Re: Impact of hardware with bad SNR & Attenuation

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).

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?

> 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.

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?

> 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?

> 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.

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?

> > - 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.

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?

> 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?

> 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.

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

Again - thanks for all the help.

I'm going to collect all the information in this thread into a single
document for internal use - does anyone have any objection if I put the
results online somewhere for everyone-else's benefit (properly attributed,
of-course)?

Mark Rogers,
More Solutions Ltd


.

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