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Please may I have an explanation of how the SNR affects broadband delivery?

firstalpha
Grafter
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Registered: 10-04-2007

Please may I have an explanation of how the SNR affects broadband delivery?

In November 2013, my broadband speed increased from 0.47 Mbps to a consistent 1.4 Mbps... from the middle of April, this had slipped back to about 0.98 Mbps and in July, a speedtest revealed a download speed of about 1.17 Mbps...
About 6 weeks ago there were some really bad days when the speed was between 0.02 and 0.05 Mbps... Plusnet support conducted some test and arranged for a BT Openreach engineer to call on 1 October...
The Engineer was unable to find any errors on my line at the time of his visit, about 9-10am... and he did fit a Mk2 OpenReach faceplate...
The attenuation on the line is 64.5 dB and the SN is set to 6... I believe the line did drop out and lose sync at about 12pm on that day...
He advised me that the SNR could be increased to 9 and although this would make the line more stable, there would be a drop in speed... not something I wanted to consider... he also said that equipment at the exchange is supposed to be managing the SNR for optimum performance... I have noticed that when I reboot the router the SNR is typically 7-8 dB and as the connectivity degrades, I have seen the SNR as low as 1.5... which is when a reboot becomes necessary...
I have difficulty understanding why the performance of my broadband has not returned to the stable speed I initially had between November and April 2014...
In layman's terms, please can I have a simple explanation as to what causes the SNR to fluctuate and how the exchange equipment is supposed to be managing it?
Kind Regards... Richard...
458 REPLIES
Community Veteran
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

Noise, is the simple answer. The more noise, the lower the SNRM - Signal to Noise Ratio Margin. When you resync it always sets back to the Target value,so if there is a lot of noise your speed will be lower.
If the SNRM is varying a lot, you have a noisy line. This could be due to bad joints etc. in which case you may here crackling noises when using the phone, or it could be just electrical/electronic interference sometimes called REIN. It could be severe cross-talk from another broadband line on the same cable to your DP.
What modem/router are you using? If it the TG582n can you post the TG582n Full DSL stats
Also do a quiet line test 17070 option2, do you hear any crackling or other noises?
I assume you are plugged directly into the top socket on your Mk2 vDSL plate - no extension leads?
firstalpha
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

Quote from: Anotherone
What modem/router are you using? If it the TG582n can you post the TG582n Full DSL stats

DSL Connection
Link Information
Uptime: 0 days, 0:05:06
DSL Type: G.992.5 annex A
Maximum Bandwidth (Up/Down) [kbps/kbps]: 17,897 / 2,024
Bandwidth (Up/Down) [kbps/kbps]: 444 / 1,144
Data Transferred (Sent/Received) [MB/MB]: 2.24 / 6.26
Output Power (Up/Down) [dBm]: 12.5 / 16.0
Line Attenuation (Up/Down) [dB]: 37.5 / 65.0
SN Margin (Up/Down) [dB]: 15.0 / 7.5
Vendor ID (Local/Remote): TMMB / TSTC
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 0 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 0 / 0
FEC Errors (Up/Down): 60,004 / 502
CRC Errors (Up/Down): 10,403 / 1
HEC Errors (Up/Down): 1,957 / 0
I've just rebooted and this is the stats you asked for...
The quiet test confirmed that the line is... quiet... no crackling or other noises...
Thank you for replying... I'll post the stats again later today...
My Thomson router is:
Product Name: TG585 v8
Serial Number: <snip>
Software Release: 8.2.7.7
jim:green serial number removed mod:end
Community Veteran
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Registered: 31-08-2007

Re: Please may I have an explanation of how the SNR affects broadband delivery?

