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Tech answers needed to ADSL2

maximod
Grafter
Posts: 139
Registered: 15-08-2010

Tech answers needed to ADSL2

Hi,
Yesterday my line changed over from  20cn to 21cn, and the 2700hgv duly re synced showing the attempted try at adsl2+ (G.DMT2 Annex A+) , then settled on adsl2 ( G.DMT2 Annex A).
Before the changeover my stats were 2848kb at 5db snr at 60.3db attenuation.
After the changeover my stats were 3131kb at 3.9db snr at 57.4db attenuation.
But several hours after the router itself did a hard reset (reboot), and adsl2 speeds reduced.
Why did the router do a re boot, and not a re train ?
Also the adsl2 specs show that minimum bitloading of 1 (compared to adsl1 of 2), my adsl2 shows minimum bitloading of 2 ?
Thanks,
Paul.
27 REPLIES
VileReynard
Seasoned Pro
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Registered: 01-09-2007

Re: Tech answers needed to ADSL2

That 3.9 snr is not the signal to noise ratio - it is the signal to noise margin
Most lines need about 6Db to operate without too many errors.
Your line accumulated too many errors, so BT increased the margin, your speed reduced.
How do you know your router did a hard reset?
I think it did a soft reset; i.e. a disconnect and reconnect.
BTW ADSL1 uses 256 possible slots 4Kbits wide, ADSL2+ uses 512 slots.
See http://www.kitz.co.uk/adsl/

maximod
Grafter
Posts: 139
Registered: 15-08-2010

Re: Tech answers needed to ADSL2

Yes i do know what snr margin is (at the time of posting i should have said snrm), 0db is the minimum needed to work, so 3db snrm should be no problem.
Where the figure 6db snrm allows extra headroom for fluctuations without losing sync.
At the time of the hard reset i was next to the router and saw it happen, this was not due to excessive errors, far from it !
The second part to my question regarding minimum bitloading has nothing to do with frequency bandwidth allocation, but the adsl2 (and +) spec says 1 bit minimum bitloading, i am getting 2 as in adsl1 spec.
Paul.,
Community Veteran
Posts: 19,099
Thanks: 434
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Registered: 31-08-2007

Re: Tech answers needed to ADSL2

Noise higher level after dark Paul, hence lower sync speed? Perhaps noise levels are just a bit too high for 1 bit minimum bit loading. As to what caused the reboot/resync anyone's guess -a spike of noise perhaps. How are you distinguishing between a reboot and a resync, I'm guessing maybe the power light, so perhaps it was a mains glitch.
maximod
Grafter
Posts: 139
Registered: 15-08-2010

Re: Tech answers needed to ADSL2

The router did a hard re boot, ie all lights went out then power light flashing, followed by solid power light then internet and broadband lights doing similar.
When the router does a re sync the power stays solid, but the inernet and sync go through the flash/solid phase.
As i was next to the router when this happened, i followed the sequence of events.
The router did a hard reset, the psu is not faulty as i am running a dc converter block from a 12v floated battery.
The errors were minimal rightup to the hard reset, i regularly keep a look at the bitloading.
Btw Anotherone, during the 10 day training period is the fault threshold rate set referenced from the lowest sync rate or the final sync rate ?
Regarding noise levels after dark, i am very aware to the increase in noise, but bitloading is always a min of 2, the 2700hgv router can and does automatically re allocates unused tones without re sync between night/day.
Paul.
Community Veteran
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Registered: 31-08-2007

Re: Tech answers needed to ADSL2

The FTR is 80% MSR, the lowest sync rate.
Other than that, I can only guess Paul. Have you ever seen minimum bit loading of 1 on the (a) 2700hgv?
Community Veteran
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Registered: 10-06-2010

Re: Tech answers needed to ADSL2

I think the minimum bit loading of 2 is something horribly complicated to do with trellis coding. There has to be an even number of 1 bit channels before the trellis coding is done, which get paired up, and after the whole trellis coding is done it ends up with 2 to 15 bits per channel.
Community Veteran
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Registered: 30-07-2007

Re: Tech answers needed to ADSL2

Quote
Why did the router do a re boot, and not a re train ?

