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If a PD device is connected to a PSE, first the PSE will detect there is connected a PD compliant, and then the PSE will wait the Clasification of the PD, if the PSE does not receive a clasification from PD the PSE will assume that PD is Class 0.......im right??

Hi Zurc,

Yes this is correct.

The PSE first looks for devices that comply with the Clause 33 specification. This is achieved by applying a small current-limited voltage to the cable. The PSE then checks for the presence of a 25k ohm resistor in the remote device to confirm compliance. Then checks for valid classification signature. If this load (resistor) is detected, then the 48V is applied to the cable (with or without the optional classification signature). When a link is powered, the PSE must continually monitor it to make sure a PD is still connected to the cable.

There are specifications on methods for disconnecting (one DC and one AC) and timing specifications in milliseconds for detection, connection (applying power), monitoring and disconnecting as well as detecting problems. There is really much more to this then we need to know for CWNA but if you have any interest in the electronics end of wireless it?¡é?€??s fun to read into.


(There are some tables on voltace and current values used in detection that I wanted to put here but the HTML doesn't work.)



Table 1 IEEE 802.3af PSE and Powered Device Power Classifications
Class Usage Minimum Power Levels Output at the PSE Maximum Power Levels at the Powered Device
0 Default 15.4W 0.44 to 12.95W
1 Optional 4.0W 0.44 to 3.84W
2 Optional 7.0W 3.84 to 6.49W
3 Optional 15.4W 6.49 to 12.95W
4 Reserved for Future Use Treat as Class 0 Reserved for Future Use: A class 4 signature cannot be provided by a compliant powered device







802.3-2005 CLAUSE 33 (not official ?¡é?€?¡° just my notes mike_grol)

PoE overview
The standard allows for a supply of 48 volts with a maximum current of 400 milliamps to be provided over two of the available four pairs used on Cat 3 or Cat 5 cable with a maximum available power of 19.2 watts, the losses in the system normally reduce this to just under 13 watts.

The standard Cat 5 cable has four sets of twisted pair cable, and the IEEE standard allows for either to be used for 10Base-T and 100Base-T systems. The standard allows for two options for Power over Ethernet: one uses the spare twisted pairs, while the second option uses the wires carrying the data. Only one option may be used and not both.

When using the spare twisted pairs for the supply, the pair on pins 4 and 5 connected together and normally used for the positive supply. The pair connected to pins 7 and 8 of the connector are connected for the negative supply. While this is the standard polarity, the specification actually allows for either polarity to be used.

When the pairs used for carrying the data are employed it is possible to apply DC power to the center tap of the isolation transformers that are used to terminate the data wires without disrupting the data transfer. In this mode of operation the pair on pins 3 and 6 and the pair on pins 1 and 2 can be of either polarity as a full wave rectifier (bridge rectifier) is used to ensure that the device receives the correct power polarity.





Within the 802.3-2005 Clause 33 standard two types of device are described: Powered Devices (PD) and Power Sourcing Equipment (PSE)

Powered Devices (PD)

- Must be able to accept power through either the data pairs or the unused pairs
-Must reply with a ?¡é?€??Detection Signature?¡é?€?? (25 Ohm)
-May reply with ?¡é?€??Classification Signature?¡é?€?? (optional)

This is equipment that receives power from the Power Source Equipment (PSE) over the Ethernet cable.

The powered device must be able to operate within the confines of the Power over Ethernet specification of IEEE 802.3-2005 Clause 33 standard. It receives a nominal 48 volts from the cable, and must be able to accept power from either alternative A or B, i.e. using spare or data cables. Additionally the 48 volts supplied is too high for operating the electronics to be powered, so an isolated DC-DC converter is used to transform the 48V to a lower voltage. This also enables 1500V isolation to be provided for safety reasons.




Power Sourcing Equipment (PSE)

-Provides power for the connected device
-Searches for PDs with DC detection signal
-Withholds power until PoE compliance is determined
-Classifies the PD if it presents a valid classification signature
-Continuously checks that powered device is present
-Monitors for conditions such as short circuits
-Scales power back when no longer required

This is the equipment that supplies power to the Powered Device (PD) over the Ethernet cable. The IEEE 802.3-2005 Clause 33 standard puts all of the intelligence into the PSE, which possesses processing power and a management interface. This allows the PD functionality to be implemented as a simple, inexpensive hardware solution.

PSE provides a number of functions apart from simply supplying the power over the Ethernet system. The PSE first looks for devices that comply with the Clause 33 specification. This is achieved by applying a small current-limited voltage to the cable. The PSE then checks for the presence of a 25k ohm resistor in the remote device. If this load (resistor) is detected, then the 48V is applied to the cable, but it is still current-limited to prevent damage to cables and equipment under fault conditions.
The PSE will continue to supply power until the Powered Device (PD) is removed, or the PD stops drawing its minimum current.

PSEs may be placed in two locations with respect to the link segment, either with the DTE/Repeater or midspan. A PSE that is located with the DTE/Repeater is an Endpoint PSE. A PSE that is located within a link segment that is separate from and between the MDIs is a Midspan PSE.

1) Endpoint or Endspan
Located with the DTE/Repeater.
An Endspan solution is a switch with integrated power-supplying equipment, or more specifically, a switch with a PSE. The switch can supply the power via its physical ports using either the data pairs or the unused conductors. Endpoint PSEs may support either Alternative A or B. An Endspan solution is compatible with 10BaseT, 100BaseTX, and 1000BaseT cabling and is the only option that will work with Gigabit Ethernet.

2) Midspan- PSE Device
Located mid-span between the switch and the powered device.
It is connected to a standard Ethernet switch and sends power over standard UTP cat 5 cable. A Midspan solution is a pass-through device with integrated power-supplying equipment (single-port or multiport). With a Midspan solution the PSE device resides inside an injector that is located between the non-PSE switch and the Powered Device (PD). The Midspan device does not regenerate the Ethernet signal and must not disrupt the Ethernet signal. Midspan devices can send power only over the unused twisted pairs on the Ethernet cable. Midspan PSEs use Alternative B and will work with 10BaseT and 100BaseTX but not Gigabit Ethernet.




PSE - Power Sourcing Equipment
DTE - Data Terminal Equipment
MDI - Media Dependent Interface
PI ?¡é?€?¡° Power Interface

Alternative A - Power on data pairs pins 1, 2 Tx (orange pair) & 3, 6 Rx (green pair) or unused by data pairs ?¡é?€?¡° pins 4, 5 (blue pair) & 7, 8 (brown pair)
Alternative B - Power on pairs unused by data pairs ?¡é?€?¡° pins 4, 5 (blue pair) & 7, 8 (brown pair)

10BASE-T - Uses two pairs for data
100BASE-TX - Uses two pairs for data
1000BASE-T - Uses four pairs for data

Thanks mike! you always are helpfull, where did you get that info? i have the CWNA guide and it dont have so much info for PoE.....