For those Network techie geeks

Discussion in 'NZ Computing' started by Bok, Jan 6, 2004.

  1. Bok

    Bok Guest

    Is this some kind of trick question?

    You haven't specified the nature of the in-wall cabling. If the in wall cabling is 10base2 (thin net), then it
    doesn't sound valid to me (but then again I'm not a network techie geek
    If the in-wall cable is 10base2, then my answer to this is close to ZERO -
    the center of the connector T should be connected as close to the NIC as
    possible for reliable operation.

    The situation will be different if there are active devices in the wall
    such as a media converter (tranceiver) or repeater of some form - please
    Bok, Jan 6, 2004
    1. Advertisements

  2. Bok

    TEX Guest

    System running 10base2, BNC interconnect. Computers are connected from
    the nic to the wall via a single cable. NOT a T connector. Basically you
    are extending the center of the T connector in a normal simple

    How long must the cable be from the wall to the computer???

    TEX, Jan 7, 2004
    1. Advertisements

  3. Bok

    Lennier Guest

    1 x D' + 3" + H where D = the distance between the computer and the wall
    and H = the distance between the floor and the plug on the wall...


    Lennier, Jan 7, 2004
  4. Bok

    TEX Guest

    No this is not a trick question. Just an interesting thing I discovered
    today. A friend was try to explain to me the finer details...but got
    lost half way through it. Thought someone might actaully know WTF I am
    taking about and actually explain to me in laymans terms. I know it is
    old technology now, but was an interesting learning experience.
    The inwall cabling is 10base2, and it utilises a "AMP" thinnet TAP
    connector to interface to the NICS. Effectivly the the thinnet TAP is a
    T connector, in which the center of the T is cabled to the NIC. Saves
    the hassle and tangle of dual leads and T on the end of each NIC

    As you have said the the center of the T has to be as close to the NIC
    as possible for reliable operation, but since I have a 6ft long thinnet
    tap cable, this answer is not quiet right.

    TEX, Jan 7, 2004
  5. I have seen a system in some place where we were ripping it out where the wall
    plate allowed for a plug to be removed and bridged the connection when it was
    taken out, and the plug actually ran 2 pieces of coax up to the PC where they
    terminated in a single BNC plug. Could this be what you have there?
    Richard Malcolm-Smith, Jan 7, 2004
  6. Bok

    TEX Guest

    Nope. The setup was originally used for a Novell LAN, years and years
    ago. Definatly a single core coax cable connected to the NIC.

    TEX, Jan 7, 2004
  7. It sounds like the old Novell 75 ohm 2Mb/s standard.
    Uncle StoatWarbler, Jan 7, 2004
  8. Bok

    Bok Guest

    The individual lengths of the drop cables in the AMP system are not
    critical. Take a look at the description, it's not a single coax. Quoting
    from the above PDF
    "The drop cables are made from dual thinwire cable with an
    overall PVC sheath".
    This makes all the difference - each side of the coax within the drop
    cable is essentially inlined on the main cable. This is not the same
    as adding a single coax extension between a T adaptor and NIC where the
    length would be critical (to avoid inteference from reflected signal back
    on the main bus).

    However, because it's dual coax, you need to allow for double the drop
    length when computing the total bus length, to stay within the recommended
    recommended 185meter. limit for 10base2.

    To emulate this electically with single core coax you need to run two
    lengths of coax from the wall to a T connector connected directly to the

    The advantage of the AMP system is that you can plug and unplug the
    drop cable without disrupting the LAN because the AMP connector is make
    before break, which retains continuity.
    Bok, Jan 7, 2004
  9. Bok

    bok Guest

    The inwall cabling is 10base2, and it utilises a "AMP" thinnet TAP
    That's quite different - we had that type of cabling in one of the buildings
    here at work years ago. The drop cables for thinnet taps consisted of TWO
    coax cables, which loops the signal to the node, then back to the other side
    of the tap. There is a microswitch in the tap assembly that allows the two
    sides of the coax to be inserted creating an unbroken network which is
    'electrically equivalent' to connecting with a traditional BNC T connector.
    If it's lan tap, then you are not connecting to a T connector in the wall.
    Thinnet tap drop cables usually had a square looking connector into the wall
    NOT a BNC connector. It's not quite right to say:

    "Basically you are extending the center of the T connector in a normal
    simple configuration."

