Allied Telesis AT-FS708/POE - Manual (Page 24)

Chapter 1: Overview
24
Implementation A standard Ethernet twisted-pair cable contains four pairs of strands for a
total of eight strands. 10/100 Mbps network traffic requires only four
strands (1, 2, 3, and 6), leaving four strands in the cable unused (4, 5, 7,
and 8).
The PoE standard, IEEE 802.3af, describes two alternative ways for
delivering power to a powered device (PD) over twisted-pair cabling.
Alternative A uses the same strands that carry the network traffic.
Alternative B uses the spare strands. The PoE implementation on the
AT-FS708/POE Fast Ethernet switch is Alternative A, where power is
transmitted over strands 1, 2, 3, and 6.
IEEE 802.3af compliant PD’s typically support both power delivery
methods. So long as a PD conforms with the standard, it should be able to
receive its power from the switch with either a straight or cross-over cable.
The PoE feature on the AT-FS708/POE Fast Ethernet switch will work
with legacy PD’s as long as the device can be powered on pins 1, 2, 3,
and 6. A legacy device is a node that was manufactured before the
IEEE 802.3af standard was completed and may not be compliant. If this is
the case, a straight (MDI) cable may be needed to insure that the DC
polarity is correct.