Re: Broadband technical data (a bit OT?)


"John Hill" <nemo@xxxxxxxxxxxxxxx> wrote in message
news:1ja549o.zgtavy9s4fsN%nemo@xxxxxxxxxxxxxxxxxx
Another thread has prompted me to ask:

My ADSL modem offers me the following information:
Noise margin: 14 dB (downstream) / 22 dB (upstream)
Line attenuation: 25 dB (downstream) / 12 dB (upstream)

These figures vary from day to day. What is their significance? Should
they be high or low? For example, 14-15 dB is typical of the downstream
noise margin, but I have seen values as low as 3 and as high as 20.

I have been having a lot of bother with my telephone connection lately.
Sometimes the crackling is so loud that it is impossible to hear a
caller, whilst at other times it is perfectly clear. When asked, BT
invariably tell me that the line tests perfectly OK.

So I'm anxious to find out if these figures offer any clue as to what is
going on.

The noise margin indicates by how much the line is better than the minimum
required to support an ADSL service of a given speed. The SNR margin is
traded against speed - if the SNR (signal to noise ratio) drops by 3dB the
speed will be reduced by about 800 kbits/sec. A target SNR margin is set,
usually at 6 dB.

The maximum speed is limited by the technology in the exchange, normally at
about 8Mbit/s sec. If you are achieving 8Mbits/sec then the relatively high
noise margin indicates the "headroom" of the line - i.e. the potential to
operate at a higher speed. If you are achieving less that 8 Mbits/sec then
the fact that the noise margin is more than 6dB indicates the line suffers a
varying level of noise, and the margin has been adjusted upwards to ensure a
reliable connection. This would be consistent with your report of noise
heard during voice calls. Note that the SNR margin may vary according to
the make and model of router - some (typically the Draytek Vigor products)
will negotiate a high noise margin to ensure reliability, while others will
try to maximise speed but may drop the connection more frequently.

The line attenuation is the loss on the telephone line and is related to its
length - which may well be much more than the direct distance between you
and the telephone exchange if the cable takes a tortuous route. Very
crudely it's about 10dB per kilometer in the downstream direction. So we
could guess that you are about 2.5 kilometers from the exchange ...?

I have seen ADSL services work with attenuation figures up to 80dB. At such
high levels of attenuation the achived speed is generally about
250kbits/sec - so better than dial-up!

As the line length increases the bandwidth and SNR both reduce, so the
available speed reduces.

What speeds are you achieving?

What you need to do is to ring BT faults every time you hear the crackling
noise. Make notes, so that you can report whether the noise is affected by
wind or rain, or time of day. Once you can get their faults people to hear
the noise, they will send out a technician to try to resolve the problem.

At this stage do not mention anything whatever about your broadband service.

When the line is has been crystal clear and quiet for about a month, review
your router statistics (keep a log over that month). Report back to us.
If you then have significant unreliability and poor speeds only it would be
appropriate to take the matter up with your ISP.

--
Graham J



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