Power management is possible with 802.3af standard. In order to protect non-PoE compliant devices from receiving unexpected power, an identification process is performed during connection time. Un-attached PSE ports are un-powered in their dormant state. When a device is first attached to a PSE port, it must present itself as a nominal 25k ohm resistance. This is called its Detection Signature, which the PSE tries to identify by applying up to 10.1 volts while measuring the current. Once identified, the PD may or may not participate in a classification process, which is used to determine the power that must be reserved for the PD. There are currently four classes of power (0 through 3) and one reserved class (class 4). Class 0 is the default class indicating that the PD may use a minimum of 0.44 watts to the maximum allowed, which is 12.95 watts. With the default setting, there is no power management. Notice the minimum power requirement. The PSE will remove from devices that draw less than the minimum required. Classes 1 through 3 offer a range of power as shown in Table 1. The PSE will apply a higher voltage (up to 20.5 volts) in order to determine the class of device by measuring the resulting current. This current signifies the class and is called the Classification Signature. If none of the anticipated currents are measured, the device is deemed to be a class 0 device and no power management is possible. In either case, the output voltage is then increased to its nominal 48 volts while its output current is monitored.

PD Power Classification
Class	Usage	Range of maximum Power used by the PD
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

Table 1 — Four power classes are currently available. However, only three classes have the possibility of being managed.

Injectors and Taps

Some PoE schemes were developed before the 802.3af standard was ratified and are still in use. These schemes may or may not be compliant to the standard. They utilized the spare pairs to power legacy devices, however, the concept is still applicable today. Usually an injector is used to apply power to an Ethernet segment, and a tap is used to extract the power from the segment before it appears at a non-PoE end station. To follow the standard, the injector must apply a nominal 48 volts, however, products exist that provide non-compliant voltages (such as 24 volts). There are two types of taps. The passive tap simply removes the power before the end station and then feeds it directly to the power input of the legacy device. The regulated tap will adjust the voltage on the cable to exactly match the requirements of the legacy device. This approach can be very effective with single station applications, however, to be truly compliant to the 802.3af standard, the tap must participate in the signature detection process and the injector must comply with all the requirements of a mid-span PSE. This may not be the case.

Power and Network Management

Each port on a PSE device must be capable of delivering 350 mA at 44 volts. This yields 15.4 watts of power, which is the maximum for a port. However, the PD cannot draw more than 12.95 watts. Power drops are expected with 100 m of twisted-pair cable — accounting for the difference. For most automation applications, 13 watts of power is quite adequate. Although each port can handle 15.4 watts, it does not mean that the PSE needs to be rated for the maximum at each port. This is where power management comes in. By having PDs report only the power necessary, it is possible to undersize the power supply and reduce its expense for the number of ports that are supported.

Another possibility is to equip the PSE with the Simple Network Management Protocol (SNMP). By introducing Management Information Base (MIB) data for each of the powered ports, power cannot only be monitored but also controlled. This information may then be used for security purposes or remote troubleshooting. Powered end-stations can then be rebooted remotely by the removal and application of power.

Summary

IEEE 802.3af-2003 Amendment to IEEE 802.3-2002.
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