What is Power over Ethernet (PoE)?
As our Professional CCTV Experience
Power over Ethernet (PoE) is a method whereby power is transmitted to
Ethernet-connected equipment (VoIP telephones, WLAN transmitters, security
cameras) from the central switch. By using the existing CAT-5 cabling, the need
for AC power (and wiring costs) can be eliminated. The switch is also able to
control power distribution to the powered devices allowing sophisticated
uninterruptible power management for vital systems.
Operation
Fundamentally,
a PoE load or Powered Device (PD) must fulfill three functions in order to act
in conjunction with the sending end Power Sourcing Equipment (PSE). The
functions are Detection, Classification, and Under-voltage Lockout.
Detection Phase
When a PoE-enabled Ethernet cable is
plugged into a PD, the PSE interrogates the device to determine if it is
PoE-enabled. This period is termed the detection phase. During the detection
phase, the PSE applies a voltage ramp to the PD and looks for a characteristic
impedance from the load (25 kO). If the correct impedance is not detected, the
PSE assumes that the load is not PoE-enabled and shuts down the PoE sending
end. The system then operates as a standard Ethernet connection. If the
signature impedance is detected, the PSE moves on to the classification phase.
The signature identification voltage is a ramp voltage between 2.5 V and 10 V.
A 24.9 kO resistor provides the correct signature impedance for detection (see
Figure 1).
The PSE continues to ramp the voltage to the PD. Between 15 V and 20 V, the classification phase occurs. During this voltage transition, the PD must draw a specified current to identify the device class (see Figure 2). The simplest class (Class 0) is also implemented by the use of the 24.9 kOhm signature resistor. The classification current describes the amount of power the PD will require during normal operation. It is this information that is fed to the controller by the PSE, which allows the system to determine power budget requirements. A table of classification current and operating PD power requirements is shown in Table 1.
Turn on phase
By this process, the PSE and PD work together to determine the nature of the
load and apply power only to PoE enabled equipment. The system controller at
the central location can determine load requirements and allocate power
according to an operational needs hierarchy during power failure from its
available UPS budget
For additional information about driving PoE compatible load equipment and
circuits for implementing Class 1 through Class 3 classification, see Design
Ideas DI-70 and DI-88.
