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Contention and congestion

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Diagram showing the two places where contention can occur on IPStream adsl.


Contention is defined as follows:-

Competition by users of a system for use of the same facility at the same time. The term contention ratio applies specifically to the number of people connected to an ISP who share a set amount of bandwidth.

When too many users are using their connection at the same time, speeds will start to slow down for all those users sharing the set amount of bandwidth. This is known as congestion.

IPStream adsl has never been intended to be used for constant downloading and you share your bandwidth with a number of other adsl users.

BTw SIN 386 4.1 states:-

... the End Users require occasional fast but ‘bursty’ access to private network facilities and / or the Internet (via the Customer). The products are not suitable for End Users who require continuous bit-rate, fullbandwidth services.

How many other users you share your connection with is shown as a ratio.
Home users are normally quoted a contention ratio of 50:1. Business users normally purchase 20:1 which is more expensive, but fewer users share the same bandwidth and therefore the theory is that adsl Office is less likely to see congestion.

If you expect to see constant top notch speeds on which you can download 24/7 and no contention then you should instead be looking at a 1:1 dsl connection which will cost in the region of £600 per month or £1,000 for a leased line.

The two main areas where congestion can occur are:-
 ~ On the Virtual Path at your Local Exchange
 ~ On the BT Central pipes which are controlled by your ISP.

With effect from October 2005 BTw seems to have moved away from quoting actual contention ratios and instead seem to be moving more towards what is classed as an acceptable speed for IPStream adsl when the network is busy. The following figures are taken from the BTw speed tester and should be considered as a guide for the minimum acceptable speeds:-

2 Mbps
400 kbps
800 kbps
1 Mbps
200 kbps
400 kbps
512 kbps
100 kbps
200 kbps

Most exchanges will see some form of congestion at some point or other. You can check your local exchange status from the Plusnet UserTools Exchange Checker.
It should be bourne in mind that these lists are only updated every few weeks by BT and therefore may not show the true current status. In my own experience BTw tend not to mark an exchange as red until speeds of under 400 kbps are seen on a 2Mb connection :/

If you are experiencing slow speeds then its best to check out my checklist here and also reading this page which discusses acceptable speeds.


Contention - a toilet analogy

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