Enterasys 7100G Series, Hardware Installation Manual

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Enterasys 7100G-Series Hardware Installation Guide D-1
D
About PoE (Power over Ethernet)
This appendix provides an overview of Power over Ethernet technology and how it is
implemented in relation to the 7100G-Series devices.
Overview
Power over Ethernet (PoE) refers to the ability to provide operational power through the same
Ethernet cabling to a PD (powered device) connected to a data network. Modern Ethernet
implementations employ differential signals over twisted pair cables. This requires a minimum of
two twisted pairs for a single physical link. Both ends of the cable are isolated with transformers
blocking any DC or common mode voltage on the signal pair. PoE exploits this fact by using two
twisted pairs as the two conductors to supply a direct current. One pair carries the power supply
current and the other pair provides a path for the return current. While several proprietary legacy
implementations of PoE have been deployed by LAN equipment vendors, in 2003 the IEEE
published the IEEE 802.3af-2003 specification, which is part of the 802.3 suite of standards.
The 7100G-Series devices that support PoE are fully compliant with the IEEE 802.3af and 802.3at
standards. They support the standard resistor-based detection method, as well as AC disconnect
capability.
Each PD has a PDC (Powered Device Classification) that is transmitted to the device for power
management purposes.
Table D-1 lists the classifications and the associated power ranges based
on the 802.3af standard. This table will be updated with 802.3at class information when the 802.3at
draft is ratified.
Proprietary PD Detection
7100G-Series devices support a subset of the currently deployed proprietary PoE methods. This
includes support for Cisco PDs, including a proprietary capacitor based detection scheme.
Table D-1 Powered Device Classifications
Class Usage PD Maximum Power Range Usage
0 Default 0.44 to 12.95 Watts
1 Optional 0.44 to 3.84 Watts
2 Optional 3.84 to 6.49 Watts
3 Optional 6.49 to 12.95 Watts
4 Not Allowed Reserved for Future Use