dslam problem?

hello all. hoping someone may have an idea what is happening with my vdsl2 . when installed i sync at a good 72 minimum but over the last two years this has now dropped to 52 (handback is 67)zen have been sending openreach engineers to check the line but find it is perfect condition and could not understand the loss, after 2 visits engineer rang and told me there was some kind of error with the card or config on the line? and it would not go above that speed. he did not know what to do and would escalate to another team, two days later he rang back to say they where about to change card and if that did not work nothing they could do. alas i lost 2 more meg on the change

Dropping from 72Mb to 52Mb is very consistent with crosstalk.
Crosstalk is simply interference caused by other lines on the DSLAM.

Pretty much every single VDSL2 line loses some speed over time as their cabinet fills up.
The early adopters will have lost the most.

There's possibly no error at all.

Unless you're lucky enough to have a BDUK funded cabinet that's got vectoring.

In reply to a post by Grimers:

Unless you're lucky enough to have a BDUK funded cabinet that's got vectoring.

Indeed, but even lines with Vectoring lose some sync speed over time.
Vectoring attempts to mitigate the effect of crosstalk but it doesn't cancel it out completely.

OpenReach's deployment of Vectoring is not exactly efficient (that's being polite).

Their free for all on what modems are used on the VDSL DSLAM's makes things even worse with many modems (including some modems supplied by big ISP's) not supporting Vectoring.
Many of them aren't even Vectoring friendly**

A customer on a cabinet with Vectoring with a modem that fully supports Vectoring can still lose 10-20Mb off their sync speed from a single rogue modem that doesn't support Vectoring.

If any of the BDUK cabinets with Vectoring reach their capacity and need a 2nd cabinet (or VDSL Sidepod) installed, the 2nd DSLAM does not support Vectoring.
Considering the Huawei DSLAM's support Node Level Vectoring (Vectoring across multiple DSLAM's) it's a poor deployment on the part of OpenReach.
Every single line connected to the 2nd DSLAM can cause crosstalk on lines connected to the Vectoring DSLAM.

Taking all that into account and the very small amount of cabinets that actually support Vectoring, there's a high chance that most drops in sync speed over time are due to crosstalk.

Whether that's the case for the OP's line or not we can't say, but it's a strong possibility.

**Vectoring friendly is when a modem doesn't mitigate crosstalk on an end users line, but still supplies the Vectoring group (the DSLAM) with the info necessary to help cancel crosstalk on other lines.

I see. Thanks as always for your detailed explanations John, it's greatly appreciated.

thankyou for your full description of what may be happening, i did not think you could lose so much speed from crosstalk, one question if i may, i am considering going to 300mb gfast, would i have the same problem there or is the routing/technology separate from vdsl

What does the BT Broadband Availability Checker show for your address?
Does it even show G.Fast as a possibility in your estimates?

https://www.broadbandchecker.btwholesale.com/#/ADSL/...

G.Fast has a very short range, much shorter than VDSL2.
Usually if you aren't close enough to the cabinet to receive the full 80Mb (or very close to 80Mb) then you aren't close enough for G.Fast.

With your current sync I would be very very surprised if you could order G.Fast.

G.Fast has Vectoring like VDSL2, but it is mandatory.
It is much better deployed than on VDSL2.

i am 80 meters from cabinet, checker shows 71 expected on vdsl2 , i can order gfast 300 expected 245, but will i have same problem

have just run the test to update
vdsl clean 80m
vdsl impacted 80 meg
gfast clean 330
gfast impacted 328

G.fast requires vectoring to function, so if your issues are being caused by crosstalk (it sounds like they are), then moving to G.fast will help.

Your choice of ISPs becomes quite limited, though the big players are all there.