Chapter 1: Overview
24
Power Budgeting
The AT-8000/8POE Fast Ethernet Switch provides a maximum of 15.4 W
of power per port on six of the eight ports for a total power consumption of
95 W, while at the same time furnishing standard 10/100 Mbps Ethernet
functionality.
The AT-8000/8POE smart power management functionality supports any
combination of Ethernet ports (1-8) that supply power for IEEE 802.3af
Class 0, 1, 2, or 3 powered devices up to a maximum of 95 watts, as
described in Table 5.
Note
Power is supplied to the powered devices in the order that the ports
are connected or on a first-come-first-served basis until the 95 watt
limit is reached. If the switch is power cycled after the PoE devices
are connected to the switch ports, the power is supplied to ports 1
through 8 in that order.
A port connected to a network node that is not a powered device (that is, a
device that receives its power from another power source) functions as a
regular Ethernet port, without PoE. The PoE feature remains enabled on
the port but no power is delivered to the device.
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-8000/8POE Fast Ethernet Switch is Alternative B, where power is
transmitted over strands 4, 5, 7, and 8.
PD’s that comply with the IEEE 802.3af standard typically support both
power delivery methods. So long as a PD is compliant with the standard, it
Table 5. IEEE 802.3af Class vs. Power Levels
Class
Usage
Minimum Power
Levels Output at
the PSE
Maximum Power
Levels Output at
the PD
0
Default
15.4W
0.44W to 12.95W
1
Optional
4.0W
0.44W to 3.84W
2
Optional
7.0W
3.84W to 6.49W
3
Optional
15.4W
6.49W to 12.95W
