Impact of hardware with bad SNR & Attenuation

I'm struggling to find comprehensive (or at least convincingly accurate)
information on what I'm sure must be a FAQ:

- What are the "good" values for SNR/attenuation for
different line speeds, and more importantly:
- what are the symptoms of borderline low SNR?
- what are the symptoms of borderline high
attenuation?
- what (in each case) if the upstream figures
are good and downstream bad?
- Vice versa?

- Regarding CRC errors:
- what causes them?
- what is a "good reading? (Obviously 0 is good
but at what figure should I worry about them?)
- what impact do they have?

- What impact does hardware play in making up for
borderline figures? Eg:
- having a crap filter?
- having crap cable?
- having a crap modem?
- having too much or crap telephone equipment
hung on the same telephone line?

- What does environmental conditions (eg weather)
affect:
- the SNR and attenuation readings?
- the impact of bad values?

To put the whole question another way: I want to have a reasonable idea if I
visit a customer with ADSL issues, when I look at the information the router
can tell me, what I should do. In which cases will changing the hardware
help (and in those cases what should I be changing).

--
Mark Rogers




Mark Rogers

On Wed, 31 Aug 2005 11:56:45 +0100, "Mark Rogers"
<mark-(E-Mail Removed)> wrote:

>I'm struggling to find comprehensive (or at least convincingly accurate)
>information on what I'm sure must be a FAQ:
sounds like you need a training course ;-)

>- What are the "good" values for SNR/attenuation for
6 dB of SNR margin is a typical minimum for reasonable service, 10 dB
or higher should be robust.

> different line speeds, and more importantly:
> - what are the symptoms of borderline low SNR?
disconnections, lots of errors.

> - what are the symptoms of borderline high
> attenuation?
none, unless it results in low SNR margin

> - what (in each case) if the upstream figures
> are good and downstream bad?
> - Vice versa?
BT's 512k products will slow down the data rate if the upstream margin
is too low, should show up as a low upstream speed (fairly rare). 1M
and above will fail to sync if inadequate upstream SNR margin.

>- Regarding CRC errors:
> - what causes them?
> - what is a "good reading? (Obviously 0 is good
> but at what figure should I worry about them?)
> - what impact do they have?
errored seconds (seconds with one or more errors) can be useful, 5 in
an hour isn't a problem, is. CRC errors arise when the signal is
corrupted by interference or by being too weak for the modem.

>- What impact does hardware play in making up for
> borderline figures? Eg:
> - having a crap filter?
> - having crap cable?
> - having a crap modem?
> - having too much or crap telephone equipment
> hung on the same telephone line?
all of these things can reduce the SNR, attenuation is more robust.

>- What does environmental conditions (eg weather)
> affect:
> - the SNR and attenuation readings?
> - the impact of bad values?
thunderstorms generate noise that reduces SNR. Rain can affect damaged
cables.

>To put the whole question another way: I want to have a reasonable idea if I
>visit a customer with ADSL issues, when I look at the information the router
>can tell me, what I should do. In which cases will changing the hardware
>help (and in those cases what should I be changing).

if you are going in as a diagnostic expert I would suggest you carry a
"golden modem" known to work well on a battery powered laptop. Take
along a known good filter and an RJ11-BT adaptor too. Diagnosis by
changing / removing things seems the most practical.

The most common problem area IMO is the domestic extension phone
wiring. Use the test socket (where available) to separate this out
http://www.adslguide.org.uk/newsarchive.asp?item=2182

Phil
--
Tiscali - dialup speeds at Broadband prices, see
http://bbs.adslguide.org.uk/postlist...&Board=tiscali

AOL - the unlimited ISP of choice for heavy downloaders.

Is this for your course assignment?

