The BT wholesale availability checker gives a result from the BT wholesale database of what speed your line is likely to achieve.
Currently this test also provides information of which green cabinet you are attached to & estimate of fibre speeds. The ability to identify which PCP your phone line goes via is often useful information if you are getting FTTC.
If you want more detailed information about your line and in particular your exchange, then also use the broadband checker.
'Impacted' only seems to show on FTTC lines and it is assumed that this is for lines which may suffer from small line faults such as a bridge tap, or possibly cross talk. If you are attaining speeds in this range, BT don't consider it as a line fault.
As of 2014 more lines seem to be showing the effects of crosstalk as more users are added to the FTTC cabinet. Although BT started using the term 'impacted' in 2013, it wasn't until cross-talk actually reached the stage of possibly impacting on my connection speed (rather than max headline) that the 'impact' figures appeared in my results. 15Mbps seems to be a fair average for most lines. Over a 6 month period I have lost 20Mbps from my own max headline speed due to cross talk.
How BT estimate anticipated speeds
"We use the individual line characteristics to match a line to similar lines in our network that already have GEA-FTTC installed. With this information, we display the range of speeds that the actual end users with similar lines are getting.
Because we don’t know the actual line details until a line is installed, the top speed we display is the speed that 20% of end users are getting on similar lines and the bottom speed we display is the speed that 80% of end users are getting. We call these the 80th and 20th percentile speeds.
There are a number of factors that can impact an end user’s actual sync speed. These factors are naturally represented within the range of speeds we display since they are represented in the live network:
- Line length (the distance of the copper wire from the cabinet where the broadband equipment is, to the end user’s premises);
- Line quality (eg: age of the copper and whether any part of the line is actually aluminium, rather than copper, etc.);
- The number of other GEA data services provided on the local cabinet (known as cable-fill).
When we initially estimate a new line before the GEA service is installed on it, we use the actual line length from the cabinet to the local distribution point (eg: the telephone pole that feeds the houses on the street) and an average line length value for the “final drop” because we know that this is usually up to 50 meters.
In a very few cases, the final drop can be much longer; even up to a kilometre, and is usually the reason why an end user’s actual speed might fall outside the range of speeds that we provide at the point of sale. But this situation doesn’t happen often and end users are usually still happy with the service because it’s typically still faster than ADSL where the length from the home all the way back to the local exchange affects the speed.
The amount of traffic that is running in the same copper cables from the cabinet to the local distribution point, before the “final drop” to the end user’s home can sometimes create noise, called cross talk, which can also reduce the speed that an end user can get. "
Last updated Oct 2014
to add estimated speed info.
Credit BT wholesale checker