"Ralph Wade Phillips" wrote in message
...


1000BaseT is 2 pair, and most Gigibit uses 2 pair.

If they didn't they couldn't do Power Over Ethernet (PoE) injection
on the first pair, 4&5 B)

RwP

Power over Ethernet has TWO OPTONS
It can work on the 2 spare pairs with 10 and 100 Mb Ethernet (which uses 2
pairs and has 2 spare pairs)
OR on 2 data pairs when used with 1 Gb Ethernet (which uses ALL 4 pairs).
Read it yourself.

http://www.poweroverethernet.com/art...?article_id=52
Part 4: How Power is Transferred Through the Cable

A standard CAT5 Ethernet cable has four twisted pairs, but only two (2) of
these pairs are used for 10BASE-T and 100BASE-T.

The specification allows two options for using these cables for power, shown
in Figure 2 and Figure 3:

a.. The spare pairs are used. Figure 2 shows the pair on pins 4 and 5
connected together and forming the positive supply, and the pair on pins 7
and 8 connected and forming the negative supply. (In fact, a late change to
the spec allows either polarity to be used).
b.. The data pairs are used. Since Ethernet pairs are transformer coupled
at each end, it is possible to apply DC power to the center tap of the
isolation transformer without upsetting the data transfer. In this mode of
operation the pair on pins 3 and 6 and the pair on pins 1 and 2 can be of
either polarity.

gb