Line Attenuation/Noise Margin requirements for BT 8Mbps ADSL ?

Anyone know how low they need to to be to get 8Mbs or even 4?


Wallace Murray

Wallace Murray Wrote:
> Anyone know how low they need to to be to get 8Mbs or even 4?

Wallace,
8mb and 4mb are over 1km and 2km respectively, which I guess equates t
about 10db and 20db.

Cheers,
Jane Moc

--
mochj

> Anyone know how low they need to to be to get 8Mbs or even 4?

Depends on modem, not only the line...

m.

In an earlier contribution to this discussion,
Marcin Lukasik <(E-Mail Removed)> wrote:

>> Anyone know how low they need to to be to get 8Mbs or even 4?
>
> Depends on modem, not only the line...
>
> m.

Anyone know why it is that different modems and routers report *vastly*
different noise margins on the *same* line?

For example, I have a Zyxel router which reports 23.5 db, an internal PCI
modem which reports about 18db, and a 3Com wireless router which reports
between 10 and 13 db - on the days when it condescends to work at all!

Any comments?
--
Cheers,
Tim
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On Thu, 01 Sep 2005 13:31:51 +0100, Wallace Murray
<(E-Mail Removed)> wrote:

>Anyone know how low they need to to be to get 8Mbs or even 4?

one opinion at http://whatismyspeed.sourceforge.net/demo/index.php

Phil
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On Thu, 1 Sep 2005 17:29:28 +0100, "Tiscali Tim" <(E-Mail Removed)>
wrote:

>Anyone know why it is that different modems and routers report *vastly*
>different noise margins on the *same* line?

a) they may be using different frequencies and hence experience
genuine differences in noise margin.
b) the measurement technique may be inaccurate
c) the user interface you are using may be wrong, even though the data
is correct
d) there may be interference from the PSU of the device or
neighbouring equipment reducing the margin - one guy got much better
margins with his laptop on batteries than on the mains

Phil
--
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Phil Thompson wrote:
> On Thu, 1 Sep 2005 17:29:28 +0100, "Tiscali Tim" <(E-Mail Removed)>
> wrote:
>
>> Anyone know why it is that different modems and routers report
>> *vastly* different noise margins on the *same* line?
>
> a) they may be using different frequencies and hence experience
> genuine differences in noise margin.
> b) the measurement technique may be inaccurate
> c) the user interface you are using may be wrong, even though the data
> is correct
> d) there may be interference from the PSU of the device or
> neighbouring equipment reducing the margin - one guy got much better
> margins with his laptop on batteries than on the mains
>

That's not suprising my works laptop can't even use a dial up access if
the PSU is plugged in...

In an earlier contribution to this discussion,
Phil Thompson <(E-Mail Removed)> wrote:

> On Thu, 1 Sep 2005 17:29:28 +0100, "Tiscali Tim" <(E-Mail Removed)>
> wrote:
>
>> Anyone know why it is that different modems and routers report
>> *vastly* different noise margins on the *same* line?
>
> a) they may be using different frequencies and hence experience
> genuine differences in noise margin.
> b) the measurement technique may be inaccurate
> c) the user interface you are using may be wrong, even though the data
> is correct
> d) there may be interference from the PSU of the device or
> neighbouring equipment reducing the margin - one guy got much better
> margins with his laptop on batteries than on the mains
>

That's extremely interesting - particularly item(d).

I have been having lots of problems with a 3Com wireless router - which
would work for about 30 minutes when first powered up from cold, after which
the SNR would plummet, CRC errors would go sky high - and synch would then
be lost. I even got 3Com to replace the router (but not the power supply!)
under warranty.

Prompted by the above reply, I dug out an old universal power supply - which
has controls to turn for several voltage steps, plus polarity - and set it
up to 12v with the correct polarity - and tried that with the router. The
results are amazing! The router is reporting a much higher SNR than it
*ever* did with 3Com's power supply - and no longer dies after a few
minutes.

I presume the difference is that the 3Com device is a switching power
supply - whereas the one I'm now using just a multi-tap transformer and
rectifier. I assume that the noise comes from the switching - but it must
surely be possible to make noise-free switching supplies - or nothing would
ever work?

Looks like I'm going to have to get 3Com to replace the power supply,
although - based on other people's experiences - I may be in for a fun time!
[I'd carry on using the other one - but it's running pretty close to its
1000mA limit - and getting a bit hot!]
--
Cheers,
Tim
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On Sat, 3 Sep 2005 14:30:47 +0100, "Tiscali Tim" <(E-Mail Removed)>
wrote:

>I presume the difference is that the 3Com device is a switching power
>supply - whereas the one I'm now using just a multi-tap transformer and
>rectifier. I assume that the noise comes from the switching - but it must
>surely be possible to make noise-free switching supplies - or nothing would
>ever work?

I've seen papers by electronics designers about this, and yes you can
make a decent one (or maybe they put it in an earthed can and filter
its output).

related reading :-
http://www.commsdesign.com/design_co...cleID=16503637
http://www.telepermit.co.nz/nl143.html#sec3

Phil
--
Tiscali - dialup speeds at Broadband prices, see
http://bbs.adslguide.org.uk/postlist...&Board=tiscali

AOL - the unlimited ISP of choice for heavy downloaders.

In an earlier contribution to this discussion,
Phil Thompson <(E-Mail Removed)> wrote:

> On Sat, 3 Sep 2005 14:30:47 +0100, "Tiscali Tim" <(E-Mail Removed)>
> wrote:
>
>> I presume the difference is that the 3Com device is a switching power
>> supply - whereas the one I'm now using just a multi-tap transformer
>> and rectifier. I assume that the noise comes from the switching -
>> but it must surely be possible to make noise-free switching supplies
>> - or nothing would ever work?
>
> I've seen papers by electronics designers about this, and yes you can
> make a decent one (or maybe they put it in an earthed can and filter
> its output).
>
I suspect in my case that the noise is carried on the 12v input rather than
being radiated. At any rate, I can have this power supply switched on when
using a *different* modem/router with no ill effects. [My wireless router
still works OK as a WAP - using this suspect power supply - even though the
modem part doesn't work]
--
Cheers,
Tim
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