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models for simulating a dsl link

Discussion in 'Electronic Design' started by Eeyore, May 25, 2007.

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  1. 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.
     
  2. That depends on what you consider to be 'short'. See
    http://en.wikipedia.org/wiki/Loading_coil
    Yes, they are.
    Correct.
     
  3. 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.
     
  4. Bob Evans

    Bob Evans Guest

    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.
     
  5. Tim Shoppa

    Tim Shoppa Guest

    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.
     
  6. stephen

    stephen Guest

    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....
     
  7. 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.
     
  8. Eeyore

    Eeyore Guest

    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
     
  9. Eeyore

    Eeyore Guest

    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
     
  10. Franc Zabkar

    Franc Zabkar Guest

    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
     
  11. Franc Zabkar

    Franc Zabkar Guest

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

    - Franc Zabkar
     
  12. Eeyore

    Eeyore Guest

  13. Ross Herbert

    Ross Herbert Guest


    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
     
  14. Eeyore

    Eeyore Guest

    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
     
  15. Ian

    Ian Guest

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

    Regards
    Ian
     
  16. Eeyore

    Eeyore Guest

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

    Graham
     
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