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uk.telecom.broadband (UK broadband) (uk.telecom.broadband) Discussion of broadband services, technology and equipment as provided in the UK. Discussions of specific services based on ADSL, cable modems or other broadband technology are also on-topic. Advertising is not allowed.

Diagnosing a strange broadband problem - wide variety of sync speeds



 
 
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  #1  
Old July 15th 14, 10:43 AM posted to uk.telecom.broadband
NY
external usenet poster
 
Posts: 492
Default Diagnosing a strange broadband problem - wide variety of sync speeds

I asked to help someone whose broadband had stopped working. The DSL light
on their router came on but the internet light never came on. Sadly this
make of router had no logging facility to show how far it was getting - eg
whether it was even trying to log on using the ADSL username and password or
whether it was failing to authenticate with those details, and it didn't
report attenuation, noise margin or sync speed. Pretty useless to have a
router that either works or doesn't but gives no clues as to why it doesn't
work :-( After checking for wiring errors (eg unfiltered phone) and trying
in the test socket of the master socket, I checked what happened with my
known-good router.

And that exhibited a strange fault: occasionally it would successfully
connect, but many times it wouldn't. I kept unplugging the DSL cable to
force it retry the connection. And there was a correlation between DSL sync
speed and ability to connect: if the router happened to sync at a fairly
slow speed (eg 1 Mbps) it would connect fine. If it happened to sync at a
higher speed (eg 2 or 3 Mbps) it would fail. Attenuation remained the same
but noise margin was worse as sync speed increased.

Can anyone think of a line fault that would cause a router to sync at a wide
variety of different speeds and only to connect successfully if it happened
to sync at a slow speed? Wouldn't you expect a router always to sync at the
same speed +/- a fairly small latitude, rather than trying speeds as far
apart as 1 Mbps and 4 Mbps?

Intriguingly when a BT Openreach engineer was called to investigate, he
diagnosed a fault in the customer's router rather than a line fault - so
maybe the behaviour of my test router was a red herring. I checked my router
on my own line later and it always synced at the same speed, give or take a
small latitude, so it hadn't started playing silly buggers.

  #2  
Old July 15th 14, 01:04 PM posted to uk.telecom.broadband
Peter Able
external usenet poster
 
Posts: 114
Default Diagnosing a strange broadband problem - wide variety of syncspeeds

On 15/07/2014 10:43, NY wrote:
I asked to help someone whose broadband had stopped working. The DSL
light on their router came on but the internet light never came on.
Sadly this make of router had no logging facility to show how far it was
getting - eg whether it was even trying to log on using the ADSL
username and password or whether it was failing to authenticate with
those details, and it didn't report attenuation, noise margin or sync
speed. Pretty useless to have a router that either works or doesn't but
gives no clues as to why it doesn't work :-( After checking for wiring
errors (eg unfiltered phone) and trying in the test socket of the master
socket, I checked what happened with my known-good router.

And that exhibited a strange fault: occasionally it would successfully
connect, but many times it wouldn't. I kept unplugging the DSL cable to
force it retry the connection. And there was a correlation between DSL
sync speed and ability to connect: if the router happened to sync at a
fairly slow speed (eg 1 Mbps) it would connect fine. If it happened to
sync at a higher speed (eg 2 or 3 Mbps) it would fail. Attenuation
remained the same but noise margin was worse as sync speed increased.

Can anyone think of a line fault that would cause a router to sync at a
wide variety of different speeds and only to connect successfully if it
happened to sync at a slow speed? Wouldn't you expect a router always to
sync at the same speed +/- a fairly small latitude, rather than trying
speeds as far apart as 1 Mbps and 4 Mbps?

Intriguingly when a BT Openreach engineer was called to investigate, he
diagnosed a fault in the customer's router rather than a line fault - so
maybe the behaviour of my test router was a red herring. I checked my
router on my own line later and it always synced at the same speed, give
or take a small latitude, so it hadn't started playing silly buggers.


