Re: Impact of hardware with bad SNR & Attenuation

Is this for your course assignment?

"Mark Rogers" <mark-news@xxxxxxxxxxxxxx> wrote in message
news:3nlgo5F21c4aU1@xxxxxxxxxxxxxxxxx
> 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
>
>


.

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