Ah, didn't expect to find you about at this time, just come on myself having had a bit of an issue.
Not a good idea to reboot on a long line after dark, you will end up with slower speeds because background MW/AM propagation is higher.
Luckily you are on a 21CN connection so your profiles update quicker when you get an increase in sync speed. Always best at least an hour after sunrise and at least an hour before sunset.
The best way of doing a resync, or anything that involves loss of sync is to use the following Disconnect method -
Log in to the Modem/Router, in the Internet box, click Disconnect to drop the PPP Internet session (this is not the sync), wait about a minute and then power down the Modem/Router. After about another minute you can unplug it from the line.
Do whatever is needed, but stay offline for at least 10 minutes. When you re-connect and power up, Login to the Modem/Router and click Connect to establish a new PPP session.
Don't do this more than 5 times in an hour, then leave it for the rest of the day. This method is to help stop the Exchange DLM from thinking your sync loss was a dropped connection.
If you have a modem/router that doesn't have an interface for you to drop the PPP session, then pull the power plug and wait a minute or two before unplugging from the line.
One further point, especially applicable to very long lines, you will get very small differences in sync speed every time you sync, so just because you get a marginally better sync speed in one situation, it doesn't necessarily mean that situation is better than another situation.
PS. You can do a Disconnect & Connect (without a power down) to Gateway hop.
firstalpha
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

DSL Connection
Link Information

Uptime: 0 days, 1:19:42
DSL Type: G.992.5 annex A
Maximum Bandwidth (Up/Down) [kbps/kbps]: 17,926 / 1,568
Bandwidth (Up/Down) [kbps/kbps]: 444 / 1,144
Data Transferred (Sent/Received) [MB/MB]: 17.29 / 40.92
Output Power (Up/Down) [dBm]: 12.5 / 16.0
Line Attenuation (Up/Down) [dB]: 37.5 / 65.0
SN Margin (Up/Down) [dB]: 15.0 / 3.5
Vendor ID (Local/Remote): TMMB / TSTC
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 0 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 5 / 0
FEC Errors (Up/Down): 60,012 / 51,351
CRC Errors (Up/Down): 20,806 / 28
HEC Errors (Up/Down): 1,957 / 28
As you can see, after being connected for just over an hour, the SNR has dropped to 3.5... is that significant?
The reason I rebooted at 1:30am was because I lost the connection and the SNR was down to 2.0...
I'll try your suggestion later... I'm off to bed now...
KRs... Richard...
Community Veteran
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

The SNRM change is quite large and we need to establish the cause. It may just be because you have a long line, but I have seen much better on lines of a similar length.
Are you plugged in at the Master socket, and do you have any extension sockets? Is the filter plugged directly into the socket and the modem/router directly into the filter without the use of extension leads?
First try the quiet line test with your phone (a corded one ideally). Dial 17070 Option 2. The line should be absolutely silent except possibly a very very quiet hiss. If you hear any crackles or other noise or have any problems with incoming or outgoing calls, you need to report a phone fault (not mentioning the broadband)and tell them to note it's intermittent.
firstalpha
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

DSL Connection
Link Information
Uptime: 0 days, 6:12:17
DSL Type: G.992.5 annex A
Maximum Bandwidth (Up/Down) [kbps/kbps]: 17,897 / 1,660
Bandwidth (Up/Down) [kbps/kbps]: 444 / 1,144
Data Transferred (Sent/Received) [GB/MB]: 188.76 / 92.06
Output Power (Up/Down) [dBm]: 12.5 / 16.0
Line Attenuation (Up/Down) [dB]: 37.5 / 65.0
SN Margin (Up/Down) [dB]: 15.0 / 4.5
Vendor ID (Local/Remote): TMMB / TSTC
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 0 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 4,864 / 0
FEC Errors (Up/Down): 60,012 / 665,227
CRC Errors (Up/Down): 343,299 / 700
HEC Errors (Up/Down): NA / 646
OK... it's now 5 hrs later and the SNR is being reported as 4.5
The quiet test is perfect... just silence... I remember the engineer being impressed with his initial test results...
I've disconnected on the router's home page and reconnected after about 1 or 2 mins...
Now the DSL Connection Link Information looks like this:

Uptime: 0 days, 6:30:29
DSL Type: G.992.5 annex A
Maximum Bandwidth (Up/Down) [kbps/kbps]: 17,897 / 1,760
Bandwidth (Up/Down) [kbps/kbps]: 444 / 1,144
Data Transferred (Sent/Received) [GB/MB]: 188.76 / 97.99
Output Power (Up/Down) [dBm]: 12.5 / 16.0
Line Attenuation (Up/Down) [dB]: 37.5 / 65.0
SN Margin (Up/Down) [dB]: 15.0 / 5.5
Vendor ID (Local/Remote): TMMB / TSTC
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 0 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 1 / 0
FEC Errors (Up/Down): 60,081 / 1,667,432
CRC Errors (Up/Down): 343,299 / 1,234
HEC Errors (Up/Down): NA / 741
The SNR is showing as being 5.5... I've not performed this type of disconnect/reconnect before...
firstalpha
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Registered: 10-04-2007

Re: Please may I have an explanation of how the SNR affects broadband delivery?