"Mark Rogers" <mark-(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I'm struggling to find comprehensive (or at least convincingly accurate)
> information on what I'm sure must be a FAQ:
>
> - What are the "good" values for SNR/attenuation for
> different line speeds, and more importantly:
> - what are the symptoms of borderline low SNR?
Increased [bit] error rates, longer start up synchronisation, hiccups in
transmission.
> - what are the symptoms of borderline high
> attenuation?
The same, plus possibly periods without connection. Eventually resets and
possibly data loss.
> - what (in each case) if the upstream figures
> are good and downstream bad?
> - Vice versa?
Unlikely to be asynchronous, however the use of lower bit rates for upstream
is a common way of reducing the impact of this problem (e.g. 75/1200 and
ADSL)
>
> - Regarding CRC errors:
> - what causes them?
Bits dropped out, high noise, interference, loss of signal, poor receiver
etc. etc.
> - what is a "good reading? (Obviously 0 is good
indeed
> but at what figure should I worry about them?)
> - wat impact do they have?

Frames / packets with bad CRC are discarded. Missing data frames will be
noticed from the incorrect sequence and retransmission requested. This may
take some time. If supervisory frames are missed they may or may not have a
major impact depending on the protcol being used.

>
> - What impact does hardware play in making up for
> borderline figures? Eg:
> - having a crap filter?
> - having crap cable?
> - having a crap modem?
> - having too much or crap telephone equipment
> hung on the same telephone line?
All of these can reduce signal and / or increase noise.
>
> - What does environmental conditions (eg weather)
> affect:
> - the SNR and attenuation readings?
> - the impact of bad values

Water in cables increases noise and often reduces signal. Lightening can
cause spikes of interference.

>
> To put the whole question another way: I want to have a reasonable idea if
> I
> visit a customer with ADSL issues, when I look at the information the
> router
> can tell me, what I should do. In which cases will changing the hardware
> help (and in those cases what should I be changing).

Take a spare and swap it in. If the fault goes then the equipment was
faulty, if not then look to the wiring.

>
> --
> Mark Rogers
>
>

In article <(E-Mail Removed)>, mark-(E-Mail Removed)
says...
> I'm struggling to find comprehensive (or at least convincingly accurate)
> information on what I'm sure must be a FAQ:
>
> - What are the "good" values for SNR/attenuation for
> different line speeds, and more importantly:
> - what are the symptoms of borderline low SNR?
> - what are the symptoms of borderline high
> attenuation?
> - what (in each case) if the upstream figures
> are good and downstream bad?
> - Vice versa?

While I don't have any concrete information for higher speeds the
general guidelines (from BT) are as follows

Attenuation:

512kb I've seen working up at around 75dB downstream attenuation - BT
will activate anything on an enabled exchange and attempt to get it
working. If it doesn't work on long lines initially an appointment may
be required to fit a filtered faceplate or swap pairs for one with more
favourable routing, etc.

1Mbit <=60dB is the limit set by BT, but again I've seen this working
higher. It depends on the SNR that is obtained, more on that later.

2Mbit <= 43dB is BT's limit for this. Again, some lines can be
absolutely fine at 2Mbit at significantly higher attenuation, even
though it's a logarithmic scale.

In regards to the higher speeds I gather the attenuation limits will be
about the same (for ~4Mbit anyway), it will depend on your SNR on what
actual line speed will be stable. Naturally, the lower your attenuation
the harder it is for noise to impact on the signal.

The upstream attenuation should be less than 80% of the downstream
attenuation on non-faulty lines, for downstream attenuation above 20dB.
Typically it's about 60% of the numeric value in dB.

SNR Margin:

BT will classify a line to be possibly faulty when the upstream or
downstream signal-to-noise ratio margin is 5dB or less. However, many
modems won't cope very well when the SNR drops below around 8dB. This
borderline nature may cause intermittent loss of synch, loss of
connectivity (PPP drops), slow speeds, or all three. Essentially what
you need to know is even if a line is out of attenuation limits for a
service, doesn't mean it's the cause of the problem you're
investigating. If the upstream/downstream SNR is fine (say, above 10dB)
then there may well be another problem.

SNR will roughly drop by around 6dB per doubling of the line speed on
in-limits lines. So if you have 12dB on a 2Mbit line, you'd be very
lucky to have a stable 4Mbit circuit.

Even if the line doesn't have low SNR or high attenuation, it can still
be faulty so you can't always blame the modem. At PlusNet we find around
25% of faults to be caused by Customer Premesis Equipment (CPE), so
modems, wiring, filters, or sometimes even dodgy washing machines or
flatbed scanners generating radio frequency interference.


