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models for simulating a dsl link

P

Paul Hovnanian P.E.

Jan 1, 1970
0
Eeyore said:
Yes. The modem connection is simply a pass-through.

Correct. One of the biggest problems I've seen with DSL installations is
the condition of the premises wiring, between the telco interface and
the various wall jacks. Over the years, splices and taps get added in
walls, many of questionable quality. If you depend on locating every
telephone device and installing a filter on every one, one day, someone
will find a forgotten jack and plug a phone in there, without a filter.

The best practice I've seen is to install a tap just after the telco
interface and run that as a single run to a dedicated DSL outlet. Then,
immediately following that tap, install the DSL filter and feed the
existing premises circuit(s) from the output of that.

The interesting analysis in this case would be how that filter behaves
as its load varies.
 
P

Paul Hovnanian P.E.

Jan 1, 1970
0
Eeyore said:
'Create the impedance'. Can you elaborate on that ? I don't think it's that at all
unless I'm very mistaken. In any case the impedance of a filter will be all over the
place with frequency.


More like a different resistance actually. At *audio* frequencies the short run of cable
from the exchange doesn't really impose any characteristic impedance value.

That depends on what you consider to be 'short'. See
http://en.wikipedia.org/wiki/Loading_coil
At the DSL frequencies the issue will come into play but the DSL filter's not in that
path so the DSL modem / router has to deal with that one just for itself.

Yes, they are.
I've heard of them but know almost nothing more about them. I gather DSL won't work if
there's one on the line. I suspect also they may have been there to help with the volume
level on long lines.

Correct.
 
Eeyore wrote:

8<


If you are going to simulate the line then you are going to have big
problems.
The BT network uses lots of different cable and the characteristics vary a
lot.
A lot of the old twisted pair cable had few twists while the newer stuff has
more twists.
The twist rate varies depending on which pair you use in the cable as does
the impedance.
A typical connection will have several of the varying cable segments and
some of the older joints may no maintain the twists very well at junction
points.
Have fun.
 
B

Bob Evans

Jan 1, 1970
0
8<


If you are going to simulate the line then you are going to have big
problems. The BT network uses lots of different cable and the
characteristics vary a lot.

Any given line will almost certainly present multiple impedance
discontinuities and this is one reason why [in the UK] ADSL uses an
adaptive DMT modulation scheme.

A good starting point for a study of UK line characteristics is
<http://www.sinet.bt.com>, particularly SIN351 and SIN346.
 
T

Tim Shoppa

Jan 1, 1970
0
Eeyore said:
A couple of recent threads about the effects of DSL (micro)filters gave me the
idea of analysing their effect under various conditions in a simulation package.

I've got several examples of filters of varying designs here and I'd really like
to know how much difference and indeed what kind of differenc(es) they can make.

As some know from previous threads I'm a bit sceptical about how much they
affect the DSL signal itself.

Before I can do this I need some suitable 'models' for the various bits of
telecom gear involved. Can anyone point me in the right direction ?

The "DSL filter" has a low-pass filter. The low-passed POTS-only
bandwidth is provided to all the POTS devices in the house, and their
complex load impedances (ringers) and not-well-impedance controlled
cabling.

There may be a high-pass port on some DSL filters to go to the DSL
modem, but usually all that functionality is contained in the modem's
circuitry itself.

Now, the DSL modem itself does really really advanced funky analog AND
digital domain filtering that automagically adapts the modems on both
ends to the line and all its imperfections. This is pretty fancy shit,
way more than just somebody turning the knobs on their graphic
equalizer till it sounds good, and if you can get yourself on the real
books on DSL modem chipsets and all that they do you'll be impressed
IF YOU CAN READ IT ALL. I've worked in a mix of telecom, data
acquisition, networking, etc. for decades and what those little few-
dollar DSL modem chipsets do is just mind-boggling. 30 years ago the
same functions took not only detailed manual adjustment and fancy test
gear but several racks and hundreds of thousands of dollars of
electronics! (And then we were doing it all just to get a long-
distance 9600 baud link!)

Tim.
 
S

stephen

Jan 1, 1970
0
Eeyore said:
But not at all frequencies. I don't deny it must have an effect of some kind of
course.



Not at the lengths used for the local loop at audio frequencies. This is a total

Twisted pair stuff has a characteristic impedance of around 100 ohms (when it
becomes relevant which it isn't at audio frequencies in the local loop).



That is indeed what matters. As I said, it's about 100 ohms for LLU
wiring.

lots of bt tech info @
http://www.sinet.bt.com/

i only usually need the digital data stuff, so not sure what kind of phone &
ADSL stuff is in there....
 
