GE -DSH-73 User Manual page 173

Chapter 8: Power Over Ethernet Overview
The signature resistor lies in the PD's PoE front-end, isolated from the rest of the PD's
circuitries till detection is certified.
Classification
Once a PD is detected, the PSE may optionally perform classification, to determine
the maximal power a PD is to consume. The PSE induces 15.5-20.5 VDC, limited to
100 mA, for a period of 10 to 75 ms responded by a certain current consumption by
the PD, indicating its power class.
The PD is assigned to one of 5 classes: 0 (default class) indicates that full 15.4 watts
should be provided. Classes 1-3 indicate various required power levels and 4 is
reserved for future use. PDs that do not support classification are assigned to class 0.
Special care must be employed in the definition of class thresholds, as classification
may be affected by cable losses.
Classifying a PD according to its power consumption may assist a PoE system in
optimizing its power distribution. Such a system typically suffers from lack of power
resources, so that efficient power management based on classification results may
reduce total system costs.
Start-up
Once line detection and optional classification stages are completed, the PSE must
switch from low voltage to its full voltage capacity (44-57 Volts) over a minimal
amount of time (above 15 microseconds).
A gradual startup is required, as a sudden rise in voltage (reaching high frequencies)
would introduce noise on the data lines.
Once provision of power is initiated, it is common for inrush current to be experienced
at the PSE port, due to the PD's input capacitance. A PD must be designed to cease
inrush current consumption (of over 350 mA) within 50 ms of power provision startup.
Operation
During normal operation, the PSE provides 44-57 VDC, able to support a minimum of
15.4 watts power.
Power Overloads
The IEEE 802.3af standard defines handling of overload conditions. In the event of an
overload (a PD drawing a higher power level than the allowed 12.95 Watts), or an
outright short circuit caused by a failure in cabling or in the PD, the PSE must shut
GE-DSH-73/DSH-82 and DSH-82-PoE User Manual 169