>
> - Regarding CRC errors:
> - what causes them?
> - what is a "good reading? (Obviously 0 is good
> but at what figure should I worry about them?)
> - what impact do they have?

CRC (sometimes known as HEC) errors on ADSL are caused by an ATM packet
getting corrupted enroute - possibly due to the modem not being able to
correctly decypher the information from the noise.

You can essentially ignore these unless you're having problems of the
nature described in the section above. They'll be higher if the
connection is in constant use than if it's been idle, and you can't
really easily guage this. Extremely high counts (eg thousands per hour)
can cause problems of slow speeds and packet loss, due to
retransmissions having to be requested.

>
> - What impact does hardware play in making up for
> borderline figures? Eg:
> - having a crap filter?
> - having crap cable?
> - having a crap modem?
> - having too much or crap telephone equipment
> hung on the same telephone line?

This is the wrong way of asking this question - the hardware you're
using can be *causing* the borderline figures reported by the modem. Or
the modem may be misreporting the statistics - they can vary
significantly between modems. The type of filter and wiring you use can
mean a difference in the SNR you get, the attenuation is generally not
something you can change from within the premesis because it largely
depends on the line's length from the exchange.

Faulty or unfiltered equipment sharing the same line can have an impact
on the SNR or cause the ADSL to not work at all. The easiest way of
eliminating internal wiring and devices entirely is to just unscrew the
master phone socket's faceplate and plug your ADSL modem into the test
socket behind that to see how the figures vary. In some cases this will
cause your SNR to be quite a lot higher, as internal wiring can pick up
all sorts of electrical noise and feed it back to the master socket,
affecting the ADSL signal.

It's then a case of either finding out what bit of wiring is causing the
problem (might be a loop, faulty bell wiring, polarity, second master as
an extension, DECT phone on an extension...), or fitting a filtered NTE5
faceplate with dedicated CAT5 cabling laid to the place you want the
ADSL modem to be fitted.

>
> - What does environmental conditions (eg weather)
> affect:
> - the SNR and attenuation readings?
> - the impact of bad values?


As mentioned above, the attenuation shouldn't really change much with
the weather conditions. SNR will vary according to weather, time of day,
phases of the moon, you name it. Thunderstorms are the thing that affect
it the most significantly, as you might expect from the electrical noise
generated. This will be all the more significant on longer lines - some
people find they can't obtain a reliable ADSL service during a storm.

A leaky junction box along the line's route could mean the ADSL drops
out in wet weather and miraculously recovers when it dries out.
Similarly, SNR can be affected by the time of day - the neighbours may
have an outside light that only comes on at certain times, halving your
SNR for instance. A TV transformer or central heating pump may be
faulty, giving the same effect. When we get customers with time-related
problems we try to identify anything in their premesis or nearby (e.g
street lighting) that may come on at the time the problem is
experienced.

>
> To put the whole question another way: I want to have a reasonable idea if I
> visit a customer with ADSL issues, when I look at the information the router
> can tell me, what I should do. In which cases will changing the hardware
> help (and in those cases what should I be changing).

As eluded to above, you can't always tell if it's the modem's fault,
however you can try to eliminate that possibility. Trying the connection
in the master socket (ideally the test socket), and changing the filters
are the first steps you'd be required to do before you could report this
to your ISP (or your customer's ISP rather), who can run some further
tests and report the fault to BT.

With sporadic connection drops on good quality lines, if no fault is
found initially, the first BT engineer will most likely turn up, confirm
the attenuation and SNR with their own hardware, make sure everything at
the exchange still checks out, and then claim the line is ok and ask you
to test with a different modem, so it's probably best to start with this
after doing the wiring checks.

Hope that helps.

