how does poe affect my test tool
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March 2004 Version 1.3
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Copyright 2004 Fluke Networks Corp. E-mail: [email protected]
What is Power over Ethernet (PoE) and how does it work?
Jim Torres Fluke Networks
E-Mail: [email protected]
Tel: 1 800 283 5853 (toll free)
Intl: + 1 425 446 4519
March 2004 Version 1.3
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Copyright 2004 Fluke Networks Corp. E-mail: [email protected]
Introduction:
Power over Ethernet (PoE) is defined in the IEEE Standard 802.3af-2003 and describes the elements of PoE such as:
• PSE = Power Sourcing Equipment (Switch, Power Adapter)
• PD = Powered Device (IP Telephone, Access Point)
• RJ45 Pin Outs for Power
• Detection of a PD
• Scaling of Current from a PSE
For more information and greater detail on Power over Ethernet you will need to reference the standard itself.
What is PoE?
PoE is the ability to provide required power to a PD by searching the link, supplying power to the link, if a PD is detected and scaling back power when power is no longer requested or required.
PSEs may be placed in two locations with respect to the link segment, either at the end of the link, like a switch or using a power adapter which is referred to as a “Midspan PSE”.
With respect to the question of; “On which pairs will power be applied?”, the following graph used from the standard, shows:
• Alternative A: shows the PSE being a Switch/Hub and that power is being applied to the Tx (1,2) and Rx (3,6) pairs.
• Alternative B: shows the PSE being a Switch/Hub and that power is being applied to pairs unused for 10/100 BaseT (4,5 & 7,8). Note that for Gigabit Ethernet those pairs are used for data.
• The last configuration shows the PSE being a Power Adapter and it is Midspan and wired using Alternative B.
March 2004 Version 1.3
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Copyright 2004 Fluke Networks Corp. E-mail: [email protected]
Detecting a Powered Device (PD):
The following steps outline the detection and disconnect process used for PoE and some details with respect to the individual steps.
Step 1:
• The PSE applies a voltage to the power pairs between 2.8 and 10 volts. If there is no PD attached then impedance is infinite and no current will flow. If a PD is attached then current will flow based on the input resistance of the PD.
Step 2:
• A PD will be detected, when the PD applies circuitry, for the PSE, and the applied voltage, from the PSE, creates current flow, and the amount of current will define the power requirements of the PD, this indicates the “Class” of PD. The following table defines the PD classes with the required power.
Class Usage Minimum power levels at output of PSE
0 Default 15.4 Watts
1 Optional 4.0 Watts
2 Optional 7.0 Watts
3 Optional 15.4 Watts
4 Reserved for future use Treat as class 0
The amount of current, that flows, will change based on the “Class” of the PD. The table below shows the unique currents for each class of PD.
Parameter Conditions Minimum Maximum Unit
Current for Class 0 14.5 to 20.5V 0 4 mA
Current for Class 1 14.5 to 20.5V 9 12 mA
Current for Class 2 14.5 to 20.5V 17 20 mA
Current for Class 3 14.5 to 20.5V 26 30 mA
Current for Class 4 14.5 to 20.5V 36 44 mA
Step 3:
• Operational voltage will be maintained until the PD changes the circuitry and changes the current flow to a specified input current for disconnect. The PSE will detect the change and disconnect power to the PD.