Allied Telesis GS900/8POE manual Minimum Power, Maximum Power, Class, Usage, the PSE, the PD

AT-GS900/8POE Gigabit Ethernet Switch Installation Guide

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 65 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.

Table 5. IEEE 802.3af Class vs. Power Levels

		Minimum Power	Maximum Power
Class	Usage	Levels Output at	Levels Output at
		the PSE	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

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). 1000 Mbps network traffic uses all eight strands.

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-GS900/8POE Gigabit Ethernet switch is Alternative A, where power is transmitted over strands 1, 2, 3, and 6.

PD’s that comply with the IEEE 802.3af standard typically support both power delivery methods. If a PD is compliant with the standard, it should be able to receive its power from the switch while using either a straight or cross-over cable. The PoE feature on the AT-GS900/8POE Gigabit Ethernet switch should also work with most 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, consequently, may not adhere to the standard. If this is the case, a cross-over (MDI-X) cable may be needed to insure that the DC polarity is correct.

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