It seems quite rational that a low negotiated sync speed would increase
the likelihood of making a connection, so perhaps the question is
primarily "why the differing sync speeds?". This is where a
differential-input oscilloscope comes in handy. It would clarify if the
problem is (intermittent) HF noise on the line.

PA

  #3  
Old July 15th 14, 01:46 PM posted to uk.telecom.broadband
NY
external usenet poster
 
Posts: 492
Default Diagnosing a strange broadband problem - wide variety of sync speeds

"Peter Able" [email protected] wrote in message
o.uk...
On 15/07/2014 10:43, NY wrote:

It seems quite rational that a low negotiated sync speed would increase
the likelihood of making a connection, so perhaps the question is
primarily "why the differing sync speeds?". This is where a
differential-input oscilloscope comes in handy. It would clarify if the
problem is (intermittent) HF noise on the line.


Yes, I sort-of asked that question (although in a rather roundabout way) in
my question: why would a router sync at a variety of different speeds rather
than always syncing at the same speed on successive connection attempts.

I realise that if the router happens (for whatever reason) to sync at too
high a speed for the noise on the line, the connection will probably fail.

A USB oscilloscope that plugged into a laptop and displayed traces on the
screen would be lovely (please, Santa!) but they are horrendous amounts of
money for the few occasions when I'd use one.

  #4  
Old July 15th 14, 03:15 PM posted to uk.telecom.broadband
The Natural Philosopher
external usenet poster
 
Posts: 2,728
Default Diagnosing a strange broadband problem - wide variety of syncspeeds

On 15/07/14 13:04, Peter Able wrote:
On 15/07/2014 10:43, NY wrote:
I asked to help someone whose broadband had stopped working. The DSL
light on their router came on but the internet light never came on.
Sadly this make of router had no logging facility to show how far it was
getting - eg whether it was even trying to log on using the ADSL
username and password or whether it was failing to authenticate with
those details, and it didn't report attenuation, noise margin or sync
speed. Pretty useless to have a router that either works or doesn't but
gives no clues as to why it doesn't work :-( After checking for wiring
errors (eg unfiltered phone) and trying in the test socket of the master
socket, I checked what happened with my known-good router.

And that exhibited a strange fault: occasionally it would successfully
connect, but many times it wouldn't. I kept unplugging the DSL cable to
force it retry the connection. And there was a correlation between DSL
sync speed and ability to connect: if the router happened to sync at a
fairly slow speed (eg 1 Mbps) it would connect fine. If it happened to
sync at a higher speed (eg 2 or 3 Mbps) it would fail. Attenuation
remained the same but noise margin was worse as sync speed increased.

Can anyone think of a line fault that would cause a router to sync at a
wide variety of different speeds and only to connect successfully if it
happened to sync at a slow speed? Wouldn't you expect a router always to
sync at the same speed +/- a fairly small latitude, rather than trying
speeds as far apart as 1 Mbps and 4 Mbps?

Intriguingly when a BT Openreach engineer was called to investigate, he
diagnosed a fault in the customer's router rather than a line fault - so
maybe the behaviour of my test router was a red herring. I checked my
router on my own line later and it always synced at the same speed, give
or take a small latitude, so it hadn't started playing silly buggers.


It seems quite rational that a low negotiated sync speed would increase
the likelihood of making a connection, so perhaps the question is
primarily "why the differing sync speeds?". This is where a
differential-input oscilloscope comes in handy. It would clarify if the
problem is (intermittent) HF noise on the line.


I wouldn't make that my tool of choice.


PA



--
Everything you read in newspapers is absolutely true, except for the
rare story of which you happen to have first-hand knowledge. - Erwin Knoll
  #5  
Old July 15th 14, 06:01 PM posted to uk.telecom.broadband
Peter Able
external usenet poster
 
Posts: 114
Default Diagnosing a strange broadband problem - wide variety of syncspeeds

On 15/07/2014 13:46, NY wrote:
"Peter Able" [email protected] wrote in message
o.uk...
On 15/07/2014 10:43, NY wrote:

It seems quite rational that a low negotiated sync speed would
increase the likelihood of making a connection, so perhaps the
question is primarily "why the differing sync speeds?". This is where
a differential-input oscilloscope comes in handy. It would clarify if
the problem is (intermittent) HF noise on the line.


