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Author Topic: ADSL2 / 21CN 9dB _upstream_ target SNRM question  (Read 1021 times)

Weaver

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ADSL2 / 21CN 9dB _upstream_ target SNRM question
« on: March 27, 2019, 01:40:19 AM »

My ADSL2 / 21CN line #3 has been having problems due to upstream SNRM variation within each 24 hour cycle, jumping up and down by ~4dB upstream SNRM. The SNRM was dropping from the 6dB upstream target down to 2dB and then possibly sinking even lower as part of the usual gentle daily variation. At an upstream SNRM that low, I am worried about significant CRC-fail errors. It seems that the results of such a low SNRM are even worse than might be expected, as I suspect - based on looking at the stats- that there is no ‘PhyR’ L2 retx operating in the upstream direction, for some reason. Don’t know why not, but it is very disappointing.

*
So : My attempt at a band-aid has been to set a 9dB target on the upstream SNRM since I see an option for this in AA’s clueless.aa.net.uk control server.

Q: It says ‘ADSL2+ only’, but I am hoping this also includes ADSL2, is that correct?

Q: Hoping that it actually means ‘not available in ADSL-Max/G.992.1/G.Dmt/‘ADSL1’/20CN mode’ ?

* However, recently the link resynched but went straight back to an initial upstream SNRM of 6dB, not to the hoped-for target of 9dB. That might have been a resynch initiated by me by hand, with me doing the modem CLI adsl connection --up.

Q: Is that something expected? That is, when I executed the manual resync command, what initial SNRM will it aim for? And how does it ‘know’?

I re-executed an SNR margin reset at 9dB upstream on clueless.aa.net.uk and by magic a resync got triggered, and it was at the correct targets: 3dB downstream / 9 dB upstream.
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burakkucat

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #1 on: March 27, 2019, 09:46:35 PM »

Hmm . . . Your questions really need to be asked of A&A. I wouldn't like to make a guess.  :no:

However your post prompts me to ask if all four circuits show identical last three output lines when the following command is executed --

xdslctl info --vendor

Taking the output that I harvested, on the first of this month, as an example --

> xdslctl info --vendor
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:   8000
Last initialization procedure status:   0
Max:   Upstream rate = 1004 Kbps, Downstream rate = 6072 Kbps
Bearer:   0, Upstream rate = 987 Kbps, Downstream rate = 5119 Kbps

ChipSet Vendor Id:   IFTN:0x71c8
ChipSet VersionNumber:   0x71c8
ChipSet SerialNumber:   

 >


If you do not see identical "ChipSet" data for all four circuits then we must assume that they are not all connected to the same MSAN. If that is the case, it may explain the absence of US PhyR on the one circuit.
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Weaver

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #2 on: March 27, 2019, 11:10:27 PM »

> Your questions really need to be asked of A&A.

Oh, I didn’t know. I realised that I don’t understand the general situation where a modem in the CPE and the DSLAM/MSAN resync and get instructions - somehow, from somewhere [?] - on either what explicit individual parameters to use in the new connection, or else gets told a criterion or constraint, such as ‘achieve 6dB up and 9dB downstream minimum target (initial) SNRM’, and has to then somehow work out a solution that satisfies that constraint.

I regarding this 9dB upstream option, I assumed it is a BT 21CN thing rather than an AA one as it has that delivered-by-BT feel somehow, whatever on earth that means. It’s a bit odd that it’s so limited, with just the two possibilities for upstream target SNRM: 6dB or 9dB or errm nothing else.
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Weaver

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #3 on: March 27, 2019, 11:12:12 PM »

I realise that I have misled you, or have written something wrongly. All lines are identical in not having upstream PhyR.

The vendor check gives the same results for all four lines.

Code: [Select]
VMG1312-B10A
Login: admin
Password:
 > xdslctl info --vendor
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    8000
Last initialization procedure status:   0
Max:    Upstream rate = 492 Kbps, Downstream rate = 3128 Kbps
Bearer: 0, Upstream rate = 528 Kbps, Downstream rate = 3004 Kbps

ChipSet Vendor Id:      BDCM:0xa3a7
ChipSet VersionNumber:  0xa3a7
ChipSet SerialNumber:   
 >
« Last Edit: March 27, 2019, 11:29:35 PM by Weaver »
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burakkucat

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #4 on: March 27, 2019, 11:48:04 PM »

Ah, I see. Thank you. I was misinterpreting the situation with regards to US PhyR on your four circuits.  :-[
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Weaver

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #5 on: April 04, 2019, 04:37:28 PM »

I had a (potential) problem situation of this type again today. I noticed a small amount of ‘packet loss’ (poss incl loss of CQM PPP LCP echo requests or responses) and took a look into it. There had been a spike in downstream SNRM resulting in loss of sync. After the modems reconnected, my modem was synced at 6dB initial SNRM upstream, rather than the 9dB target SNRM desired. This proved to be a problem later, because of the continuing cycle of weirdness whereby the upstream SNRM dropped down from 6dB to 2dB. A healthier situation would have been a drop of the same size but from 9dB to 5dB, which was what was intended. See screenshot: https://ibb.co/hKFNNKH
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Weaver

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #6 on: April 07, 2019, 12:04:26 PM »

> Your questions really need to be asked of A&A.

