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Author Topic: Dying gasp (again)  (Read 6364 times)

Weaver

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Dying gasp (again)
« on: March 02, 2016, 09:14:02 AM »

@kitz - I remember you saying that you thought that dying gasp has no input into the DLM process. I'm sure we've talked about this before. Do you think that's still true?

Another point. I wonder how many modems now have dying gasp support? I wonder if the fraction has gone up?

Assuming you're right, then the BT logic is presumably that : DLM is required to somehow do "the right thing™" even where there is no support in the modem for dying gasp anyway. So, the logic goes that there's no point doing something different for one breed of modems. Why not allow better modems to give the user the better experience of being more DLM-friendly? What do you think?
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kitz

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Re: Dying gasp (again)
« Reply #1 on: March 02, 2016, 09:59:36 AM »

These days all modems should support the dying gasp and the DSLAMs are all capable of recording this information, its just that BT decided not to use it within their DLM.  I think that the early DLM system used it but for some reason they decided it wasnt accurate enough and to come up with something different and started using MTBR.

MTBR uses activity in the previous 15 min bin to determine whether or not if the retrain was forced or not.  This is used in conjunction with monitoring for what they call Wide Area Events.   The theory being that if a pile of neighbouring lines are also getting lots of resyncs due to say power outages or lightening, then DLM can detect its not the individual line.

Yes they still use this system of ignoring dying gasp.  We see evidence of it every so often.   Like the number of users who may reboot several times when putting a new modem on the line and configuring it.   If DLM used dying gasp then it would detect power cycles and user initiated reboots etc. 

GoldenFibre/MaxDSL got caught out last year when messing with his new router.  Plusnet supplied his RADIUS report and I think we counted from that 7 resyncs triggered the NGA DLM.    However last summer BT also released notification that they were increasing the MTBR as they recognised that it was perhaps a bit too aggressive.  The figures are on the main site, but Im just about to go out and am dashing.

ISPs should also be able to access your MTBR records. They also have MTBR thresholds available to them.   They just choose not to publish it.

[Moderator edited to correct a typo.]
« Last Edit: March 02, 2016, 09:41:33 PM by burakkucat »
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ejs

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Re: Dying gasp (again)
« Reply #2 on: March 02, 2016, 11:23:52 AM »

Plusnet removed the dying gasp support from their ADSL 582n models when they removed the USB port, which also resulted in those devices emitting a quiet high pitched noise when connected to the ADSL (which did get fixed in later batches of devices).

Would a reboot even send a dying gasp, since it doesn't actually lose power, it just restarts? I think there's also some sort of "orderly shutdown" procedure that could be used instead.
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tickmike

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Re: Dying gasp (again)
« Reply #3 on: March 02, 2016, 05:27:09 PM »

BTor chap last week while doing his tests just unplugged the DSL line with the modem still powered up  :'(

Remind me what MTBR stands for.  :hmm:
« Last Edit: March 02, 2016, 08:56:46 PM by tickmike »
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ejs

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Re: Dying gasp (again)
« Reply #4 on: March 02, 2016, 05:42:26 PM »

MTBR stands for mean time between retrains (retrain being a more technical term for what often gets called a re-sync).
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tickmike

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Re: Dying gasp (again)
« Reply #5 on: March 03, 2016, 12:55:53 AM »

MTBR stands for mean time between retrains (retrain being a more technical term for what often gets called a re-sync).
Thank you.
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kitz

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Re: Dying gasp (again)
« Reply #6 on: March 04, 2016, 01:28:17 AM »

Quote from: ejs
Would a reboot even send a dying gasp, since it doesn't actually lose power,

You have a valid point there.   I can recall many years ago (10+) when DLM was first introduced and supposedly used dying gasp, that it was always recommended to  do an orderly shut down via the router.  Thinking about it though dying gasp is supposed to be primarily for sudden loss of power such as if the power cord is pulled.  I dont know much about in detail about the early DLM as I never looked into it in depth other than to write this which was based on BT documentation about maxdsl.

Just had a look on Cisco and it says:-

Quote from: Cisco
Dying Gasp—One of the following unrecoverable condition has occurred:

    Interface error-disable
    Reload
    Power failure or removal of power supply cable

Quote from: ejs
I think there's also some sort of "orderly shutdown" procedure that could be used instead.