Regards,
--
| Alex Crosby Broadband Solutions for
| Broadband Faults Analyst Home & Business @
| PlusNet plc www.plus.net
+------ PlusNet - The smarter way to broadband ------

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Alex Crosby wrote:
> In article <(E-Mail Removed)>, mark-(E-Mail Removed)
> says...
>
>>I'm struggling to find comprehensive (or at least convincingly accurate)
>>information on what I'm sure must be a FAQ:
>>
>>- What are the "good" values for SNR/attenuation for
>> different line speeds, and more importantly:
>> - what are the symptoms of borderline low SNR?
>> - what are the symptoms of borderline high
>> attenuation?
>> - what (in each case) if the upstream figures
>> are good and downstream bad?
>> - Vice versa?
>
>
> While I don't have any concrete information for higher speeds the
> general guidelines (from BT) are as follows

<snip>

Interesting read - thanks!


- --
Adam Piggott, Proprietor, Proactive Services (Computing).
http://www.proactiveservices.co.uk/

Please replace dot invalid with dot uk to email me.
Apply personally for PGP public key.
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Mark Rogers <mark-(E-Mail Removed)> wrote:
> I'm struggling to find comprehensive (or at least convincingly accurate)
> information on what I'm sure must be a FAQ:
>
> - What are the "good" values for SNR/attenuation for
> different line speeds, and more importantly:
<snip>
<snip>
> - What impact does hardware play in making up for
> borderline figures? Eg:
> - having a crap filter?
> - having crap cable?
> - having a crap modem?
> - having too much or crap telephone equipment
> hung on the same telephone line?

Crap filters can be very bad.
For example, changing from a 99p ebuyer one to a faceplate filter from
http://www.solwise.co.uk/ added some 10dB to my SNR margin, when
other devices (cordless phones) were plugged in.

Interesting what you say Alex about BT and their processes. Friend of mine
living locally was activated but he could not receive any adsl signal. He
reported the fault to his ISP. BT then apparently did the check without
going to the house and according to the BT W checker this number is now
showing as not capable of receiving broadband at any speed following an
engineer visit!.He is about 5km from exchange and others further down the
cable seem to be getting 512 ok. BT dont seem to have made any attempt to
switch pairs. They also dont seem to have told his ISP yet!
Alan

