of competitive options or understanding of the services and technology. Implementation of a bandwidth cap is sometimes termed a fair access policy, fair usage policy, or usage-based billing.