Yes, I sort-of asked that question (although in a rather roundabout way)
in my question: why would a router sync at a variety of different speeds
rather than always syncing at the same speed on successive connection
attempts.

I realise that if the router happens (for whatever reason) to sync at
too high a speed for the noise on the line, the connection will probably
fail.

A USB oscilloscope that plugged into a laptop and displayed traces on
the screen would be lovely (please, Santa!) but they are horrendous
amounts of money for the few occasions when I'd use one.


I happen to have one and so can simultaneously observe A, B and A-B, but
a simple oscilloscope transformer-coupled to the line will suffice.
Sometime I must try transformer-coupling a spectrum analyser to my line.

It's all down to the perk of having an office facing onto the corridor
between the firm's calibration department and the Biffa bin ;-}}

PA

  #6  
Old July 15th 14, 08:29 PM posted to uk.telecom.broadband
Graham J[_2_]
external usenet poster
 
Posts: 723
Default Diagnosing a strange broadband problem - wide variety of syncspeeds

NY wrote:
I asked to help someone whose broadband had stopped working. The DSL
light on their router came on but the internet light never came on.
Sadly this make of router had no logging facility to show how far it was
getting - eg whether it was even trying to log on using the ADSL
username and password or whether it was failing to authenticate with
those details, and it didn't report attenuation, noise margin or sync
speed. Pretty useless to have a router that either works or doesn't but
gives no clues as to why it doesn't work :-( After checking for wiring
errors (eg unfiltered phone) and trying in the test socket of the master
socket, I checked what happened with my known-good router.

And that exhibited a strange fault: occasionally it would successfully
connect, but many times it wouldn't. I kept unplugging the DSL cable to
force it retry the connection. And there was a correlation between DSL
sync speed and ability to connect: if the router happened to sync at a
fairly slow speed (eg 1 Mbps) it would connect fine. If it happened to
sync at a higher speed (eg 2 or 3 Mbps) it would fail. Attenuation
remained the same but noise margin was worse as sync speed increased.

Can anyone think of a line fault that would cause a router to sync at a
wide variety of different speeds and only to connect successfully if it
happened to sync at a slow speed? Wouldn't you expect a router always to
sync at the same speed +/- a fairly small latitude, rather than trying
speeds as far apart as 1 Mbps and 4 Mbps?

Intriguingly when a BT Openreach engineer was called to investigate, he
diagnosed a fault in the customer's router rather than a line fault - so
maybe the behaviour of my test router was a red herring. I checked my
router on my own line later and it always synced at the same speed, give
or take a small latitude, so it hadn't started playing silly buggers.



Normally, if the sync speed increases you would see the SNR margin
reduce, to settle at about 6dB - depending on whether the service is
ADSL, ADSL2, or ADSL2+.

The converse occurs when there is noise on the line. The SNR margin
increases and the sync speed decreases.

This is indeed what you saw. The implication is that at the higher
speeds the noise margin was too small to allow reliable communication,
and the login process may have been corrupted by the noise.

It follows that the router should have shown you errors. This is where
the choice of router as a diagnostic tool becomes important. Some will
only report SNR margin and loop attenuation for the downstream
direction; others will report it in both directions.

Some routers will report uncorrected errors (which require that the data
packet is retranmitted) and others will also report corrected errors
(where the Forward Error Correction [FEC] mechanism allows the router to
identify and correct errors).

For example the (no longer current) Vigor 2600 and 2800 series routers
report both corrected and uncorrected errors. I have seen cases where
the corrected error count rises to about 300 errors per second, and
stays there; but a re-sync causes the corrected count to drop back to
nearly zero. In this instance I suspect the processor in the router
cannot keep up, thus while correcting one packet it misses the start of
the next packet and so has to correct that packet also. A more modern
router (the TP-Link TD-W8960N) on the same line shows a high corrected
error count at times, (maybe 1,000 errors per second) but it always
drops back to nearly zero as the intermittent noise fades away.

