Bandwidth cap
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A bandwidth cap, also known more precisely as a download cap or bit cap, limits the transfer of a specified amount of data over a period of time. The cap is commonly applied when a channel intended to be shared by many users becomes overloaded, or may be overloaded, by a few.
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[edit] Standard cap
Many ISPs engineered their facilities in the 1990s to use dynamic capacity allocation to serve multiple bursty users. Each user is 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 for file sharing or 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.
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.
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.
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, imposing an excess burden on the ISP and reducing the performance of other users.
Channel capacity is a finite resource. Using huge amounts of it can be deemed an abuse. Dial-up ISPs often published policies that tried to clarify the difference between "unmetered" and "unlimited". An analogy to water supplied to an apartment makes the distinction clear. Water comes with the apartment, and is unmetered, but not unlimited. If one uses water in a way that a typical apartment user would, one will have no problems. If however, one leaves all of the faucets running day and night, one can expect the water connection to be turned off or the water pressure to be greatly reduced.
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 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.
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.
[edit] Lowered cap
Another type of capping refers to the reported phenomenon of an ISP reducing an individual user's bandwidth cap, without notifying that user, as a defensive measure and/or as a punishment for heavy use, especially for upstream traffic. "Servers" tend to use upstream bandwidth heavily, and violate most service agreements. (Cable and other broadband services tend to be asymmetrical, making upstream capacity scarcer than downstream.) Somehow the ISP detects that the user is an offender — perhaps by analyzing traffic to detect the activity of a server, or perhaps by comparing the user's long-term data rate against an unpublished limit. If a user gets tagged as an offender, then the ISP imposes a lower bandwidth cap upon that user, and/or restricts other services.
The implementation of a sustained rate limit might appear as a reduced cap. If the data rate improves automatically after idle time, this would confirm the operation of a sustained rate limit.
Reports claim that the intervention of an administrator is required to remove a lowered cap.
Many broadband Internet Service Providers in North America and Europe introduced bandwidth caps in the early 21st century. The same practice has been in place in Australia, New Zealand, Malaysia and South Africa since the release of broadband.
[edit] Notes
As stated, bandwidth cap is not an accepted term[citation needed], and should not be confused with Subscriber Traffic Management or Traffic Shaping .
