[Gllug] ADSL problem

[Chris Bell]

On Fri 25 Feb, will wrote:

> The problem is that when I have the microfilter in the chain (with or 
> without the Linksys plugged in) the phone rings constantly.  Not a 
> standard 'ring ring' like when someone calls but a constant, distorted 
> ring sound[0].  This happens only when the microfilter is in the chain. 


   The microfilter is there to prevent the high frequency ADSL from reaching
your ordinary phones, but instead ensure that the maximum amount of high
frequency ADSL signal travels between the ADSL modem and the exchange
equipment. The ADSL modem should be connected straight across the signal
pair that arrives from the master line termination, and everything else
should be fed through the microfilter. Do not connect the ADSL modem to the
"local phone" side of the filter as you will be feeding high level carrier
from your modem straight into your phones. Cheap "ADSL filters" are
available that in my opinion are incapable of doing the job correctly, there
is just not enough space inside to fit suitable components.

   The special faceplate as installed by BT is in my opinion by far the best
system to use as it correctly terminates the line using a balanced
multi-stage filter which is capable of working correctly with the full range
of signal levels. It is possible to obtain a replacement from Solwise, and
they are only slightly more expensive than the standard cheap filters
available elsewhere.

http://www.solwise.co.uk

   The input signal level and quality, line frequency response and
attenuation, (what goes in at one end compared with what falls out of the
other), the in-band noise, (unwanted content that is present with the output
signal and within the signal bandwidth), and out-of-band noise, (other
unwanted content), all affect the quality of the data transmission.
   The minimum attenuation for a given length is set by the line design and
construction, but the actual attenuation depends on joint quality, cable
damage, corrosion, damp, etc. Every imperfection will cause signal loss at
the output and will usually also result in signal reflections, so multiple
imperfections result in multiple reflections in both directions. Similar
reflections occur if the line is not correctly terminated at both ends.
   Differential frequency response results in pulse distortion, but the
effects can be reduced by filtering the input signal to remove out-of-band
components.
   The effects of out-of-band noise at the output are also reduced by
careful filtering.
   There will be a minimum received signal level and signal-to-noise ratio
for the input circuitry of the actual equipment used. You can make the most
of a marginal line by using a sensitive modem connected as close as possible
to the line termination, and fitting a high quality balanced ADSL filter
which prevents the ADSL signal from entering the rest of the local wiring.


-- 
Chris Bell

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