topic Can FTTC sync speed be affected by quality of VDSL RJ11 cable? in Fibre

Can FTTC sync speed be affected by quality of VDSL RJ11 cable?

cranhpc1 — Sun, 05 Oct 2025 09:52:27 GMT

We have had a very reliable VDSL connection to Sagemcom FAST5364 3.00 with SG4K100208 - normal sync speeds ~75mbps down/20 up
Last weekend we experienced sudden drops of both speeds to ~2-3mbps. Resetting router restored speed for short periods.

I used the automated online test which threw up an issue which was promptly handled by OpenReach, with promise of an engineer visit (quite impressed by that!)

OR Engineer came on the Saturday with his bag of tricks, did a lot of talking then said that the issue was at the DSLAM end and he took some actions remotely,. He also replaced the V5C faceplate as a precaution.

He then spent a while looking at why his test equipment could get an 80mbps connection to the DSLAM but our router could then only manage ~70.

He then decided that it might be our VDSL cable (ours was a Belkin RJ11 cable made with Cat 5 ethernet cable). He produced an old flat RJ11 cable from his bag and that delivered a consistently better download sync speed (75mbps) than the expensive Belkin one & also the responsiveness seemed better.

I purchased another RJ11 cable but this also produced a consistently worse sync speed.

I wonder of anyone has seen this sort of thing happen before and also if they have had experience of VDSL cables made from higher spec ethernet media such as Cat 5e/Cat 6 etc. Logically they would seem to be overkill at VDSL speeds.

Re: Can FTTC sync speed be affected by quality of VDSL RJ11 cable?

Divsec — Sun, 05 Oct 2025 11:10:54 GMT

Hi @cranhpc1 glad the system worked and you got a very good service from the Openreach engineer. As for the rj11 cable impossible to give an answer other than more money isn't always better.

Re: Can FTTC sync speed be affected by quality of VDSL RJ11 cable?

Mandisa-TT — Thu, 09 Oct 2025 08:55:35 GMT

Thank you for assisting @Divsec, @cranhpc1 After any changes, continue monitoring your internet speeds to see if there are consistent improvements.

Re: Can FTTC sync speed be affected by quality of VDSL RJ11 cable?

cranhpc1 — Wed, 15 Oct 2025 09:09:51 GMT

Thanks, connection is stable at around 75/20 mbps. I guess it's a case of "if it ain't broke, don't fix it".

I do know from experience that cable spec can affect ethernet performance eg cat 5 vs Cat 5e.

It would be tempting to buy an RJ11 to RJ11 cable made with Cat 6 cable but I suspect that there might not be much point in practice.

Re: Can FTTC sync speed be affected by quality of VDSL RJ11 cable?

Mandisa-TT — Wed, 15 Oct 2025 10:45:55 GMT

You're absolutely right @cranhpc1 —if your connection is stable at 75/20 Mbps, there's no pressing need to change anything! The "if it ain't broke, don't fix it" approach definitely applies here.

You're also correct about the impact of cable specifications on Ethernet performance. While using a Cat 6 RJ11 to RJ11 cable might seem like a good idea for future-proofing, in practice, the performance might not significantly improve given your current setup.

If you ever decide to upgrade your connection speed in the future, then it might be worth considering. For now, it sounds like you’re in a good spot! If you have any other questions or need assistance, feel free to ask!