By contrast the more modern Vigor 2820 and 2830 routers show only
"errors" and do not identify the cause.

I think the exchange equipment (DSLAM or whatever) can only recognise
uncorrected errors - certainly when the router shows a high corrected
error count the ISP says there are no errors reported at their end.

When I've watched Openreach engineers monitor a line, they generally
ignore FEC errors.

I've never seen the circumstance you describe where a router behaves
erratically at a customer site yet is reliable at your home site without
also the Openreach engineer being able to see a problem at the customer
site. Did you actually watch the engineer working and ask him to
explain his conclusions? I find this approach is absolutely vital to
understanding the nature of the problem, both for you and sometimes also
for the Opnreach engineer!

--
Graham J



  #7  
Old July 16th 14, 09:25 PM posted to uk.telecom.broadband
Martin Brown
external usenet poster
 
Posts: 343
Default Diagnosing a strange broadband problem - wide variety of syncspeeds

On 15/07/2014 10:43, NY wrote:
I asked to help someone whose broadband had stopped working. The DSL
light on their router came on but the internet light never came on.
Sadly this make of router had no logging facility to show how far it was
getting - eg whether it was even trying to log on using the ADSL
username and password or whether it was failing to authenticate with
those details, and it didn't report attenuation, noise margin or sync
speed. Pretty useless to have a router that either works or doesn't but
gives no clues as to why it doesn't work :-( After checking for wiring
errors (eg unfiltered phone) and trying in the test socket of the master
socket, I checked what happened with my known-good router.


There seems to be a trend towards magic black box ADSL zero diagnostics
routers at the consumer end of the market these days

And that exhibited a strange fault: occasionally it would successfully
connect, but many times it wouldn't. I kept unplugging the DSL cable to
force it retry the connection. And there was a correlation between DSL
sync speed and ability to connect: if the router happened to sync at a
fairly slow speed (eg 1 Mbps) it would connect fine. If it happened to
sync at a higher speed (eg 2 or 3 Mbps) it would fail. Attenuation
remained the same but noise margin was worse as sync speed increased.


That is normal as is getting a worse SNR/connect speed after sunset
(which might be helpful here). You need to track SNR against connect
speed to see where you stand. A reliable connection should sit at 6dB
and if you are lucky on fastpath. Rural lines tend to be 9dB and
interleaved. I favour routerstats light as a first test - there is a
German one even better but it doesn't work with any modem that I own.

Can anyone think of a line fault that would cause a router to sync at a
wide variety of different speeds and only to connect successfully if it
happened to sync at a slow speed? Wouldn't you expect a router always to
sync at the same speed +/- a fairly small latitude, rather than trying
speeds as far apart as 1 Mbps and 4 Mbps?


If there is an intermittent fault like a dry joint where the thing is
capable of 4Mbps for short intervals long enough to calibrate the link
but not to sustain meaningful data transfer then this is sort of what
you expect. But then what should happen is that you enter a spiral of
speed downgrades due to disconnects until the SNR is sufficiently high
to maintain a reliable connection with a floor at about 256k.

A power cycling router reboot is generally a bit more consistent that
plugging and unplugging the external connection. Some brands are prone
to getting themselves into strange states after extended periods...

Intriguingly when a BT Openreach engineer was called to investigate, he
diagnosed a fault in the customer's router rather than a line fault - so
maybe the behaviour of my test router was a red herring. I checked my
router on my own line later and it always synced at the same speed, give
or take a small latitude, so it hadn't started playing silly buggers.


Local intermittent interference of some sort would be my guess. I don't
understand why you are not seeing successive retries drop in speed until
a reliable connection is established (albeit at pathetic data rates).
That is certainly what happens on lines with dodgy rectifying aluminium
wire with corroded oxide connections round here.

--
Regards,
Martin Brown
 




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