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Networks how many pairs are used in a 4 pair cable ?

Discussion in 'Hardware' started by Phil, 1 Oct 2001.

  1. Phil

    Phil What's a Dremel?

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    I've just got a whole load of plugs and wire from maplin, as well as a nice crimping tool and some strain relief boots too.

    Have been making up a few cables....

    I made the first without re-reading the various how to guides, and just assumed that so long as all the wires on one plug in the same order as the other, it would all work fine. (wait for it) and it did work fine. Despite what you're supposed to do which is split one pair (green or orange depending on type a or b) to pins 3 and 6...

    any way, I found wiring each pair in order worked, but that was probably just because it was a short patch cable, with longer cable the crosstalk would probably have become much more of an issue.

    I'm going off track here...ah yes, the question....

    I have heard, that only 2 pairs in the cable are actually used - RX+, RX-, TX+ and TX-. Is this true ? in which case why the hell are there 2 pairs unused in there ?

    Why can I not modify one cable to carry two cable's worth of data ?
     
  2. Forsaken

    Forsaken GameFaction.com

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    Think it was only for 10mb that only 2 pairs was used, but im not sure.
     
  3. Fly

    Fly inter arma silent leges

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  4. :: kna ::

    :: kna :: POCOYO! Moderator

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    Or,OrW,Gr,X,X,GrW,X,X

    They're used in a standard cable which leaves:

    X,X,Bl,BlW,X,X,Br,BrW

    Free for more data or voice should you wish to wire something in there. Yes you can use them for more data (and can in fact buy splitters which do it too) but as Fly said you may get signal degredation.

    In theory you could get 1 data and 2 phone lines down the one piece of cable if you really wanted to..
     
  5. Phil

    Phil What's a Dremel?

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    cheers kna, that's what I thought. I'll make up a double cable some time to test it.
     
  6. Phil

    Phil What's a Dremel?

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    i read that all 4 pairs are used in gigabit ethernet only...

    soo expensive for gigabit kit atm it's pointless anyway...for home users that is.

    I'm sure some Mad dude will step in at any minute and tell me they have a gigabit fiber network in their house or something....
     
  7. Ataraxia

    Ataraxia <b>OOH BABY!!!</b>

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    nar, 100meg networking uses all 4 pairs.
    I don't know what cable gigabit uses, but i do know that the 4 pairs in cat5 are all twisted at different tightnesses to eliminate crosstalk - gigabit ethernet would test that to the limit I think.

    talking about gigabit... the central switch on our network is gigabit... there's a network diagram around here somewhere....
     
  8. Phil

    Phil What's a Dremel?

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    if you're talking about university, most will have some gigabit gear in there somewhere. Ours has a gigabit backbone that feeds all the switches and routers in various parts of the uni.

    Most of these devices will be like 16 100meg ports and a couple of fiber channel uplinks - that's how it's all layed out...

    Gigabit ethernet can also use Cat5e cable, which as you've said is twisted slightly differently, and has various other performance boosting characteristics.

    But I'm still pretty damn certain that 100meg only uses 2 pairs....I must make a cable to find out for absolute certain though.....

    trouble is feeding in only 4 wires to the plug in the right places will be tricky...hmmmm
     
  9. Ataraxia

    Ataraxia <b>OOH BABY!!!</b>

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    yeah, i'm at Sheffield Uni - the network diagram I was on about has hardware descriptions and IP addresses too so I can't post it...

    but yeah, the network has a gigabit switch in the middle, with the servers around that. everything up to the last switches is 100meg, and each machine gets 10meg - I know this sounds poo, but it stops the network failing under heavy load. an ongoing project is to upgrade all desktops to 100meg

    yeah we have fibre going between sites

    i'm pretty certain it uses 4 :p so there we go... fancy a wager?
     
  10. Fly

    Fly inter arma silent leges

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    http://www.cablemeter.com/Wiring_Patch_Cords/

    Look at this URL, a few paragraphs down it tells you:

    The IEEE Specification for Ethernet 10BaseT / 100BaseT requires that two twisted pairs be used and that one pair is connected to pins 1 and 2, and that the second pair is connected to pins 3 and 6. Yes that is right - pins 4 and 5 are skipped and are connected to one of the remaining twisted pairs.


    According to the EIA/TIA-568B RJ-45 Wiring Scheme:


    It gets even more odd because wire Pair#2 (white/orange, orange) and Pair#3 (white/green, green) are the only two pairs used for 10BaseT / 100BaseT data.



    there you go, I rest my case...


    Fly
     
  11. Phil

    Phil What's a Dremel?

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    awww Fly, you could have let me make a bet with Ataraxia before you proved me right :D
     
  12. linear

    linear Minimodder

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    You use two pair for full-duplex mode, and one for half duplex. For 10baseT and 100baseT, anyhow. GigE on copper is for masochists, so I'll let someone else sort that out...
     
  13. Phil

    Phil What's a Dremel?

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    you still use two pairs for half duplex, it's just only one would be in use at any given time...
     
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