You are absolutely right, I will have to.

Again today the modem did a resync and went straight back to 6dB upstream SNRM - not the desired 9dB target - and 6dB was not good enough since this was in the good part of the cycle and went down to around 2dB when the connection went bad during the bad part of the day’s cycle.

It just is not obeying the 9dB instruction.

Is it possible that this is because I am not in fact in G.992.5 ADSL2+ (rather I am in G.992.3 ADSL2) ? Given that clueless says ADSL2+ implying ‘only’. But I didn’t take this literally. I assumed that it was ADSL2* = ADSL2 + ADSL2+ both.
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burakkucat

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #7 on: April 07, 2019, 10:32:36 PM »

Again today the modem did a resync and went straight back to 6dB upstream SNRM - not the desired 9dB target . . .

Just checking the details, I presume there was no power-cycling of the device?
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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #8 on: April 08, 2019, 12:41:37 PM »

No power-cycling. I have done a manual resync by issuing the CLI command and on another occasion the link dropped.
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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #9 on: April 08, 2019, 09:39:16 PM »

b*cat is puzzled.  ???
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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #10 on: April 09, 2019, 05:30:42 AM »

How does the whole business of controlling _target_ SNRM work at resync time anyway? I realise I haven’t ever known. It’s probably my fault because I haven’t read far enough into the G.xxx specs which might give a partial clue.

And the other thing is, perhaps the whole target thing is controlled by the MSAN anyway? And I did some CLI-initiated resynch commands which perhaps don’t count because maybe that scenario is outside the use-case where the MSAN oversees the resync. I have no idea what I am talking about there, just guessing.

Also I don’t know what determines the threshold when a modem will give up and resync - when a certain level of badness is experienced? Is it something solely determined by the modem itself? Is the criterion for resync just inside the modem alone ? Fixed or variable ? So much ignorance.

I also need to read Kitz’ DLM articles again. That line, line 3, which is the upstream good-bad-good cycler, still has DLM set on it iirc. I would have to look back but AA turned off DLM on I think one line only, and it was in line 4 iirc, not on line 3. I don’t know whether DLM needs to be considered here in this case.
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Chrysalis

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #11 on: April 09, 2019, 07:23:16 AM »

If I remember right and understand right.

Target SNR is controlled at the downstream side, so for your upstream the DSLAM would control it, for your downstream modem would control it, however the DSLAM can of course send commands/request to your modem to connect with certain parameters on your downstream.

On vdsl it seems broadcom chipsets target SNR manipulation is disabled for some reason (maybe locked in place by dslam or broadcom just never implemented it in driver).  On adsl it should function as normal but I believe only on the downstream.  However you can set a sync rate cap on the upstream which is another way of increasing the upstream snrm.

I could be wrong so dont take what I typed as some kind of fact, especially as I have never been on a BT based adsl2+ service before.

If you configured the upstream snrm using the aaisp control panel I assume there would be some lag between when you set it and for it to be actioned by the dslam.
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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #12 on: April 09, 2019, 08:41:51 AM »

If I set the upstream target SNRM using clueless.aa.net.uk, most of the time it remotely triggers a resync within a few seconds. I set the parameters as I want them and hit SNR RESET. Iirc, sometimes a resync doesn’t happen. My best guess is that it might be to do with the system’s idea of something like that ‘target margins have / have not changed’ or ‘the current values of the various fields are xxx and it’s still like that, so nothing to do’. But I’m not sure. I’m very fuzzy about this just now.

Does anyone know if there is anything written up in the various standards docs, BT SINs, The Broadband Forum (is that right?), anyway, the usual suspects etc etc ?

> for your upstream the DSLAM would control it

Understood. Thanks. Btw, is that directly or indirectly, do you know? By which I mean is it the case that the DSLAM tells the CPE modem what to do or perhaps ‘asks’ it please do x in that respect? Or is it more direct and doesn’t work like that?

That would explain a lot, would it not? The DSLAM normally controls what happens in the resync and ensures that the initial value of the upstream SNRM is at the target as expected. But then if going to the correct target at resync time relies on the DSLAM remotely controlling things, and if no persistent setting gets stored in the CPE modem as a ‘standing order’, ‘for all future resyncs’, then when I do a CLI-initiated resync, that is then outside the sphere of influence of the DSLAM perhaps. So it could be that the behaviour that has been seen is just exactly what I should have expected had I but known how things work?
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Chrysalis

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #13 on: April 09, 2019, 05:04:05 PM »

The dslam controls it directly, but of course there would be something controlling the dslam such as the DLM system.
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aesmith

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Re: ADSL2 / 21CN 9dB _upstream_ target SNRM question
« Reply #14 on: April 15, 2019, 09:53:46 AM »

If I set the upstream target SNRM using clueless.aa.net.uk, most of the time it remotely triggers a resync within a few seconds. I set the parameters as I want them and hit SNR RESET.

Hi, when you do that does it restart the 10 day learning process, and record new values for MSR/FTR?
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