An hour and a half has passed and whilst I have been out of the house, my computer is continuously connected to a forex server... with the SNR now being reported at 4 dB, does this mean that the change from 7.5 to 3.5 to 5.5 to 5 and now 4 is the result of the exchange equipment 'managing' the connection?
As I post this rely at 0 days, 8:32:14 uptime, the reported SNR is... SN Margin (Up/Down) [dB]: 14.5 / 3.5
DSL Connection
Link Information
Uptime: 0 days, 8:02:42
DSL Type: G.992.5 annex A
Maximum Bandwidth (Up/Down) [kbps/kbps]: 17,897 / 1,564
Bandwidth (Up/Down) [kbps/kbps]: 444 / 1,144
Data Transferred (Sent/Received) [GB/MB]: 188.78 / 130.80
Output Power (Up/Down) [dBm]: 12.5 / 16.0
Line Attenuation (Up/Down) [dB]: 37.5 / 65.0
SN Margin (Up/Down) [dB]: 14.5 / 4.0
Vendor ID (Local/Remote): TMMB / TSTC
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 0 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 16 / 0
FEC Errors (Up/Down): 60,312 / 1,877,779
CRC Errors (Up/Down): 343,300 / 1,264
HEC Errors (Up/Down): NA / 770
Superuser
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Registered: 22-08-2007

Re: Please may I have an explanation of how the SNR affects broadband delivery?

Hi Richard,
The variability in SNRM is nit the action of the exchange, rather it suggestive of variable electrical noise.  This could be down to a line (copper) fault or it could be interference.
Anotherone has started with this issue so I'll leave it to him to continue with.
For the time being, when you notice the SNRM is low, can you repeat the QLT as line noise can be intermittent.
Note that the disconnect you did through the router interface (as advised by AO) does not drop the xDSL session and therefore will not change the synch rate.  Given the wide variation of the SNRM, I suggest that you avoid re syncing at the moment.  Though you will gave a lower synch speed the connection will be more stable as the SNRM might not fall sufficiently to drop the connection.
I suspect that some detailed SNRM monitoring will very required to explore the possibility of REIN - always bad news I'm afraid!

@CRT,
What is the target SNRM fir this line please?
Is there a recent full CLT report you can share please?  An intrusive test might not be desirable at the moment... but as ever it's your call! Wink

Kevin
firstalpha
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

Just checking the SNR again @Uptime: 0 days, 12:02:28
SN Margin (Up/Down) [dB]: 14.5 / 7.0
WOW... up to 7 without any interaction from me!!!
Community Veteran
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

Quote from: Richard
The attenuation on the line is 64.5 dB and the SN is set to 6... I believe the line did drop out and lose sync at about 12pm on that day...
He advised me that the SNR could be increased to 9 and although this would make the line more stable, there would be a drop in speed... not something I wanted to consider...

Actually I don't think it makes much difference at the moment. The SNRM variability is sufficiently large over such a short timescale that it's a case of trying to find the root cause first before worrying about anything else. As I pointed out in my initial reply, it will be noise of one sort or another.
As Kevin has mentioned it could be line, interference etc. His suggestion of doing the Quiet Line Test when you see the SNRM very low is certainly a good one, and I'd suggest doing it a bit more regularly as well when you are seeing variability. Intermittent line faults can sometime be a pain to catch and a one-off test doesn't always show any problems.
Are you seeing similar variability during the day?
Just some further clarification on the "Disconnect" method.  As Kevin has repeated, the Disconnect of the PPP Internet Session does not affect Sync (nor SNRM by implication). When you drop the PPP session, the exchange DLM sees this as user action and so any subsequent loss of sync is generally not seen as a "dropping connection" which might otherwise cause DLM to act if you are or have been having other drops in sync due to something being faulty or serious interference.
(Note however, that a loss of sync will generally cause a loss of PPP Internet session if you haven't disconnected it yourself. There are some rarer exceptions, but that is getting more technical than we need at this time).
Now if your daytime SNRM is more stable, it suggests that the main part of the problem stems from the pickup of night-time interference, the bulk of which is MW/AM propagation. This can be caused by unbalanced lines (often caused by faults) not uncommon on long lines but not always the case, or where part of the connection doesn't use "twisted-pair" cable. This can be internal to your installation or sometimes external where the original BT cable is to a very old standard.
Quote from: Anotherone
I assume you are plugged directly into the top socket on your Mk2 vDSL plate - no extension leads?

