models for simulating a dsl link

That depends on what you consider to be 'short'. See

Yes, they are.

Correct.

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.

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 , particularly SIN351 and SIN346.

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.

lots of bt tech info @

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

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.

Would you care to give examples ?

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).

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

Graham

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.

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

Graham

On Fri, 25 May 2007 21:45:40 GMT, Eeyore put finger to keyboard and composed:

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:

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

International DAA parameters:

- Franc Zabkar

On Fri, 25 May 2007 21:11:55 GMT, Rich Grise put finger to keyboard and composed:

Here is an example:

- Franc Zabkar

Some *VALUES* !

Thank You.

Graham

simulation package.

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

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

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

Regards Ian

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

Graham