Interlogix GE-DSH-73 Series User Manual - Manual (Page 172)

Chapter 8: Power Over Ethernet Overview
168 GE-DSH-73/DSH-82 and DSH-82-PoE User Manual
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.
Stages of powering up a PoE link
Stage Action
Volts specified
per 802.3af
Volts
managed
by
chipset
Detection
Measure whether powered device has the correct
signature resistance of 15–33 kΩ
2.7-10.0 1.8–10.0
Classification Measure which power level class the resistor indicates 14.5-20.5 12.5–25.0
Startup Where the powered device will startup >42 >38
Normal
operation
Supply power to device 36-57 25.0–60.0
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.