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A bandwidth cap, also known as a band cap or a data cap,〔(Arstecnica: Comcast plans data caps for all customers in 5 years, could be 500GB )〕 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''. As of October 2015, there were 58 wired broadband providers in the US that used data caps.〔(BroadbandNow: Wired Broadband Providers with Data Caps )〕 ==Standard cap== In many situations, each user of a network has been expected to use high speed transmission for only a short time, for example to download a megabyte web page in less than a second. When use is continuous, as it might be in the case of file sharing, Internet radio or streaming video, a few users who use the connection at high rates for hours at a time may seriously impair the service of others. The concept is more relevant in cable internet where both the core network and the access network are shared, than in DSL where the core network is shared but the access network is not. It is most relevant in wireless internet, particularly satellite internet, where both the core network and the access network are shared and total network bandwidth is relatively narrow. One type of bandwidth cap, administered by an Internet service provider (ISP), simply limits the bitrate or speed of data transfer on a broadband Internet connection. The purpose of bandwidth capping is to prevent individual users from consuming the entire transmission capacity of the cable, a shared resource. Critics have charged that it is a method to charge consumers more by introducing tiered bandwidth caps.〔http://www.nctc.net/index.php?option=com_chronocontact&Itemid=100<〕 Capping might be handled by the user's cable modem. Knowledge of capping has led to attempts at uncapping. When uncapping succeeds, the resulting data transfer rate is supposed to be extremely fast, but users who are caught are said to be banned permanently by broadband ISPs. Uncapping is considered theft of service by ISPs.〔http://www.rogers.com/web/content/network_management〕 Sophistication is possible, and even required in limiting bandwidth. The simplest approach simply limits the data rate. The problem with the simplest approach is, a very active user could consume the maximum bandwidth continuously, theoretically imposing an excess burden on the ISP and possibly reducing the performance of other users. Channel capacity is a finite resource, using huge amounts of it can be deemed an abuse in countries with poor communications infrastructure. Dial-up ISPs often published policies that tried to clarify the difference between "unmetered" and "unlimited". A more sophisticated approach is called "bursting", the administrator would specify a "peak rate limit", a lower "sustained rate limit", and a "credit limit". If you continuously saturate your connection, you will only get the sustained rate. While you are idle or use less than the sustained rate, you accumulate a credit, in bytes, up to some limit. If you try to saturate your connection after idling, you will get the peak rate until your credit runs out, at which point you will again be running at the sustained rate limit. The transition from peak rate to sustained rate could be abrupt, gradual, or even an arbitrarily designed curve. If you alternately idle and saturate, your long-term average data rate will never exceed the sustained rate limit, and your short-term data rate will never exceed the peak rate limit. This is known as a Token bucket. Other schemes or models are possible to regulate bursting. Running at the peak rate could accumulate a debt. Once the debt reaches a limit, the user is held to the sustained rate limit and the debt does not change. Only running below the sustained limit (or idle time) pays back the debt. The behavior is similar. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bandwidth cap」の詳細全文を読む スポンサード リンク
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