Contention is something that appears to be only talked about in the UK, but it is a major part of the Internet. Without contention at the various points of the Internet it would be vastly more expensive to run and very few people would even have access to a 500kbps service.

Contention Ratio Explained

Contention is something that appears to be only talked about in the UK, but it is a major part of the Internet. Without contention at the various points of the Internet it would be vastly more expensive to run and very few people would even have access to a 500kbps service.

With respect to BT Wholesales Home services which run at a nominal 50:1 contention, the worst case that contention can produce is a 0kbps throughput.

There is no active management of the contention. The way the contention is managed is such that you should not expect to see below 100kbps, of course BT Wholesale and your ISP can do nothing about a website or server that supply the data slower than that.
An urban myth has grown up that the 0.5Mbps service is shared between 50 users, the contention on BT IPStream 0.5Mbps services is done very differently.

How is BT Wholesale Contention carried out

The 50:1 contention of a Home service refers to the BT Wholesale controlled part of the network, between your local phone exchange and one of 22 locations around the UK (these 22 locations are called Points of Presence or POPs), where the connection is handed onto the various Service Providers. At many exchanges and in the BT Wholesale controlled part of the network the contention is so low, that most of the visible contention is actually taking place in the ISP controlled segments.

To avoid problems with one user hogging all an exchanges bandwidth, the users are contended on a larger than 0.5Mbps pipe. The normal basic pipe for a 50:1 service is 4Mbps (though with the introduction of a 1Mbps 50:1 service this may increase), this pipe will be populated with an absolute maximum of 400 x 0.5Mbps users, it is more common that only 200 to 300 users are placed on each 4Mbps pipe. The fact that each user is capped by their connection over the ADSL to the exchange means that no one user can saturate the pipe, and therefore the chances of you receiving your full 0.5Mbps is more likely.

The pipe sizing is different for the Office 20:1 contention services, which run at 0.5, 1 and 2Mbps. In that case the normal minimum pipe size is 10Mbps. BT Wholesale does monitor the bandwidth levels in the various pipes and can adjust them up and down to cope with increases in the usage level, though it may take sometime for action to be taken. It is also possible that there may be several 50:1 pipes leaving the exchange, this can mean if you are seeing contention that not all other users on your exchange will be, and sometimes users on the same ISP and same service may actually be on a different pipe connecting to the ISP.

With the new Home 1000 product these users are running on the same pipe as the 0.5Mbps users, so we can expect to see some pipe resizing going on, if a lot of users transfer to the Home 1000 service. You may also see the pipe referred to as the backhaul, PVC or Virtual Path. BT at most exchanges has capacity installed for up to 155Mbps of traffic from ADSL users, and the smaller subtended exchange builds have around 30Mbps of bandwidth available. Which should be sufficient for most exchanges. Net Nurse provide broadband services for home and office users, enabling super-fast always-on Internet access for a flat monthly fee using your existing telephone line. Capable of receiving data up to forty times faster than a standard analogue modem, our wireless broadband ADSL is a much more suitable service for transporting multimedia data, video, audio and other large files. All while allowing the simultaneous use of both the telephone and Internet. This is particularly useful for small offices and for staff that frequently work from home.