"Alex Crosby" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> In article <(E-Mail Removed)>, mark-(E-Mail Removed)
> says...
>> I'm struggling to find comprehensive (or at least convincingly accurate)
>> information on what I'm sure must be a FAQ:
>>
>> - What are the "good" values for SNR/attenuation for
>> different line speeds, and more importantly:
>> - what are the symptoms of borderline low SNR?
>> - what are the symptoms of borderline high
>> attenuation?
>> - what (in each case) if the upstream figures
>> are good and downstream bad?
>> - Vice versa?
>
> While I don't have any concrete information for higher speeds the
> general guidelines (from BT) are as follows
>
> Attenuation:
>
> 512kb I've seen working up at around 75dB downstream attenuation - BT
> will activate anything on an enabled exchange and attempt to get it
> working. If it doesn't work on long lines initially an appointment may
> be required to fit a filtered faceplate or swap pairs for one with more
> favourable routing, etc.
>
> 1Mbit <=60dB is the limit set by BT, but again I've seen this working
> higher. It depends on the SNR that is obtained, more on that later.
>
> 2Mbit <= 43dB is BT's limit for this. Again, some lines can be
> absolutely fine at 2Mbit at significantly higher attenuation, even
> though it's a logarithmic scale.
>
> In regards to the higher speeds I gather the attenuation limits will be
> about the same (for ~4Mbit anyway), it will depend on your SNR on what
> actual line speed will be stable. Naturally, the lower your attenuation
> the harder it is for noise to impact on the signal.
>
> The upstream attenuation should be less than 80% of the downstream
> attenuation on non-faulty lines, for downstream attenuation above 20dB.
> Typically it's about 60% of the numeric value in dB.
>
> SNR Margin:
>
> BT will classify a line to be possibly faulty when the upstream or
> downstream signal-to-noise ratio margin is 5dB or less. However, many
> modems won't cope very well when the SNR drops below around 8dB. This
> borderline nature may cause intermittent loss of synch, loss of
> connectivity (PPP drops), slow speeds, or all three. Essentially what
> you need to know is even if a line is out of attenuation limits for a
> service, doesn't mean it's the cause of the problem you're
> investigating. If the upstream/downstream SNR is fine (say, above 10dB)
> then there may well be another problem.
>
> SNR will roughly drop by around 6dB per doubling of the line speed on
> in-limits lines. So if you have 12dB on a 2Mbit line, you'd be very
> lucky to have a stable 4Mbit circuit.
>
> Even if the line doesn't have low SNR or high attenuation, it can still
> be faulty so you can't always blame the modem. At PlusNet we find around
> 25% of faults to be caused by Customer Premesis Equipment (CPE), so
> modems, wiring, filters, or sometimes even dodgy washing machines or
> flatbed scanners generating radio frequency interference.
>
>
>>
>> - Regarding CRC errors:
>> - what causes them?
>> - what is a "good reading? (Obviously 0 is good
>> but at what figure should I worry about them?)
>> - what impact do they have?
>
> CRC (sometimes known as HEC) errors on ADSL are caused by an ATM packet
> getting corrupted enroute - possibly due to the modem not being able to
> correctly decypher the information from the noise.
>
> You can essentially ignore these unless you're having problems of the
> nature described in the section above. They'll be higher if the
> connection is in constant use than if it's been idle, and you can't
> really easily guage this. Extremely high counts (eg thousands per hour)
> can cause problems of slow speeds and packet loss, due to
> retransmissions having to be requested.
>
>>
>> - What impact does hardware play in making up for
>> borderline figures? Eg:
>> - having a crap filter?
>> - having crap cable?
>> - having a crap modem?
>> - having too much or crap telephone equipment
>> hung on the same telephone line?
>
> This is the wrong way of asking this question - the hardware you're
> using can be *causing* the borderline figures reported by the modem. Or
> the modem may be misreporting the statistics - they can vary
> significantly between modems. The type of filter and wiring you use can
> mean a difference in the SNR you get, the attenuation is generally not
> something you can change from within the premesis because it largely
> depends on the line's length from the exchange.
>
> Faulty or unfiltered equipment sharing the same line can have an impact
> on the SNR or cause the ADSL to not work at all. The easiest way of
> eliminating internal wiring and devices entirely is to just unscrew the
> master phone socket's faceplate and plug your ADSL modem into the test
> socket behind that to see how the figures vary. In some cases this will
> cause your SNR to be quite a lot higher, as internal wiring can pick up
> all sorts of electrical noise and feed it back to the master socket,
> affecting the ADSL signal.
>
> It's then a case of either finding out what bit of wiring is causing the
> problem (might be a loop, faulty bell wiring, polarity, second master as
> an extension, DECT phone on an extension...), or fitting a filtered NTE5
> faceplate with dedicated CAT5 cabling laid to the place you want the
> ADSL modem to be fitted.
>
>>
>> - What does environmental conditions (eg weather)
>> affect:
>> - the SNR and attenuation readings?
>> - the impact of bad values?
>
>
> As mentioned above, the attenuation shouldn't really change much with
> the weather conditions. SNR will vary according to weather, time of day,
> phases of the moon, you name it. Thunderstorms are the thing that affect
> it the most significantly, as you might expect from the electrical noise
> generated. This will be all the more significant on longer lines - some
> people find they can't obtain a reliable ADSL service during a storm.
>
> A leaky junction box along the line's route could mean the ADSL drops
> out in wet weather and miraculously recovers when it dries out.
> Similarly, SNR can be affected by the time of day - the neighbours may
> have an outside light that only comes on at certain times, halving your
> SNR for instance. A TV transformer or central heating pump may be
> faulty, giving the same effect. When we get customers with time-related
> problems we try to identify anything in their premesis or nearby (e.g
> street lighting) that may come on at the time the problem is
> experienced.
>
>>
>> To put the whole question another way: I want to have a reasonable idea
>> if I
>> visit a customer with ADSL issues, when I look at the information the
>> router
>> can tell me, what I should do. In which cases will changing the hardware
>> help (and in those cases what should I be changing).
>
> As eluded to above, you can't always tell if it's the modem's fault,
> however you can try to eliminate that possibility. Trying the connection
> in the master socket (ideally the test socket), and changing the filters
> are the first steps you'd be required to do before you could report this
> to your ISP (or your customer's ISP rather), who can run some further
> tests and report the fault to BT.
>
> With sporadic connection drops on good quality lines, if no fault is
> found initially, the first BT engineer will most likely turn up, confirm
> the attenuation and SNR with their own hardware, make sure everything at
> the exchange still checks out, and then claim the line is ok and ask you
> to test with a different modem, so it's probably best to start with this
> after doing the wiring checks.
>
> Hope that helps.
>
> Regards,
> --
> | Alex Crosby Broadband Solutions for
> | Broadband Faults Analyst Home & Business @
> | PlusNet plc www.plus.net
> +------ PlusNet - The smarter way to broadband ------

