Latest status reported from the modem shows some amazing stats for my line, 17000 (Kbps) upstream, 75370 (Kbps) downstream and SNR up and down is 0 (dB)

Sadly the speedtest is the same result as before. BT aren't due to start work on the line fault until Tuesday so I'm surprised about the above figures.

In reply to a post by joconnell:

Latest status reported from the modem shows some amazing stats for my line, 17000 (Kbps) upstream, 75370 (Kbps) downstream and SNR up and down is 0 (dB)

That looks more like a power cut. Either your substation or the fibre cabinet. Or a reboot of the cabinet DSLAM triggered by Openreach.

You will have been one of the first to reconnect, with little or no crosstalk. Then al the other modems came online and reduced your margin to zero. You are likely but not certain to have a re-sync very soon. (Some modems can hold sync as low as -1dB margin).

Yes, that's the only downside of having a quick syncing modem!

An update on the situation - the fault is fixed.

BT Openreach guy came on Tuesday and after plugging in his equipment to my master socket and running tests, agreed there was a problem but couldn't pinpoint the root cause, so he replaced the master socket faceplate with a new up-to-date version and also replaced the PCB in the small junction box where the phone line comes in from outside.

The Vigor fibre modem showed sync speed of 79995 down and 20000 up with 3dB SNR and although speed tests showed ping times of around 12ms and upload speed of around 18Mbps, the download speed was still limited to around 48Mbps, compared with the 69-70Mbps speed before the fault.

This weekend I replaced a couple of 50cm cat6 patch cables that connect my router to the fibre modem and connect the router to the main network switch that serves the LAN, replacing them with 100cm cat6 cables so that there's less cable strain. I retried the speedtest which reported 73Mbps download speed!

I haven't yet retried with the original shorter cables (they're reused elsewhere) but it does seem a coincidence that after replacing them with longer cables, the DL speed increased by so much. I wonder if the Draytek Vigor 130 modem is responsible for the increase in maximum DL speed from 69Mbps to 73Mbps?

Oddly, running the BT Wholesale speed test on my iPhone (with only WiFi active) reports DL speed of 292Mbps. On FTTC? I wish!

ISPs are supposed to shape traffic to the client based on the sync rate in order to avoid excessive traffic loss (per SIN498 section 2.1.7).

Plusnet certainly do, you can see 'Current Line Speed (Download)' in the user control panel under Broadband, AFAIK they use 92.5% of sync rate, there are encoding and encapsulation overheads to make allowances for.

After faults on a couple of different lines I have seen this value to sometimes get stuck at a figure based on the sync rate when the fault was present, occasionally dropping the PPPoE session will pick up the changes, other times it has required support calls asking them to reset their figure. In your case changing the modem - router network cable would have dropped the PPPoE session.

In reply to a post by tdw42:

ISPs are supposed to shape traffic to the client based on the sync rate in order to avoid excessive traffic loss (per SIN498 section 2.1.7).

Plusnet certainly do, you can see 'Current Line Speed (Download)' in the user control panel under Broadband, AFAIK they use 92.5% of sync rate, there are encoding and encapsulation overheads to make allowances for.

After faults on a couple of different lines I have seen this value to sometimes get stuck at a figure based on the sync rate when the fault was present, occasionally dropping the PPPoE session will pick up the changes, other times it has required support calls asking them to reset their figure. In your case changing the modem - router network cable would have dropped the PPPoE session.

I wasn't aware of that, it does explain the maximum speeds I'm getting then.

I don't think the actual cables would cause the lower speed but as tdw42 stated the disconnection would cause the PPPoE session to be restarted, this could have updated the BT Wholesale IP profile and the Plusnet Current Line speed profile.

The Current Line Speed profile only affects users with a Static WAN IP address, due to different network equipment used for them.

In any case your speeds seem to be fixed now.