PSE
Endspan PoE Switch
UTP Switch
UTP
UTP w/ PoE
FIGURE 1: Example of midspan and endspan PoE.
UNDERSTANDING THE POWER OF DELIVERY
There are two ways power is delivered from the PSE to
the PD: through an endspan device or a midspan injector
as illustrated in Figure 1. An endspan device is usually
located in the telecommunications room (TR) in the
form of a switch with PoE capabilities that sends both
power and data over the horizontal cable to the device.
A midspan injector is inserted into the horizontal link
between the TR and end device and injects power onto
the cable from that point forward. This allows PoE
to be added onto any Ethernet link that does not
employ power from a PoE switch in the TR.
As a network designer, there are two types of
infrastructure topologies to decide upon early in the
planning stages depending on whether deploying an
endspan device in the TR or closer to the PDs. The
two topologies are either centralized or decentralized,
respectively. The deciding factors include the amount
of power required at the PD, the required transmission
speed of the PD, the distance from the PSE, and cost.
It is important to note that a decentralized topology
is not recognized within most industry structured
cabling standards.
In a centralized topology, all the processing power
and active equipment (i.e., PoE-enabled switches) are
in a centralized location. From there, the horizontal
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100 Meters
cabling is installed to the powered device, creating a
point-to-point scenario. This centralizes all the cabling
and connectivity in the TR, enabling all the moves, adds
and changes (MACs) to be performed at patching fields
through a cross-connect or interconnection to the active
equipment. When a new application or PD is added,
another horizontal cable would then be installed to
the PD.
A decentralized topology puts the DC power closer
to the PDs. This can be accomplished with a PoE switch
located in the plenum space between the TR and
the PD. The decentralized PoE switch manages both
network computing power as well as DC power to the
device, eliminating individual connection from each
PD to a switch in the TR. “A plenum-rated PoE switch
can connect to the device or to a node and allows you
to originate several runs aggregated on one uplink,”
explains Luis Suau, technical leader, Industrial Products,
Cisco Systems. “A digital building switch is an active
component that actually regenerates the signal and
allows more cables to provide downstream connectivity
to the node or the devices themselves, either in a pointto
point or daisy-chained topology, dependent upon the
power and bandwidth required,” he adds. (Note that as of
this writing, PoE switches used for this application are limited
to 10/100 Mb/s applications, so they may not be suitable
AC
AC AC
PSE
Midspan PoE Injector
100 Meters
PD
PD