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Aspiring Contributor
Posts: 5
Registered: ‎28-05-2017
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Downstream SNR Margin too high after previous disconnects

After a spate of disconnects a few weeks ago, my SNR Margin is now 19.1 db.

Before the disconnects my modem was able to hold 3db fine, so my speed is now half what it was before. I've had my modem up for 4 days and 22 hours (below are the stats) but DLM has not reduced the SNR margin at all. Also I've tried a spare modem but that still connects at 19.

How can I go abouts getting the SNR margin reset to at least default.

Thanks in advance,

Luggles

 

+-------------------+-----------------+
| Connection Time   | 4 days 22:37:08 |
+-------------------+-----------------+
| Connection Status | connected       |
+-------------------+-----------------+
| Negotiation       | Success         |
+-------------------+-----------------+
| Authentication    | Success         |
+-------------------+-----------------+

 

+--------------------------------------------------+
| Line Status                                      |
+--------------------------------------------------+
|                          | Downstream | Upstream |
+--------------------------+------------+----------+
| Attainable rate (kbit/s) | 20472      | 1216     |
+--------------------------+------------+----------+
| SNR margin (dB)          | 19.1       | 9.5      |
+--------------------------+------------+----------+
| Line attenuation (dB)    | 25         | 11.3     |
+--------------------------+------------+----------+
| Output power (dBmV)      | 0          | 12.8     |
+--------------------------+------------+----------+
+--------------------------------------------+
| Statistics                                 |
+--------------------------------------------+
|                    | Downstream | Upstream |
+--------------------+------------+----------+
| Line rate (kbit/s) | 9727       | 1109     |
+--------------------+------------+----------+
| CRC errors         | 317        | 0        |
+--------------------+------------+----------+
| FEC errors         | 11         | 50       |
+--------------------+------------+----------+
| HEC errors         | 160        | 81       |
+--------------------+------------+----------+

 

 

 

 

Distinguished Sage
Posts: 61,271
Registered: ‎21-06-2010
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Re: Downstream SNR Margin too high after previous disconnects

[ Edited ]

Why has my speed suddenly dropped?
 
ADSL connections (see notes about Infinity at the end), and the notes by RogerB
Changes in your connection speed are automatically managed by the exchange equipment, using a process called
Dynamic Line Management (DLM).
Its purpose it to give you the best speed possible, without introducing a lot of errors back into the BT network.
These errors impact on other users, and can slow their connection down, as your data packets have to keep being resent.
This is a continous process, but certain things can cause it to make drastic changes to your connection speed.

These can include the following:-
You could have also been recently moved from ADSL max to ADSL2+, this would mean that the line
would have to re-train again, and must be left undisturbed  for at least 10 days.
 
If you keep restarting or disconnecting your BT Home hub, then the exchange equipment will see this as a fault condition, and reduce your connection speed, in an attempt to stabilise your line.
You have recently had a fault on your line which has just been fixed.

Sometimes a single interruption to your line can cause this too, especially if it generates a large error burst.
The effect is that you get placed in a low banded speed profile, and you will notice that your downstream margin is high, usually over 15dB.
 
The lower the banded speed profile, the quicker it will recover, but it could take up to a month before the fullspeed is recovered.

If you want your speed to recover, then these are the steps you need to take.
 
DO NOT restart or disconnect the power to your BT Home Hub.
DO NOT disconnect your BT Home hub from your phone line.
 
Leave everything alone for at least 10 days, so that the exchange can record a stable connection.

During this time, the exchange may drop the connection, and re-establish a faster connection.
If you use the BT Speedtester, then do not restart the BT Home Hub, even if it tells you to.
If after 10 days your speed does not increase, then please return to your forum post.
showing  that you have 10 days of connection time, then the problem can be escalated.


 
Infinity connections
A similar process is applied to the connection between the Infinity cabinet, and the Openreach modem.
You cannot view that connection information, but the same thing applies, do not restart or disconnect the Openreach modem.
The BT home hub only acts as a router, and restarting it will not affect your physical connection speed to the cabinet.

 

 

Additional information by forum member RogerB
BT broadband is line adaptive, if you have a good line, you get good broadband , BUT consider this, you only get good broadband
 
from the outset, if you let the exchange "train" the connection, when it is new, simply because it has to try various methods of modulation.
 
