Chapter 5 The Poe Provision Process; Line Detection; Classification; Start-Up - Planet POE-1200 User Manual

Chapter 5
THE POE PROVISION PROCESS
While adding PoE support to networked devices is relatively painless, it should be realized
that power cannot simply be transferred over existing CAT-5 cables. Without proper
preparation, doing so may result in damage to devices that are not designed to support
provision of power over their network interfaces.
The PSE is the manager of the PoE process. In the beginning, only small voltage level
is induced on the port's output, till a valid PD is detected during the Detection period.
The PSE may choose to perform classification, to estimate the amount of power to be
consumed by this PD. After a time-controlled start-up, the PSE begins supplying the 48
VDC level to the PD, till it is physically or electrically disconnected. Upon disconnection,
voltage and power shut down.
Since the PSE is responsible for the PoE process timing, it is the one generating the
probing signals prior to operating the PD and monitoring the various scenarios that may
occur during operation.
All probing is done using voltage induction and current measurement in return.

5.1 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 KΩ 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 the PD's
circuitries till detection is certified.

5.2 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.

5.3 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).
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