In article <4315d170$0$306$(E-Mail Removed)>, (E-Mail Removed)
says...
> Interesting what you say Alex about BT and their processes. Friend of mine
> living locally was activated but he could not receive any adsl signal. He
> reported the fault to his ISP. BT then apparently did the check without
> going to the house and according to the BT W checker this number is now
> showing as not capable of receiving broadband at any speed following an
> engineer visit!.He is about 5km from exchange and others further down the
> cable seem to be getting 512 ok. BT dont seem to have made any attempt to
> switch pairs. They also dont seem to have told his ISP yet!
> Alan
>

Hi there,

While I couldn't comment on this specific case it does really depend on
the state of play in regards to the line itself. BT's data may suggest
the line is routed in a specific way meaning there's no point sending an
engineer out, or it's TPON, or various other possibilities. I'd suggest
you get your friend to find out from his ISP why they can't supply ADSL
- the ISP if necessary will be able to call BTw and obtain further
information. It's entirely possible that BT have made a mistake and
misclassified the line.

Regards,
--
| Alex Crosby Broadband Solutions for
| Broadband Faults Analyst Home & Business @
| PlusNet plc www.plus.net
+------ PlusNet - The smarter way to broadband ------

Alex Crosby wrote:
> Even if the line doesn't have low SNR or high attenuation, it can
> still be faulty so you can't always blame the modem. At PlusNet we
> find around 25% of faults to be caused by Customer Premesis Equipment
> (CPE), so modems, wiring, filters, or sometimes even dodgy washing
> machines or flatbed scanners generating radio frequency interference.

You forgot the infamous Xmas tree lights/rope lights, I've also known
electrical relays to cause dropouts, one case in particular all the ADSL
circuits in the building dropped when the relay controlling the carpark
lighting switched over. In the end they had to live without the lights
as we couldn't pin it down to a voltage spike or RF spike causing the
problems & the end user wasn't incline to investigate the car park
wiring & the customer was more worried about the broad band circuits
than having some halogen spot lamps on ....

Hi Alex,
Thanks for the response. Would you mind if I asked my friend to contact you
direct as he is with your ISP and this matter has been dragging on since the
6 Aug when our lines locally were activated. He is getting increasingly
frustrated with the lack of action from BT.

Alan

"Alex Crosby" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> In article <4315d170$0$306$(E-Mail Removed)>, (E-Mail Removed)
> says...
>> Interesting what you say Alex about BT and their processes. Friend of
>> mine
>> living locally was activated but he could not receive any adsl signal.
>> He
>> reported the fault to his ISP. BT then apparently did the check without
>> going to the house and according to the BT W checker this number is now
>> showing as not capable of receiving broadband at any speed following an
>> engineer visit!.He is about 5km from exchange and others further down the
>> cable seem to be getting 512 ok. BT dont seem to have made any attempt to
>> switch pairs. They also dont seem to have told his ISP yet!
>> Alan
>>
>
> Hi there,
>
> While I couldn't comment on this specific case it does really depend on
> the state of play in regards to the line itself. BT's data may suggest
> the line is routed in a specific way meaning there's no point sending an
> engineer out, or it's TPON, or various other possibilities. I'd suggest
> you get your friend to find out from his ISP why they can't supply ADSL
> - the ISP if necessary will be able to call BTw and obtain further
> information. It's entirely possible that BT have made a mistake and
> misclassified the line.
>
> Regards,
> --
> | Alex Crosby Broadband Solutions for
> | Broadband Faults Analyst Home & Business @
> | PlusNet plc www.plus.net
> +------ PlusNet - The smarter way to broadband ------