1. Traditional Power System on
Remote Ethernet Device
All of electronic devices need
power to perform its function
including Ethernet devices. In
the point of data communication
networking topology, the head
end equipment (Local Device) can
connect more than one remote
device, like one 4 ports
Ethernet switch can link 4
Wireless Access Point (AP)
simultaneously. It means five
power source (AC/DC) are needed
for operation of this Local Area
Network, i.e., five AC outlets
are mandatory facility.
Sometimes critical positions,
like roof, tree or trellis, they
are very difficult for AC power
outlets layout.
In exited power system, two
cables are essential materials,
power cable (adaptor) and LAN
cable (Cat.5, Cat.5e, even
Cat.6). The flexibility of
layout for remote device is also
limited by AC outlet. It means
that is not easy for expansion
and reallocation.
2. Benefit of Power over
Ethernet (PoE)
– One LAN cable can deliver power and
Ethernet data at same time
-
Flexibility for
distribution
-
Lower setup cost, running cost and
maintenance fee than traditional way
-
Safety mechanism. IEEE 802.3af /
IEEE 802.3at can provide short
circuit protection, under voltage
protection, over current protection,
…etc.
-
Ease of installation. Specially in
critical environments, like roof,
tree and public area (air port,
library and electric poles on
street)
-
Central monitoring and control. Some
remote devices need to be managed by
a server or central controller, like
PoE clock, PoE digital signature,
PoE digital display, …etc.
3. Power over Ethernet
(PoE)
Power over Ethernet, IEEE802.3af, is
constructed by
Power
Sourcing
Equipment
(PSE), Injector and
Powered
Device
(PD), Splitter. PD extracts power from
PSE via conventional Cat. 5 cable and
energizes remote device. The most
different with traditional power system
is that power and Ethernet data are
delivered by a single Cat.5 cable
between local device, Ethernet switch,
and remote device, wireless access
point, IP camera,
Network
Attached
Storage
(NAS), ...etc. PoE offers less
maintenance fee than traditional method
because of no AC outlet and power
adaptor on remote side. Two typical
connections are as follow.
4.
Power over Ethernet Introduction for
IEEE802.3af Compliant Device
A.
PoE
Link Detection
PSE shall issue a detection signal to
detect PD if PD can pass PD signature
detection and then do PD power
classification (Class 0 ~ 4) to define
the minimum and maximum power
requirement for this PD. PSE shall offer
48Vdc to PD when PD signature and power
classification is passed and finished.
PSE shall always repeat PD detection
process when the load is attached on PD
side but not IEEE802.3af compliant PD,
it means this PD can’t pass PD signature
detection.
Table 1
PSE Power Classifications
|
Class |
Usage |
Minimum Power Levels at Output
of PSE |
|
0 |
Default |
15.4W |
|
1 |
Optional |
4.0W |
|
2 |
Optional |
7.0W |
|
3 |
Optional |
15.4W |
|
4 |
Reserved For
Future Use |
Treat as Class 0 |
Table 2 PD Power Classifications
|
Class |
Usage |
Range of Max. Power Used By The
PD |
|
0 |
Default |
0.44W ~ 12.95W |
|
1 |
Optional |
0.44W ~ 3.84W |
|
2 |
Optional |
3.84W ~ 6.49W |
|
3 |
Optional |
6.49W ~ 12.95W |
|
4 |
Not Allowed |
Reserved For
Future Use |
B.
Isolated And Non-isolated PDs
b1.
Isolated PD
It can
provide a 1500V isolation protection
from PD input to PD output. This
isolation protection can efficiently
enhance short circuit protection
whatever short circuit is happened in
PSE side or PD side.
b2.
Non-isolated PD
It is
weak than isolated PD for short circuit
condition. Short circuit condition is
possible to damage both of PSE side and
PD side because of PSE and PD power
output side sharing same power ground,
return path.
5.
Mid-span
& End-point Connection
A.
End point Connection
B. Mid-span Connection
6.
IEEE802.3AT - PoE+
System
IEEE802.3AT is the advanced high power
PoE solution, so it is called PoE+.
It supports PSE output power up to 25.5W
and 4 pairs deliver 48Vdc to PD. PoE+
can provide up to 50W at the output of
PSE when uses two PSE channels on a
single RJ45 and applies 48Vdc on 4 pairs
simultaneously.
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