G

Gordon Hudson

Jan 1, 1970
0
Absolutely not. If a cable wasn't twisted pair that might make a
difference
though. I wouldn't call that 'quality' myself. I like to see things called
by
their correct names.

I think you are a bit wrong there.
The quality of the cable (what its made from and the connectors and how they
are attached) will all have an effect on losses.
I don't know exactly whats being sent across an ADSL cable though so I can't
say what effect it might or might not have.
 
E

Eeyore

Jan 1, 1970
0
If you are going to simulate the line then you are going to have big
problems.
The BT network uses lots of different cable and the characteristics vary a
lot.

Would you care to give examples ?

A lot of the old twisted pair cable had few twists while the newer stuff has
more twists.
The twist rate varies depending on which pair you use in the cable as does
the impedance.

As mentioned before, at the short length in use for the local loop, the
characteristic impedance of the cable is not relevant at audio frequencies

For DSL to work it has to be assumed that the cable is ~ 100 ohms at RF - and it
is.

DSL modems present an essentially resistive 100 ohm load to the line (info
recently found).

A typical connection will have several of the varying cable segments and
some of the older joints may no maintain the twists very well at junction
points.

None of this nullifies the reason for doing this little 'experiment'.

Graham
 
E

Eeyore

Jan 1, 1970
0
Gordon said:
I think you are a bit wrong there.
The quality of the cable (what its made from and the connectors and how they
are attached) will all have an effect on losses.

Aluminium wire will have higher resistance than copper.

Different insulaltion materials may have slighty varying dielectric constants
which would alter the characteristic impedance of the cable at RF very slightly.

The biggest influence would be mechanical construction as in wire dia and
conductor spacing. But all of these will have their effect at RF not AF.

I don't know exactly whats being sent across an ADSL cable though so I can't
say what effect it might or might not have.

Traditional audio plus digital data modulated onto multiple carriers at
frequencies between 30kHz and 1MHz.

Graham
 
F

Franc Zabkar

Jan 1, 1970
0
Not so sure about impedance matched. What impedance is a phone anyway, never
mind 4 in parallel ( the max load for a line and a single filter is *supposed*
to cope ) ?

Graham

AIUI the impedance of an off-hook telephone is either "real" or
"complex". The former is typically 600 ohms, the latter varies between
the many telecom authorities and usually involves an RC combination.

See fig 25, table 5, page 37 of this document:
http://www.midcom-inc.com/Tech/pdf/TN69.pdf

Some time ago I found the following data in a Silicon Labs datasheet.

International DAA parameters:
http://groups.google.com/group/comp.dcom.modems/msg/232247c06425cc1d?dmode=source&hl=en

- Franc Zabkar
 
F

Franc Zabkar

Jan 1, 1970
0
My DSL provider gave me a handful of filters - just see if you can get
somebody to give you one, and hack it and find out what's actually inside
the thing.

By Occam's razor, it's probably just an ordinary LC low-pass filter,
probably balanced and impedance-matched to the line(s).

Good Luck!
Rich

Here is an example:
http://www.uoguelph.ca/~antoon/circ/dslfilt.html

- Franc Zabkar
 
R

Ross Herbert

Jan 1, 1970
0
A couple of recent threads about the effects of DSL (micro)filters gave me the
idea of analysing their effect under various conditions in a simulation package.

I've got several examples of filters of varying designs here and I'd really like
to know how much difference and indeed what kind of differenc(es) they can make.

As some know from previous threads I'm a bit sceptical about how much they
affect the DSL signal itself.

Before I can do this I need some suitable 'models' for the various bits of
telecom gear involved. Can anyone point me in the right direction ?

Graham


I can't recall having come across any software modelling packages for
dsl simulation. I think most manufacturers of dsl filters use hardware
simulators such as http://www.epl.co.uk/X600Manual.pdf
 
E

Eeyore

Jan 1, 1970
0
Ross said:
I can't recall having come across any software modelling packages for
dsl simulation.

Well no. Any electronics simulation package can do it once you have workable
models. It's not rocket science.

Thanks for the link, I'm gathering a fair bit of useful info now.

Graham
 
I

Ian

Jan 1, 1970
0
Eeyore said:
Yes. The Capacitance values are printed on the caps btw.



Not from the exchange end. I don't have access to that for one thing ! The
whole
entity of the line from exhange to subscriber is what I intend to
simulalte.
It's quite simple as long as I can get the numbers to punch in for various
pieces of kit such as phones.

Build your own TDR, then you don't need access to the exchange end.

Regards
Ian
 
E

Eeyore

Jan 1, 1970
0
Ian said:
for > various pieces of kit such as phones.


Build your own TDR, then you don't need access to the exchange end.

I don't plan on building anything. That's the whole point about simulations.

Graham
 
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