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PoE system architecture
The specification of PoE typically requires two devices: the
Powered Source Equipment (PSE) and the Powered Device
(PD). The PSE is either an end-span or a mid-span, while the
PD is a PoE-enabled terminal, such as IP phones, wireless
LAN, etc. Power can be delivered over data pairs or spare
pairs of standard Cat5 cabling.
Powered Source Equipment (PSE)
Power sourcing equipment (PSE) is a device such as a switch
that provides (sources) power on the Ethernet cable. The
maximum allowed for continuous output power per cable in
IEEE 802.3af is 15.4 W. A later specification, IEEE 802.3at,
offers 25.50 W. When the device is a switch, it is commonly
called an end-span (although IEEE 802.3af refers to it as
endpoint). Otherwise, if it is an intermediary device between a
non PoE capable switch and a PoE device, it is called a mid-
span. An external PoE injector is a mid-span device.
Powered Device
A powered device (PD) is a device powered by a PSE and thus
consumes energy. Examples include wireless access points,
IP phones, and IP cameras. Many PDs have an auxiliary
power connector for an optional, external power supply.
Depending on the PD design, some, none, or all power can be
supplied from the auxiliary port, with the auxiliary port
sometimes acting as backup power in case of a PoE power
failure.
How power is transferred through the cable
A standard Cat5 Ethernet cable has four twisted pairs, but only
two of these are used for 10BASE-T and 100BASE-TX. The
specification allows two options for using these cables for
power, shown in Figure 10 and Figure 11.
The spare pairs are used. Figure 10 below shows the pair on
pins 4 and 5 connected together and forming the positive
supply, and the pair on pins 7 and 8 connected and forming the
negative supply (either polarity can be used).
Figure 10: Power supplied over spare pins
The data pairs are used. Since Ethernet pairs are transformers
coupled at each end, it is possible to apply DC power to the
center tap of the isolated transformer without disrupting the
data transfer. In this mode of operation, the pair on pins 3 and
6 and the pair on pins 1 and 2 can be of either polarity.
6 / 8
Figure 11: Power supplied over the data pins
Troubleshooting
This section contains issue-solving information. If the gigabit
Ethernet switch is not functioning properly, ensure that it was
set up according to the instructions in this manual.
Issue
The link LED does not
illuminate.
The 1000BASE-T port link
LED illuminates, but the
traffic is irregular.
The gigabit Ethernet switch
isn't connected to the
network
A PoE device connected to
the gigabit Ethernet switch
is not receiving power
How can the power output of
each PoE port be
determined?
Solution
Check the cable connection and
try swapping out a cable.
Ensure that the attached device is
not set to full duplex. Some
devices use a physical or software
switch to change duplex modes.
Auto-negotiation may not
recognize this type of full-duplex
setting.
Check the LNK/ACT LED and/or
try another port on the gigabit
Ethernet switch. Ensure that the
cable is installed properly and is
the correct type. Turn off the
power and then, after a while, turn
on the power again.
•
Check the cable type making
the connection to the device.
The cable should be an 8-wire
UTP, Cat5 or above, and
EIA568 cable within 100
meters. A 4-wire, short loop
cable, or a cable over 100
meters, affects the power
supply.
•
Ensure that the device is fully
compliant with IEEE
802.3af/IEEE 802.3at
standards.
•
Each PoE port supports 53 V-
54 DC, 600 mA, and a
maximum of 30 W of power
output. Detect and inject by the
IEEE 802.3at standard.
•
Each PoE port supports 53 V-
54 DC, 300 mA and a
maximum of 15.4 W of power
output. Detect and inject by the
IEEE 802.3af standard.
P/N 1073337-EN • REV A • ISS 19JUL17
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