Why 4 pairs in 100Base-T

Telephony and Power over Ethernet.

In comp.dcom.lans.ethernet Charles Lindsey wrote: < Connections between desktop computers and hubs/routers customarily us RJ45 < copnnectors and cat5 cable carrying 4 twisted pairs. < But, AIUI, only 2 of those pairs are used for the protocol, so what are < the other two for? given that cat5 cable does not come cheap, surely < 2-pair cable would be far more economical. < So why not use 2-pair cable and leave the other RJ45 pins unconnected? < Would that work?

I have some patch cables with only two pairs that came with

100baseTX cards. Otherwise, they are pretty rare.

You need all four pairs for gigabit, though. It may cost a little more now, but saves the cost of replacing all the cables when updating to gigabit.

I have some patch cables with only two pairs. Otherwise, they are pretty rare.

You need all four pairs for gigabit, though. It may cost a little more now, but saves the cost of replacing all the cables when updating to gigabit.

Otherwise, I don't think you can easily buy two pair Cat 5. The cables I have might only be Cat 3, though if short enough will likely work for 100baseTX.

-- glen

In comp.dcom.lans.ethernet Charles Lindsey wrote in part:

Yes, for 100baseTX . But not for 1000baseT (Gigabit) which needs all four pairs.

Why two pair won out is a bit of a historically interesting question. There was 100baseT4 that required and used all four pairs but only needed Cat3 cable quality. I think it required more sophisticated electronics to handle the pair slew (much like GigE needs).

One driver for two-pair was the original 10baseT spec that only used two-pair and left two pair for phone.

As for "why 4 pair cable?", the answer is more obscure. One simple answer is for easy parallelling rather than pulling new cable. Cable cost is only about 10% of total-installed system cost. Connectors are more (~15%), and installation labour is the most (~75+%) . Retrofits are even more labour-intensive.

-- Robert

OK, so if I'm making up my own cables, for whatever reason, then I may as well use just two pairs (assuming I can find someone to sell me such cable in bulk).

Sure, but most applications (including mine) would be happy on 10BASE-T, so I don't think I need worry about gigi-stuff.

In comp.dcom.lans.ethernet Charles Lindsey wrote: (then I wrote) In comp.dcom.lans.ethernet Charles Lindsey wrote: < So why not use 2-pair cable and leave the other RJ45 < pins unconnected? Would that work? I have some patch cables with only two pairs that came with

100baseTX cards. Otherwise, they are pretty rare. < OK, so if I'm making up my own cables, for whatever reason, < then I may as well use just two pairs (assuming I can find < someone to sell me such cable in bulk).

As I understand it, Cat 5 requires four pairs. For short patch cables you can probably get away with Cat 3 cable. That would be fairly likely if that was the only cable in the connection, and much less likely if there was 90m in between. You need all four pairs for gigabit, though. It may cost a little more now, but saves the cost of replacing all the cables when updating to gigabit. < Sure, but most applications (including mine) would be happy < on 10BASE-T, so I don't think I need worry about gigi-stuff.

You originally asked about 100baseT (that should have been more specific, as 100baseTX or 100baseT4. 100baseT4 uses four pairs of Cat 3 cable, but is very rare.) I believe that two pair 10baseT patch cables were not that rare. My favorite is one that came with a 3com PCMCIA NIC with a very small outer diameter.

-- glen

Just be certain to label the cables well since likely as not the outer insulator/jacket will have markings claiming the wires inside are suitable for GbE.

rick jones

Cat 5 is a specification for the electrical and mechanical characteristics of the wire pair. While cables may commonly be built with four pairs of Cat 5, there is no requirement that they be this way. You can make a two-pair Cat 5 cable, if that meets your needs. The pairs are still Cat 5.

FWIW, my cat's name is Cat 3, and I have not seen any reason to upgrade him.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

In message , Charles Lindsey writes

You'd probably find that it would be more expensive than the 4 pair as I suspect a cable with only two pairs, characterised for Ethernet, is going to be very specialist and as such quite rare.

Plus, you may find it harder to make up a cable with only two pairs, it's fairly easy to hold 8 wires in a way that ensures they push into the right hole on the plug, 4 wires would be a little more awkward.