Installation guide

DFE-Platinum Series Hardware Installation Guide C-1
C
About PoE (Power over Ethernet)
ThisappendixprovidesanoverviewofPoweroverEthernetTechnologyandhowitis
implementedinrelationtothe7G4285‐49and7G4205‐72DFEmodules.
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7G4285‐49and7G4205‐72arefullycompliantwiththeIEEE802.3af
standard.They
supportthestandardresistorbaseddetectionmethod,aswellasACdisconnectcapability.
The7G4285‐49and7G4205‐72arealsocapableofsupplyingthemaximumspecified
currenttoallportssimultaneously.
EachPDhasaPDC(PoweredDeviceClassification)thatistransmittedtothe DFEmodule
forpower
managementpurposes.Table C‐1liststheclassificationsandtheassociated
powerranges.
Table C-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