    This is not a valid 10base2 configuration and will be unreliable.

    bok, Jan 7, 2004
  10. Bok

    bok Guest

    If it's lan tap, then you are not connecting to a T connector in the wall.
    Just to clarify this a bit. The tap assembly plus the drop cable are
    equivalent to a T connector. It is invalid to lengthen the drop cable with a
    length of coax and BNC coupler, which would be the same as inserting a
    length of coax between the centre of a T adaptor and the network node (NIC).

    hope this helps
    bok, Jan 7, 2004
  11. Bok

    bok Guest

    Sorry, I replied to your other post before I read this. I thought you might
    have the same thinnet tap technology Richard referred to, which is the
    variety I'm familiar with. Was the wall connection BNC? If it was, then this
    cabling doesn't conform to most 10base2 standards I've come across.
    bok, Jan 7, 2004
  12. Bok

    bok Guest

    Nope. The setup was originally used for a Novell LAN, years and years
    Could be, but that's not 10base2, which by definition is 10Mb/s.
    bok, Jan 7, 2004
  13. The untrained eye wouldn't be able to tell the differnce.
    Uncle StoatWarbler, Jan 7, 2004

  14. This _is_ the Novell 2Mb/s system.

    Drop cables must be a multiple of 1/2 the wavelength, that way they
    present the same impedance at the T as is on the end of the tail - and an
    open circuit is seen as an open circuit.
    Uncle StoatWarbler, Jan 8, 2004
  15. Bok

    TEX Guest

    Correct. It is a square connector that plugs into the wall, with a bit
    of cable and male BNC on one end. If you look inside the wall jack, it
    isn't (as you correctly put it) a T connector, but is wired exactly the
    same way.

    When you say "The tap assembly plus the drop cable are equivalent to a T
    connector", how would that be different to T with BNC coupler and
    extension cable??

    The situation was, that I inserted a T connector in the main line, and
    extended the line of the center tap, with coupler and cable. (To emulate
    the thinnet TAP configuration, since you can't get connectors any more)
    But found that only certain lengths of cable would work.

    After hours of mucking around, I was then told since, the cabling is
    tuned to a freq, the length must be proportional to half the wavelength
    of the travelling freq on the cable. Hence why some of the cables
    worked, and some didn't.

    Is this correct or totally wrong?? How does the thinnet tap system
    differ from the T and coupling?? They are both electrically the same.

    TEX, Jan 8, 2004
  16. Bok

    TEX Guest

    Yes the system was used to run a novell network ages ago. It just
    utilises the same 50ohm cable and 50ohm terminators as per 10base2 and
    has been happily running 10mbs for the last 8 years, using the premade
    drop cables. I had to buy a cable not too long ago, and they can in
    lengths of 6ft and 8.2ft.

    Yes my non computer techy friend (but electronics geek) said something
    about if the cable is not 1/2 wavelength the impedance potential at the
    junction (T) would appear as if not terminate. He then raved on about
    something about calculating the length of the drop cable etc using a
    formula, but by that stage I was lost.

    He said this sort of thing is also important in arial tuning for TV's
    and RF. Apparently it is a big field, but would never have thought it
    would come into use in networking.

    Would be keen on finding some more info on how this 2mbs novell system
    works and how it differs from 10base2.

    TEX, Jan 8, 2004
  17. Bok

    bok Guest

    Correct. It is a square connector that plugs into the wall, with a bit
    It is different! Take a look the "Thinnet Tap" system described at the
    bottom of this page:

    The pictures might help to determine if the system you are dealing with is
    the same as this, which is a standard 10base-2 configuration.

    Notice the warning at the bottom of this page:
    "Never attempt to lengthen a Drop Cable with a length of coaxial cable and a
    BNC coupler. This is the same as locating a "T" adapter away from the node,
    and will cause major network problems!"
    This isn't going to emulate a thinnet tap system, refer to web page.
    That's correct, but if you don't use the correct drop cables it won't be a
    standard 10base-2 configuration. If your system is as shown on the web page
    above and you are running 10Mb/s you are better off trying to get the
    correct drop cables from somewhere.
    Not quite. With a T plus extension cable you will get standing waves
    propagated on the extension that will reflect back on the main cable - the
    inteference between the waves will result in a a suboptimal signal.
    bok, Jan 8, 2004
  18. Bok

    Rudy Seoa Guest

    Here's a data sheet on the AMP system:

    The drop cables are only available in 2.5, 3.6, 4.8, 7.6m. These set
    lengths have probably something to do with the RF-side of things, but
    the theory to confirm this is beyond me.
    Rudy Seoa, Jan 8, 2004
  19. Bok

    pete Guest

    I know of some cabling similar - I think it was proprietary 3Com. At the
    network card end the cable has a single BNC connector, with no T piece. At
    the wall socket end is a odd looking plug. We have a few cables at work to
    support a client who still uses this stuff. The benefit is that you can
    remove a workstation without dropping the whole ine. The flylead cable in
    effect is a very long T piece. It pretty old stuff now. And yes, this
    client is using a Novell lan (the servers are 486 ICLs, the workstations
    are a mix of 386 and 486 diskless PCs, and on the whole, it all works
    pretty well)
    pete, Jan 8, 2004
  20. This is one of the reasons the system was 2Mb/s.

    It's actully more like ARCnet than Ethernet - but it's not ARCnet either.
    Uncle StoatWarbler, Jan 8, 2004
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.