I would tend to agree.  We know that most of the decent routers show a Last Retrain Reason followed by a code no.  Ive never heard of any DLM system using them though.   I wonder if it could be down to the fact that differing manufacturers may use different codes?   
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sevenlayermuddle

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Re: Dying gasp (again)
« Reply #7 on: March 04, 2016, 07:51:42 AM »

BTor chap last week while doing his tests just unplugged the DSL line with the modem still powered up  :'(


I suspect that may be part of the problem.  We (geeks on Kitz's forums) all know it's wrong but there will be a lot of people who don't, and there's no law against it yet.   

If I were designing DLM, I'd want an algorithm for ignoring user-initiated events that would work for everybody, not just the expert users who bother to read the small print.
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tickmike

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Re: Dying gasp (again)
« Reply #8 on: March 04, 2016, 02:08:36 PM »

BTor chap last week while doing his tests just unplugged the DSL line with the modem still powered up  :'(


I suspect that may be part of the problem.  We (geeks on Kitz's forums) all know it's wrong but there will be a lot of people who don't, and there's no law against it yet.   

If I were designing DLM, I'd want an algorithm for ignoring user-initiated events that would work for everybody, not just the expert users who bother to read the small print.
He was a BT OR trainer so he knew what he was doing, I pulled him up on it (as I would  ;D) he said the DLM will catch up in a few hours  ???.
I think after his 5hrs of testing the interleving had gone to 64 down 8 up
then went to 32 / 4 soon to 8 / 2 now it's gone to fast track 1 / 1  :D
« Last Edit: March 04, 2016, 05:27:08 PM by tickmike »
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kitz

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Re: Dying gasp (again)
« Reply #9 on: March 04, 2016, 02:25:04 PM »

Quote
he said the DLM will catch up in a few hours

Yep.  Sure it will...  and of course it does...  although it may be detrimental too!   :(     
Its ok for 20/21CN as the ISP can request a DLM reset.  Different matter on NGA DLM (fttc) where only the OR engineer can request a reset and if he forgets then you're a bit snookered. 

Saying that, the fact that since the DLMs ignore dying gasp and doesnt appear to do anything with orderly shutdown codes.   As previously pointed out by ejs, then an orderly shut down probably wont be sending a dying gasp message anyhow.    So it appears to be pointless worrying about it.

DLM will ignore several resyncs (depending on the profile selected by your ISP), so therefore the engineer just doing the odd one shouldnt make any difference.    Last week when I was testing the TD-W9970 I did about 5 or 6 reresyncs due to taking off my Zyxel,  also putting on the HG612 to get some benchmark stats, as well as any config changes whilst I was poking around in the TD-W9970 GUI.    I didnt bother waiting the 30mins as I just wanted to get on with the task in hand and otherwise I would have wasted too much time and half the evening would have been gone.     I drew the line at 6 though. 
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kitz

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Re: Dying gasp (again)
« Reply #10 on: March 04, 2016, 03:31:55 PM »

Quote
If I were designing DLM, I'd want an algorithm for ignoring user-initiated events that would work for everybody, not just the expert users who bother to read the small print

Good point.
I should imagine the first problem to overcome with this would be to standardise reason for resync codes.  I think we've seen on here that even the BCM based modems can show 2 different sets of codes.   iirc it was eric that found the codes used by current BCM 6338/63xxx variants, but even some of those are confusing.

On reflection... using the 15min bin periods is quite a novel approach.   It certainly covers the people who shut off their routers overnight type situations.    They also appear to have gone one stage further with monitoring for Wide Area Events which should also detect thunderstorms and local power issues.

The only thing its doesnt detect its cant see if the EU is at the other end pulling the power chord or issuing a reboot router command... or even if the modem has flipped its lid and power cycling.     

I wonder if the current DLM system could be combined with certain errors such as LoS.    The current system tends to look at MTBR and MTBE as two separate parameters and it takes action on either one.   I suppose it would be fairly easy to say monitor LOS count within 15min periods which all modems should be capable of.   The hard part would perhaps be determining if a LOS happened x seconds before a retrain.    Modems only record a count per 15 min bin but not the actual time it occurred to later try tie up with retrain time.     

The reason why we have access to more accurate data is because the likes of DSLstats take a reading say every minute and this info is processed on the local machine and we can see that count has increased by 'x' during the one min monitoring period from the current bin.     Modems dont do that..  their count unit is in 15 min periods.  15 mins is a standardised measurement across the board for all modems/routers.     

   
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aesmith

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Re: Dying gasp (again)
« Reply #11 on: March 04, 2016, 03:49:38 PM »

Just had a look on Cisco ...

Judging by the clicks, Ciscos have a relay to disconnect from the line during resets and retrains.   That might be why they've specifically added DG support to various soft operations, not just in case of power failure.
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