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


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