Power-over-Ethernet (PoE) enables the use of a single Ethernet cable for
simultaneous delivery of power and data, eliminating a significant
number of electrical receptacles and circuits. The benefits include
simplified infrastructure management, reduced power consumption and
operational costs, improved safety, greater flexibility regarding
placement of devices, and higher reliability (with less infrastructure
there is less opportunity for error). However, a lack of understanding
of how and where power is sourced and used in a PoE environment may
result in unanticipated downtime. This can be caused by insufficient
available power or inefficiency due to improperly sized cooling in the
equipment room.
Understanding the basics of PoE and the implications
of its use promotes favorable decisions concerning electrical and
mechanical system design for spaces where this equipment may be
installed.ethernet switch design
PoE Components, Terminology, and Capabilities
Before delving too deeply into specific power and cooling needs for PoE
applications, one must first understand the basic required components
for PoE applications and the typical types of devices that take
advantage of its benefits.
Figure 1 illustrates the components and basic connectivity found in
every PoE application.A PoE injector is the Power Sourcing Equipment
(PSE) that supplies power into the powered device (PD). PSEs generally
fall into two categories: mid-span and end-span. A mid-span PSE is a
unit that injects power downstream, after the network switch. It is
typically connected in-line, between the network switch and the patch
panel, adding power to the line. The other type of PSE is the end-span,
which injects the power, along with the data stream, at the network
switch itself. Although this document specifically addresses the power
and cooling requirements for PoE switches (end-span PSEs), the same
concepts and results apply equally where mid-span PSEs are utilized.
Figure 2 – Typical Mid-span PoE injectors Originally, typical
applications for PoE included IP phones and wireless access points since
the IEEE PoE Standard (IEEE Std. 802.3af) permitted a maximum of 15.4
Watts of power delivered over the twisted-pair data cabling. However,
the drive to achieve greater energy efficiency, integrate building
systems, and reduce costs created demand for devices that require more
power. In October 2009, the IEEE approved the “PoE+” Standard (IEEE Std.
802.3at). This allows up to 30 Watts of power delivered over four pairs
of data cabling (category 5 or better). PoE+ supports IP-based security
camera and card access readers, radio-frequency identification (RFID)
tag readers, bar code scanners, print servers, and Building Automation
System (BAS) devices. With the onset of PoE+, a whole spectrum of
additional, higher-powered devices become possible – some still
emerging.
