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.
This fourth article within Detektor Security Academy will, with the help of EtherWAN, explain the most important facts that everyone dealing with PoE should know.
The PoE scheme
|A PoE enabled IP security camera is an example of a powered device (PD) equipped to accept low voltage power transmitted over Ethernet cabling.|
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
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.
|PoE means passing electrical power on Ethernet cabling, along with data. The IEEE standard for PoE requires Category 5 cable or higher for high power levels.|
Cables The IEEE standard for PoE requires Category 5 cable 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 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.
|In EtherWAN's quality assurance lab every PSE needs to pass the PSE Conformance Test Suite offered by Sifos Technologies.|
EtherWAN, 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.
- Ethernet – Ethernet is a family of computer networking technologies for local area networks (LANs) commercially introduced in 1980. Standardised in the IEEE.
- Cat 5 – Category 5 cable (Cat 5) is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. This category guarantees speeds of up to 100 megabit per second.
- IEEE – The Institute of Electrical and Electronics Engineers is the world's largest professional association dedicated to advancing technological innovation and excellence.
- UPS – An uninterruptible power supply is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails.
- Phantom power – A technique used to allow the powered pairs in an Ehternet cable to also carry data.
- Injector – An injector is used to power PoE devices on a standard Ethernet line that is not PoE.
About the sponsor EtherWAN Systems, founded in 1996 with the US headquarters in California and the Pacific Rim headquarters in Taiwan, has become a leader in Ethernet connectivity for applications in various markets including IP surveillance, intelligent transportation systems (ITS), electric utility, oil and mining, factory automation, etc. EtherWAN specialises in designing and manufacturing fiber optic Ethernet products and Ethernet equipment made especially for harsh environments. Technical engineers are available for network topology consultancy.
For more information about EtherWAN's solutions, visit: www.etherwan.com
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