I'm afraid there's really no way to determine that. The reboot will have cleared any history.
If it helps, in my experience of 2700s ( in the office and at home ) I've only ever seen reboots happen for 2 reasons:- after a firmware upgrade and for power supply issues.
I very much doubt it's a f/w upgrade since I'm pretty sure that BT have stopped that since they don't use the 2700 anymore, and you seem to have eliminated the PSU as the cause.
I suggest just putting it down to one of those things... if it happens again then it might be the 2700 developing a h/w fault.
bill888
Seasoned Pro
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Re: Tech answers needed to ADSL2

fwiw, I have seen my 2701HGV reboot every couple of days for no apparent reason in the past.  (The logs are deleted and I caught it in the act on one occasion)
Eventually solved the problem after removing Enhanced Services as described here
http://bt2700hgv.tripod.com/163.htm

maximod
Grafter
Posts: 139
Registered: 15-08-2010

Re: Tech answers needed to ADSL2

Anotherone, i have never seen 1 bit loading on the 2700hgv, adsl1 spec says 2 bit minimum, adsl2 and + says 1 bit minimum.
Interesting ejs regarding trellis coding, i will have to look in to that, looks a bit complicated !
I am using the sbc 4.25.19 firmware, i am pretty sure this is not getting any udate .
I do have several other spare 2700 routers with v5 and v6 firmwares, but i am resisting swapping them for obvious reasons during the training period, and also i do not believe it is faulty !
Such is life,
Paul.
Community Veteran
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Registered: 30-07-2007

Re: Tech answers needed to ADSL2

Wasn't v4 a very early version of the Adsl2 modem code ?
You might be better trying a v5. Tbh I've only ever used v5 or v6 pi n 21cn connections
Community Veteran
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Registered: 31-08-2007

Re: Tech answers needed to ADSL2

Latest firmware was the reason I asked the question really. IIRC a number of modem/routers had to have upgrades to get the correct functioning with ADSL2/+. Maybe this is the reason then.
Edit: If you use the Disconnect method during daylight, I wouldn't have thought swapping would have been a problem. At the end of the day if the wotsit hit the fan you could always have a line reset.  Wink
maximod
Grafter
Posts: 139
Registered: 15-08-2010

Re: Tech answers needed to ADSL2

Quote from: MisterW
Wasn't v4 a very early version of the Adsl2 modem code ?
You might be better trying a v5. Tbh I've only ever used v5 or v6 pi n 21cn connections

I was always lead to believe that v4 firmware was not adsl2+ compatable, but my router shoiws an attempt upon a re sync, then defaults to adsl2, which on my line length i expected it to anyway even if i had a brand new model .
The router has now re synced higher, and unexpectedly routerstats shows interleaving off !
Did not expect that on this line, i checked the routers stats and fec's are 0 so looks to be true.
As the dlm sets the MSR as the lowest sync during the 10 day training period, i think i will leave any router testing till after that period, don't want to tempt fate !
Interestingly my upload sync rate changed before and after the re train, was 444kb, now 431kb, strange as it has 15.8db snrm ?
Seems a bit odd compared to adslmax where the upload stayed at 448kb all the time, except when the phone rings, and a few times dropped to 128kb, but download stayed unaffected.
Anotherone, just spotted your response as i was just about to reply above, As adsl2+ is very unlikely to work on my line attenuation, i did not think that would matter, adsl2 should be ok on v4.
Btw, do you know exactly what the the test it does at the exchange to determine if adsl2+ is used or kicked out ?
Does it look for tone usage above a certain frequency as a trigger point to accept or not use that mode ?
Paul.
Community Veteran
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Registered: 31-08-2007

Re: Tech answers needed to ADSL2

I suspect that it's because the modem/router can't see a high enough SNRM on the tones above 255 and so can't allocate any bits, ie. no usable tones. Now what can't be predicted is how things may behave if tones above 255 are/become usable and the firmware isn't up to the mark.
maximod
Grafter
Posts: 139
Registered: 15-08-2010

Re: Tech answers needed to ADSL2

I say dream on !
My line struggles to see 144 let alone 255 !