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Chapter 8: Power Over Ethernet Overview
GE-DS-82 and 82-PoE Ethernet Managed Switch User Manual 215
Line Detection
Before power is applied, safety dictates that it must first be ensured that a valid PD is
connected to the PSE's output. This process is referred to as "line detection", and
involves the PSE seeking a specific, 25 KO signature resistor. Detection of this
signature indicates that a valid PD is connected, and that provision of power to the
device may commence.
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, 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.