Quote from: Anotherone
Are you plugged in at the Master socket, and do you have any extension sockets? Is the filter plugged directly into the socket and the modem/router directly into the filter without the use of extension leads?

I did ask both these questions to be absolutely clear on the way you are or could be connected and I note you haven't commented in response to either. Could you please be very clear on how you are setup, whether you have extension sockets (or is the master socket the only socket), using extension (phone/router) leads etc.
Plusnet Staff
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

Hi,
Sorry for just picking this up a little late. SNR Margin is 6db.
If the OP is happy with me posting the copper line results here, I'll post.
Tony
 Tony
 Plusnet Support
firstalpha
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

Hey Tony... my router just rebooted itself... NO interaction from me at all...
DSL Connection
Link Information
Uptime: 0 days, 0:02:19
DSL Type: G.992.5 annex A
Maximum Bandwidth (Up/Down) [kbps/kbps]: 17,962 / 2,072
Bandwidth (Up/Down) [kbps/kbps]: 444 / 1,144
Data Transferred (Sent/Received) [MB/MB]: 1.07 / 2.68
Output Power (Up/Down) [dBm]: 12.5 / 15.5
Line Attenuation (Up/Down) [dB]: 37.5 / 64.5
SN Margin (Up/Down) [dB]: 15.5 / 7.0
Vendor ID (Local/Remote): TMMB / TSTC
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 5 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 15,186 / 0
FEC Errors (Up/Down): 1 / 91
CRC Errors (Up/Down): 458 / 0
HEC Errors (Up/Down): 11 / 0
You are welcome to post the results from my ongoing support ticket... I am just seeking to understand what it all means... like I said... between November last year and April 2014 the service was probably the best it could be... certainly better than the 0.47 Mbps I was getting before...
KRs... Richard...
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

I can hazard a guess that the sync loss was Tony doing what's called an "intrusive test" which is needed to fully check out the copper. So when he posts the results, I'll give you a bit of info on what they mean.
In the meantime can I please draw your attention to the questions at the end of my reply #10. This is extremely important in relation to eliminating potential contributions to the large SNRM variability.
firstalpha
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Re: Please may I have an explanation of how the SNR affects broadband delivery?

Quote from: Anotherone
In the meantime can I please draw your attention to the questions at the end of my reply #10. This is extremely important in relation to eliminating potential contributions to the large SNRM variability.
I have changed nothing... the router is plugged into the new Mk2 OpenReach faceplate and one telephone handset plugs directly into the phone socket... there are no sounds of any sort when running the quiet line test...
DSL Connection
Link Information
Uptime: 0 days, 7:24:06
DSL Type: G.992.5 annex A
Maximum Bandwidth (Up/Down) [kbps/kbps]: 17,962 / 1,692
Bandwidth (Up/Down) [kbps/kbps]: 444 / 1,144
Data Transferred (Sent/Received) [MB/MB]: 208.44 / 603.50
Output Power (Up/Down) [dBm]: 12.5 / 15.5
Line Attenuation (Up/Down) [dB]: 37.5 / 64.5
SN Margin (Up/Down) [dB]: 15.0 / 4.5
Vendor ID (Local/Remote): TMMB / TSTC
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 5 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 65,536 / 0
FEC Errors (Up/Down): 751 / 247,536
CRC Errors (Up/Down): 473 / 386
HEC Errors (Up/Down): 27 / NA
I'll be retiring to bed now and will check the DSL stats again later...
Thank you for your attention...