A number of readers have recently emailed us seeking qualification on the data we present within our Exchange Info facility. In the main, the query revolves around the 'Congestion Rating', being either Green, Amber or Red. So how is it worked out ? Sitting comfortably ?

A "bandwidth score" is calculated as a sum of the capacity on the circuits linking each DSLAM back to the BT collosus network verses the number of users on the DSLAM. EG: a DSLAM with a 10mb link to Collosus with 100 users comes out as high bandwidth availability to those users. A DSLAM with a 10mb link and 1000 users has a low bandwidth availability rating.

To give the Red, Amber or Green scores the "Bandwidth Score" is divided by the number of DSLAM's to work out an average. The calculations are geared to err on the side of caution in that if an exchange has 3 DSLAM's, with 1 heavily congested and the other two not, it will give an Amber rating even though for the majority of users they are not having problems since some will be.

It's further important to note that the representation of the exchange congestion is 'estimated'. BT do not supply a "congestion rating" in a manner which the Exchange Info shows. As the explanation above demonstrates, the Red, Amber or Green rating assigned, will be different according to who (read which ISP) makes that statement.

Finally, the source of the data. Virtually all of the data pertaining to exchange stats and congestion comes direct and untouched from source (NDO). Samknows.com provides us with exchange lookup facilities - but is also there for future development within this tool.