Re: Impact of hardware with bad SNR & Attenuation

Mark Rogers wrote:
> Alex Crosby writes:
>> 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.
>
> BT have just fitted a couple of faceplates here - we'll have to see
> how much difference that makes. To be fair the filters were cheap
> ones (part of the reason for asking my original question was to find
> out which things the filter will mess up).

The faceplate filters help stop the internal electronic mush, picked up
by the bell wire of the extensions feeding back & mixing with the ADSL
signal, via the the master socket
>
> On a line with no phone, where the filter is being used only as a way
> to convert from RJ11 to BT plug will there be any difference between
> a cheap (crap) filter and a good one? Ie does unplugging all other
> telephones act as a good test as to whether the filter is the problem?

Depends on how many/much extension wiring is attached, ref the above
bullet point.
>
>> 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.

In the field the engineers try to get a +10dB as in a lot of situations
the SNR will vary & normally will drop in the evening, so 10dB does give
you a bit of lee way..
>
> Reading between the lines here, a good modem is better at dealing
> with poor SNR than a cheap one. What's a good modem to go for in
> these cases? How much difference do they make - eg do they typically
> cope with 1-2dB less SNR than a poor modem, or more/less?

Not neccesarily, depends a lot on how much you want to spend (a really
good one can be £300 or there abouts) & after that you then have to get
the correct long line firmware installed as well.
>
>> 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.
>
> If the problems are with CPE (other than the mode/wiring/etc), what
> are the best solutions? Eg if the problem is a washing machine or
> scanner, is it proximity to the device that matters, or is it
> electrical noise? Some hardware better than others at coping with the
> noise? Do things like surge protectors or UPSs help?

Nope none of the above will help, what may (repeat may) help is fitting
a RF3 filter in line before the master socket, but even that isn't
guaranteed, depending on high big the RF spike is...
>
>> 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.

Speaking from personal experience you can cut down on excessive CRC (or
even HEC) errors by using the ADSL circuit in the master socket via a
SSFP, or even just disconnecting all the extensions connected to that
line..

>
> I had a situation recetly where we could get sync but no IP address
> (including from bt_test@startup_domain). The error count was
> increasing by about 1 or 2 per second (which given the lack of
> traffic without even having an IP I guess is pretty high). The SNR
> was very bad though (<3dB).
>
> If I have sync but no IP, what does this mean? For that matter, what
> does sync actually mean? What do all the "ACTIVATION" to "SHOWTIME"
> actually statuses mean?

Showtime is when the circuit is synch'd up, Activation I must be honest
I cannot recall ever seeing (but it could mean that the monem/router is
attempting to negotiate a connection with the exchange)..

>
>>> - 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 the above can & will cause problems, which is why it's always
best to start from the test socket on the NTE5 (if you haven't got one
then disconnect all your extensions) with a known, good, working filter
>>
>> This is the wrong way of asking this question - the hardware you're
>> using can be *causing* the borderline figures reported by the modem.
>
> I tend to assume that there's crap hardware which causes (or at least
> exacerbates) the problem, standard hardware that does works fine in
> normal situations and really good hardware that may work when bad
> line quality means that normal kit can't cope. Is that fair? If so,
> which hardware fits into which bracket - is it simply a case of cheap
> is crap, expensive is good?

Not neccesarily as different users in different situations can get
different performance from the same piece of kit, even the expensive kit
can be a lemon...
>
>> 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.
>
> I almost always locate the modem (ie modem/router) next to the phone
> socket and CAT5 from there to where the network is needed - I assume
> that this is preferable to extending the cable on the ADSL side?

You can run anything upto 30 metres of cat3/5 cable from the NTE
faceplate filter to where ever you place your router/modem, but doing it
your way can be simpler, easier & it does rule out one possible
problem...

>
>> 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.

Hopefully they would also test the line (from the exchange), for
capacitance balance as well as the normal battery, earth, loop & low
resistance test. Also they would check the exchange equipment under
load, using artifical line extenders (plug in dongles), personally I
always check with 80dB but some only use 60dB, which is no matter as
either way it is putting a loading onto the DSLAM which could prove a
problem into the DSLAM card you are connected to.

>
> Good advice. Are there any other good tests to perform before calling
> on BT (even before calling on the ISP) that help to say "I have
> tested this and it's almost certainly a BT/ISP problem"? So far I
> normally: - Swap routers and filters, and test each without anything
> in the phone socket
> - Make notes of SNR/attenuation in each case
> - Try bt_test@startup_domain login

DO NOT call BT for any ADSL problems. All ADSL problems have to be fed
via your ISP (yes even if it's BT Yahoo/Broadband/Openworld or
whatever). You run the very real risk or getting visit charges raised
against you & little else if you do. Raise it with the ISP, that way
you will get an ADSL engineer who will be able to test your ADSL service
& hopefully either fix the problem if it's on the BT network side or
give you some advice if it's on your private wiring




.

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