An increasing number of businesses, school districts and healthcare
facilities are delving into Smart Ethernet switches with Power-over-Ethernet
(PoE) technology—and reaping big benefits for their IP Surveillance networks. Power over Ethernet
technology describes a system to pass electrical power over Ethernet cabling,
along with data. This means that a network device can be powered and operated
using the same cable as for network connection, eliminating the need for power
outlets close to an IP camera, for example. However, in order to save money on
installation and increase the security level by using PoE, it's important to
keep certain key points in mind when dealing with the technology. This article
will guide you through equipment and standards within PoE that will help you
succeed in the field.
Power over Ethernet,
or PoE, is not a new technology and it's already widely utilised in networking,
for example in IP phones, wireless AP's and IP cameras. The first successful
design and implementation of a proprietary PoE system was accomplished by Cisco
in the year 2000. Their technique of putting 48V DC power on the LAN data cable
along with the data traffic helped them overcome their customer's objections to
wall adapter powering – and in the end also helped them to sell a lot of VoIP
phone systems.
The PoE scheme
In a PoE scheme, two
different types of devices are involved: power sourcing equipment (PSE) and powered
devices (PD). A PD is a PoE enabled network end device, such as an IP security
camera, equipped to accept low voltage power transmitted over Ethernet cabling.
A PSE on the other
hand, is a device that provides ("sources") power to the Ethernet
cable. Power is supplied in common mode over two or more of the differential
pairs of wires found in Ethernet cables and comes from a power supply such as
an Ethernet switch. There are two types of PSEs which can add PoE to your
network: endspans and midspans. Endspans are Ethernet switches that include the
power over Ethernet transmission circuitry and are commonly called PoE
switches. Midspans are power injectors that stand between a regular Ethernet
switch and the powered device, injecting power without affecting the data.
IEEE standards In June 2003, the IEEE working group
released the ratified IEEE 802.3af PoE standard. It provides up to 15.4W of DC
power (minimum 44V DC and 350mA) to each powered device. The maximum current of
IEEE 802.3af is 360mA. The output voltage range of IEEE 802.3af is from 44V DC
to 57V DC.
In 2010, IEEE ratified
a new PoE standard, 802.3at, which provides 30W of DC power to the PD. The
maximum current of IEEE802.3at is 600mA. The output voltage range varies from
50V DC to 57V DC. The IEEE 802.3at standard is also known as PoE+ or PoE plus.
The four PoE phases Every PSE is responsible for managing
four basic aspects (or phases) of PoE:
- PD detection
- PD classification
- Power-up
- Power-removal
PD detection and PD
classification are carried out through complex signaling protocols which make
sure that power is delivered to the PD according to the classification. It
prevents powering when no PD is connected and assures prompt power removal when
a PD is disconnected. The protocols also maintain a stable DC current flow at
all voltage levels. A PoE enabled PSE provides a low power signaling mechanism
that constantly monitors for a 802.3 powered device (PD) to appear at the end
of the LAN cable. If a non-powered network device is connected, the PSE can
function just as a non-PoE and perform an "ordinary" link to the
networked device. However, if an 802.3 PD is connected, the PSE will quickly
recognise this and begin the process of powering it up.
Cables The IEEE standard for PoE requires
Category 5 cable (CAT-5) or higher for high power levels, but can operate with
Category 3 cable for low power levels. But still, even high quality outdoor
Category 5 cable (CAT-5) is much cheaper than USB repeaters or AC wire.
Fault protection To minimize the possibility of damage to
equipment in the event of a malfunction, the more sophisticated PoE systems
employ fault protection. This feature is good to have and shuts off the power
supply if excessive current or a short circuit is detected.
UPS PoE can increase your security level
through a so called central UPS (Uninterrupted Power Supply) in the monitor
room or central control room where the PoE Ethernet switch is located.
In case of a power
outage, critical networking devices will become inoperable unless they are
protected by a UPS with battery backup. Having the networked equipment
distributed throughout your building or campus requires the distribution of
several UPS systems. With PoE, a single, centrally managed UPS can be used to
supply backup power to your PSE equipment. All the distributed PD networking
devices can then receive battery-backed power even in power outages.
Centrally managed
power also enables remote shutdown or remote reset capabilities. Through
managing a PoE-enabled LAN switch via a web browser or by SNMP, remote
networking devices can be easily reset or shut down saving the time and expense
of dispatching a technician.
PSE Conformance Test
is important despite the
various requirements described for PD detection signaling in the 802.3
specification, there is considerable room for design variation. In practice,
detection pulses and detection measurement schemes do vary significantly across
PSE interface technologies. The 802.3at specification leaves considerable room
for implementation dependent behaviours. Additionally, many vendors of PSE will
choose to go outside the 802.3 specification in ways that will affect the
ability to power and maintain pure 802.3at PDs. This high degree of variation
adds a number of PoE compatibility issues, such as problems with voltage
levels.
PoE challenges design
and test engineers a great deal. Evaluating the quality of a PSE comes down to
having to work with "smart" multi-channel DC power sources that are
activated and deactivated through signalling protocols operating over several
power delivery and polarity configurations. The application and management of
DC power over multiple local area network connections must be completely transparent,
safe, non-destructive, and non-disruptive to the traditional data transmission
behaviours of those network connections and associated network equipment.
for example, has over
ten years of PoE Ethernet switches design experience, and in the quality
assurance lab every PSE needs to pass the PSE Conformance Test Suite offered by
Sifos Technologies. The test suite for 802.3at produces up to 115 test
parameters depending upon PSE capabilities. These parameters are measured in 23
distinct tests that cover over 95 percent of the PSE PICS (conformance check
list items) in the IEEE 802.3at specification. The test is widely used
throughout the networking community as the industry "norm" for PSE
specification compliance.
Due to the fast pace
of this sector, with most equipment having a realistic lifespan of around five
years, it is most important to buy a fully qualified system. If you buy
equipment which is not fully conformant to IEEE 802.3at or IEEE 802.3af, you
might end up having to deal with a real nightmare in the future.
A few of the
advantages with PoE
- Equipment can be placed in the
most optimal location instead of choosing one where power is available.
- Network installations can be
accomplished cheaper, easier and faster.
- Network changes, such as adding,
removing or moving something, can be made much easier.
- Using a PoE infrastructure
enables centralised power management capabilities for critical network
devices.
- PoE can be used in security applications where USB or AC power is unsuitable, inconvenient or too expensive to use.
