Power over Ethernet: Midspans
PoE describes power transmission over a standard cat 5/6 twisted pair cable, thus not only eliminating the need for a power line, but also allowing installation by amateurs due to the safe low voltage. PoE works with voltages below 60V and is thus considered SELV (safety extra low voltage). A distinction is made between injectors (PSE – power sourcing equipment), which can in turn be designed as midspan or endspan, and consumers (PD- powered device).
Midspans use the spare pairs 4|5 und 7|8, endspans transmit power only over the data line pairs 1|2 und 3| 6.
As a rule, IEEE defines three power classes:
- IEEE802.3af (15.4W per port)
- IEEE802.3at (30W per port)
- IEEE802.3bt (up to 90W per port)
In addition, there are two special classes: ultra and mega PoE with 60 and 95W respectively per port.
It is possible for ageing networks to apply power to an existing Ethernet switch using midspan hubs. There are 4,8,16 and more channels to choose from here, with outputs of up to 90W per port, mostly available as 19" rack units with 1U height.