And once it is running at full speed and set the MSR (maximum stable rate) provided you don't keep disconnecting the router, it will stay that way.
 
So two important things:  ALWAYS leave the router on, 24/7 and DO NOT impede the training process which can take up to 10 days.
 
The reasons are vaguely the same for both scenarios:  DLM trains the connection during setup, and DLM runs the connection in a similar but different mode once it's running.
 
And the most common issue apart from line faults in causing poor broadband is FREQUENT RESETS and DISCONNECTIONS of the router or hub, because if you interupt line management it will demote the line profile if done to excess, with often time consuming efforts and long waits to get it back.
 
BT don't punish users indiscriminently by making them wait for their broadband speed to come up, but the line management is in place  to DEFEND the network against bad and corrupted data, and if the exchange sees your line being reset too often it assumes a fault,
whereby your linespeed is demoted, and often profoundly so.
 
Also, because the exchange can't tell the difference between you messing around resetting all the time, and a real line fault, it assumes (using logic) the worst and demotes the line profile.
 
If you get a real line fault and the noise margin increases, the linespeed will decrease, it's normal, again it's a defence to help give you a service  allbeit a depleted one, and at the same time stop corrupted data getting back to the exchanges.
 
But unfortunatlely it's at this point when people in general start to become frustrated,  and guess what?
 
They then start to reset the router in an attempt to "repair" the connection. And in doing so, impede the line management from promoting the linespeed again. (providing any fault is or was fixed).
 
And to summarise, if the connection is left to run unimpeded, no excess resetting, and left to its own devices, it actually benefits from the good line

history that the exchange has accumulated for your connection, simply because the attention span by the line management is not so profound when it does bite the dust, for whatever reason.
 
So regardless of the opinions of some, switching off broadband routers is NOT a good idea, not if you want consistent broadband and a good line rate.


john46 | BT.com Help Site | BT Service Status
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Aspiring Contributor
Posts: 5
Registered: ‎28-05-2017
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Re: Downstream SNR Margin too high after previous disconnects

Thanks for the info, I'll leave it up for another 5 days then and hopefully that will sort it.

Moderator
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Registered: ‎04-07-2013
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Re: Downstream SNR Margin too high after previous disconnects

@luggles How's you're connection now?

Community ModeratorNeilO
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Aspiring Contributor
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Re: Downstream SNR Margin too high after previous disconnects

Hi @NeilO

 

The connection has been stable. There was a disconect according to the logs on the 1/06/17 for 30 seconds, apart from that its again been up for another 7 days (after 10days up before that), with no issues. The SNR Margin is still really high though, no longer 19 dB but currently i am seeing 17.8 dB. Is there anyway I can get it reset to 6?

Thanks

Luggles

Moderator
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Re: Downstream SNR Margin too high after previous disconnects

@luggles I'm so sorry about the delay getting back to you. If you still need help with this please use the 'click here to contact the mods' link in my forum profile to send in your details. You can find the link by clicking on my username.

Thanks

Neil

Community ModeratorNeilO
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Aspiring Contributor
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Re: Downstream SNR Margin too high after previous disconnects

@NeilO no problem about the delay. The SNR Margin is still 17.6 dB, so I've contacted you through your forum profile as you described.

Thanks 

Luggles

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Re: Downstream SNR Margin too high after previous disconnects

@luggles Thanks, I'm sure we'll be able to get this sorted.

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Re: Downstream SNR Margin too high after previous disconnects

Hi @luggles,

 

Thank you for sending in your details. I've reset your SNR and I'll check back with you tomorrow to see how your connection is preforming.

 

Thanks

 

PaddyB

Community ModeratorPaddyB
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Re: Downstream SNR Margin too high after previous disconnects

Hi @luggles,

 

I've tested your connection and your sync speed has increased to 17435 Kbps. Your SNR is sitting at 6.5 DB. We'll need to keep an eye on your connection to make sure it remains stable. I'll check back with you in around 48 hours to see how the connection has been preforming.

 

Thanks

 

PaddyB

 
Community ModeratorPaddyB
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