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[Bald_Eagle1]

Here's a strange happening that has me baffled..............

I have recently been remotely monitoring a poor 2000m+ VDSL2 connection.

These are the resyncs since we started monitoring:-

29/01/2014 12:26 - RESYNC detected (DS 4695 Kbps, US 295 Kbps), AS = 40, Retrain Reason: 1
29/01/2014 16:50 - RESYNC detected (DS 4695 Kbps, US 304 Kbps), AS = 52, Retrain Reason: 1
30/01/2014 04:51 - RESYNC detected (DS   928 Kbps, US 416 Kbps), AS = 12, Retrain Reason: 1
30/01/2014 05:10 - RESYNC detected (DS 4695 Kbps, US 314 Kbps), AS = 51, Retrain Reason: 1
31/01/2014 04:48 - RESYNC detected (DS 6397 Kbps, US 314 Kbps), AS = 49, Retrain Reason: 1

The 04:51 resync of 30th January was at very low speed indeed.
Looking at the resync logs, it appears it was so low because it had actually resynced in G.DMT ADSL mode.

It only lasted until 05:10 though before reverting to VDSL2 mode.

I have attached a snapshot montage for the G.DMT period and another from 06:00 30th January when it was back in VDSL2 mode.

Do any of you have any ideas why/how that could have happened?

Just fiddling about, I have previously attempted to force my own VDSL2 connection into the various ADSL modes without any success.



EDIT:
The connection also exhibits symptoms of bridged tap(s) in the Hlog graphs, hardly noticed at ADSL frequencies.

[burakkucat]

A few random comments . . .

Your montage, from when the service was synchronised in G.Dmt mode, shows all the graphs with a "Bit Loading" title. 

I believe the service that you are currently monitoring is supplied via an ECI MSAN, whereas your own service is supplied via a Huawei MSAN. I wonder if an ECI Hi-FOCuS M41 has the ability to "drop back" its modulation mode under extreme circumstances? Something, perhaps, that a Huawei SmartAX MA5616 is unable to do?

If the G.Dmt signal, with which the modem had synchronised, did not originate from the cabinet equipment then how can this event be explained? Total loss of the VDSL2 signal from the ECI M41 and leakage of the exchange based ADSL signal through the cabinet based low-pass filter, perhaps?

Very puzzling. 

Edited to add --

Looking first at the Hlog graph and then at the data used to produce it (in VDSL2 mode), I name the three minima as tones 132, 404 & 780.

Working with the case of one intermediate minimum, the delta is thus 648. Using "Table 2" (from a JDSU document) for reference and performing the necessary calculation allows me to tentatively estimate the presence of a bridging tap with a length of approximately 71.5 metres.

[Bald_Eagle1]

Graphs now fixed

[burakkucat]

Graphs now fixed

Purrfect. 

[waltergmw]

Gentlefolk,

I actually observed the subcontractor (from some distance) with his inverter and modem "Test equipment" at the PCP.
Given that all human beings make mistakes, and the fact that this one was not equipped to perform any sort of pair quality test etc. etc., it is not impossible that the PCP jumpers were wrongly connected.

Kind regards,
Walter

[burakkucat]

That could be a possibility but, to be honest, I cannot think of any PCP based jumpering malfunction which would give rise to the observed occurrence. 

Suppose that there is a battery contact fault with another pair in the D-side and, let us assume the other pair carries an ADSL based service. Could that account for the spurious synchronisation in G.Dmt mode?    (It has already been suggested that such a contact fault could account for the appearance of the bridging tap in the Hlog graph / data.)

Only an examination of the D-side joints will answer the question.

[les-70]

 My neighbor has a quite severe HR fault with intermittent sync and phone access.  He on a 40/10 FTTC service which normally runs full speed.  With the fault and a sync he gets actual download speeds of either about 27 mb/s or 7mb/s , it seems to be one or the other, I assumed syncs of about 28 or 8mb/s. He is on a ECI cab but I can't confirm the sync mode as he does not have modem access.  It sounds as if it might be a similar behavior.

[Bald_Eagle1]

Just a quick update..........


A number of engineers visited & found no faults.

Eventually a known to be keen engineer visited & found the cct was "leg 'n legged" instead of proper twisted pair.


That being dealt with, the Hlog graph now looks much cleaner & performance & speed are now far more acceptable for quite a long connection.

Apart from Interleaving being applied (DLM was initially reset on a wide open profile following the repair work), the connection now seems to be perfectly stable with no recurrence of the occasional drift into G.Dmt mode.

[burakkucat]

. . . G.GMT mose.

   G.Dmt mode was intended, perhaps? 

[Bald_Eagle1]

Well spotted that man 

[burakkucat]

Now purrfect, my feathered friend! 

[Chrysalis]

bald eagle what is leg 'n legged?

My line before it permenantly stayed low for a while last year was jumping up and down between 85 attainable and about 70-72 attainable, it was sudden jumps not gradual changes of sync speeds.

I have resigned now to having to keep my line in what I believe to be a faulty state, as openreach are refusing to do any physical checks (they wont even check junction box and pole).  They even told plusnet internally that if they fixed my line (seems admittance of a fault to me) that other lines would regress, so it indicates some serious D side issues in my area imo that openreach probably for cost reasons dont want to fix.  I dont know if its related to the cabinet issue that was mentioned to me last december.

[Bald_Eagle1]

bald eagle what is leg 'n legged?

The leg bit comes form the old GPO term A leg and B leg (I think).
It means that some part of the line uses one conductor each from two twisted pairs thus creating a nasty imbalance result

[Chrysalis]

ahh so a crossed/split pair?

http://www.ospmag.com/issue/article/052013-McCarty

Interesting read I guess.

Openreach just left my neighbour's flat 10 minutes ago, he just had vdsl2 installed and has a hh5.

My sync 65.5 (after I swapped my nte5 and put mk2 back something 4 engineers failed to even bother trying to do, was 62 before).
His sync 80 with a 9db snr margin, attainable almost 90mbit.

I caught the engineer on his way out and pretended I am not a customer, said I was thinking of getting vdsl2 but not sure how good my area performs, how much did my neighbour get.

He brags full speed 80mbit

then I told him ahh I am a customer and am nowhere near that currently, asked him why I would be on 65 when am in same building.  Then he goes oh sorry he gets same speed just he is on the 80 product, so I let him go on his way.

Found of course he was fibbing as indeed my neighbour has a very easy 80mbit.

Plusnet told me officially BT wont even contemplate it as a fault on a 80/20 service regardless of estimate unless sync is below 40mbit.