Idle question

Discussion in 'Home Networking' started by Dave J., Mar 15, 2008.

  1. Dave J.

    Dave J. Guest

    Can anyone offer an explanation for the ages-back decision to use two
    different pin alignments for CAT-5/RJ45 networking plugs/sockets? You know
    the way you need a crossover when the alignments are identical and a
    straight through when they're mirrored?

    Could it have been as simple as someone just imagining that all cables
    would be straight-throughs because no one would deviate from the standard
    pattern of router->[switch/hub]->Host?

    I still remember my CCNA tutor having to stop and think when I pointed out
    that if all sockets had the same pinout and all cables were crossovers
    then there'd never be any problem..

    On further thought, I can see that even under my supposed 'common sense'
    approach, you'd still sometimes want straight-throughs for extension
    purposes. Hmm, actually, no, you wouldn't would you, just make the inline
    connectors also all crossover. So an extended link would be
    [crossover-cable]-[crossover-inline]-[crossover-cable] and it would still
    work perfectly.

    So, what was the thinking? Does anyone have an idea?

    Dave J.
     
    Dave J., Mar 15, 2008
    #1
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  2. Dave J.

    PeeGee Guest

    No real answer, but I have no problem with the setup because I come from
    the DCE/DTE era (Data Communications Equipment - eg modem - and Data
    Terminal Equipment - eg teletype [yes, I know teletypes also used 80v
    signalling for some versions]) where the interfaces were designed for
    straight though cables as required by the ISO (also EIA and ECMA)
    standards :)

    In networking, the switch (or hub) is DCE and the computer DTE, so logic
    would suggest a continuation of the practise.

    The more confusing issue IMHO is the cross-over cable crosses two pairs
    and leaves two pairs straight through :-(

    --
    PeeGee

    The reply address is a spam trap. All mail is reported as spam.
    "Nothing should be able to load itself onto a computer without the
    knowledge or consent of the computer user. Software should also be able
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    Peter Cullen, Microsoft Chief Privacy Strategist (Computing 18 Aug 05)
     
    PeeGee, Mar 16, 2008
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  3. Dave J.

    newshound Guest

    No real answer, but I have no problem with the setup because I come from
    Ah the good old days.....never went anywhere without a null modem....
     
    newshound, Mar 16, 2008
    #3
  4. Dave J.

    Rob Guest

    I reckon you probably invented firewire without knowing (which, btw,
    work great for networking under XP.)
     
    Rob, Mar 18, 2008
    #4
  5. Dave J.

    Alex Fraser Guest

    I take it Firewire is "all cables crossed" then. Not computer-related, but
    SCART is like this too (for the audio in/out and CVBS in/out).

    Back to Ethernet, links made between punch-down blocks would need to be
    crossed too. That's fine if one end is in a wiring cupboard (or whatever)
    and the other inside a wall-mounted socket in an office, but is that always
    the case? (I have no idea, but if not and you forget which way you wired the
    first end...)

    Alex
     
    Alex Fraser, Mar 18, 2008
    #5
  6. Dave J.

    Rob Guest

    Yes, IEEE1394 cables are all cross-wired:
    http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=pinconbus_ieee_1394
    (scroll down to Cables and connectors, if interested.)
     
    Rob, Mar 19, 2008
    #6
  7. Dave J.

    Dave J. Guest

    That makes sense. Still doesn't quite make it correct, but it does make
    sense.
    Oh? Does the DCE/DTE standard have more than the data cables reversed?

    Dave J.
     
    Dave J., Mar 20, 2008
    #7
  8. Dave J.

    PeeGee Guest

    DCE/DTE, as I used the terms above, are part of the ISO/EIA/ECMA
    standards (the latter are virtually identical) and the standards defined
    the pin-outs for the equipment. IIRC, apart from the physical
    obstructions, the connectors were defined so that the two units would
    plug in directly - the cable, therefore, was straight-through extension
    lead.

    The ethernet cross-over patch lead is confusing because the two data
    pairs for 10 and 100Mbps are crossed over, the other pairs are not. If
    you use such a cable with Gigbit ethernet, two data pairs are crossed,
    two are not :-(

    Thankfully, unless you are using old equipment (especially hubs) the
    interface is auto-sensing and the patch lead is no longer a factor.
    Cross-over leads are largely redundant unless you wish to connect two
    computers directly, when auto-sensing is (or is that was?) not common
    for NICs.

    --
    PeeGee

    The reply address is a spam trap. All mail is reported as spam.
    "Nothing should be able to load itself onto a computer without the
    knowledge or consent of the computer user. Software should also be able
    to be removed from a computer easily."
    Peter Cullen, Microsoft Chief Privacy Strategist (Computing 18 Aug 05)
     
    PeeGee, Mar 20, 2008
    #8
  9. Dave J.

    Dave J. Guest

    IIUYC, that had, umm.. crossed my mind..

    My thought was that the standard could cope with that by defining
    plug<->plug as always crosover but (plug or device or socket) <-> (socket
    or panel) as always straight through.

    IOW as opposed to straight-through/crossover there would be 'connector
    cable' or 'extension cable' - one defined as for connecting two standardly
    wired sockets together and the other as extending a socket, the former
    being always crossover and the latter being always straight-through. No
    difficulty telling one from the other where it's cables and easy enough
    for technicians to get used to as the wiring from devices to sockets or
    patch panels would be identical.

    I dunno, I meant it as 'idle question', just me reflecting my musings and
    wondering if anyone else had thought similarly.

    Dave J.
     
    Dave J., Mar 21, 2008
    #9
  10. Dave J.

    Dave J. Guest

    Firewire's something I know little about, for some reason (don't quite
    know why) it's always come across to me as one of those gimmicky things
    that's implemented on a few devices but is superseded within months or a
    couple of years. Or, in some examples, before it's even released..

    Dave J.
     
    Dave J., Mar 21, 2008
    #10
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