Re: Impact of hardware with bad SNR & Attenuation

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" <tma20@xxxxxxxxxxx> wrote in message
news:MPG.1d7fb8dfe12cc6b798969d@xxxxxxxxxxxxxxxx
> In article <3nlgo5F21c4aU1@xxxxxxxxxxxxxx>, mark-news@xxxxxxxxxxxxxx
> 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 ------


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