2-pair Ethernet

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 Question by George Palm posted 17 May 2005
 2-pair Ethernet
I have a 25 pair CAT-5 24 AWG cable between two buildings.
I wish to use only 2 pairs per 100 Mhz ethernet
connection to maximize use of this expensive intra-building cable. I (In the wiring plan all four pairs go from the switch to the 110 PP or from the jack to the PP, but only two pairs are cross-connected to the cable)
I have done this and it works. But any LAN tester when applied to the jack fails the drop as only two pairs are present. With the PP if the other pairs are rquired in the future (POE?) I can run another cable and connect them up as well. My question is how can I show that this setup meets LAN drop specs? And also, please comment on using 3 or 4 pairs in the 25pr for analog Voice? (I have also done this with no apparent ill effects.)
 Answer by Joseph Golan posted 17 May 2005
Dear George,
First, I am going to assume that by "LAN tester" you are refering to an instrument similar to a Fluke OMNI Scanner. If so, most of these testers have an Autotest option to test for 100BaseT which would then pick up only the active pairs (1,2 & 3,6). Lacking that there may be an option to create your own autotest (using a cat5e autotest as the base) and then turn just off the unwanted pairs (4,5 & 7,8).

Second if you run a second cable in the future to fill in the missing pairs, you may end up with a skewing problem due to manufacturer differences between the cables.

Last I am not a proponent of shared services under the same sheath as voltage differentials of signals may cause a problem that today is unknown. You may end up actually using more bandwidth due to resends and yet think that there is no apparent ill effects as the data eventually gets through.

Joseph Golan, RCDD

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