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Things You Should Know about Power-Over-Ethernet

Power-over-Ethernet is a technology formalized under the Institute of Electrical and Electronic (IEEE) standard.By combining power and communications over a single UTP cables like Cat 3 or Cat 5 cable, engineers canconstruct low-maintenance Ethernet networks quickly and inexpensively compared with installations employingseparate system, the key advantages of PoE technology are its simplicity and the fact that power is availablewherever there is a data socket.

Development of PoE

PoE first introduced as a solution to power Voice over Internet Protocol (VoIP) phones. Traditional phones tooktheir power directly from the copper wire carrying voice calls. However the increasing popular VoIP phones werenot connected to these conventional circuit, instead taking calls over the Ethernet cables of a company’s LocalArea Network (LAN). As the Ethernet cable can not carry power, so the VoIP phones had to be plugged in to themains supply via an adapter. It was a less-than-elegant solution, and if the building power was lost, the phone willstop working.

In 2000, telecommunication equipment vendor Cisco firstly mimic the traditional phone system by introducingproprietary technology that allowed Ethernet cables to carry a 48 VDC supply to power VoIP phones. PoE reallygained momentum in 2001 and 2002, as other manufacturers, particularly Wireless Access Point makers, tookadvantage of the technique.

In 1983, the IEEE organization, think it is necessary to create a standard version of PoE so that any manufacturercould make their products workable with PoE. The work was assigned to a working subcommittee of the IEEE802.3 Ethernet Committee and named “802.3af”. By June, 2003 the working subcommittee ratified the IEEE802.3af PoE Standard. In 2009, a second standard, IEEE 802.3at, was ratified which defined a similar technologythat could handle more power.¹

IEEE PoE StandardPoE technology used as single, standard RJ45 connector and Cat 5 cable, and can handle tens of watts. Once theEthernet network is installed for communication, it can also be served as power, saving on materials, labor,installation time, and ongoing maintenance costs.

Power may be transmitted on the used conductors of the Ethernet cable, since only two of the four pairs of Cat 5cabling is required for the typical 10 to 100 Mbps physical layer. Power also may be transmitted on the dataconductors of the cable by applying a common-mode voltage to each pair. As Ethernet users differential signaling,this does not interfere with the cable’s data transmission.

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802.3af Possible Devices

IEEE 802.3af defines two types of PoE devices, Power Sourcing Equipment (PSE) and the Powered Device (PD).A PSE draws power from its own conventional power supply and then manages the power sent over the Ethernetcable network to the PD, which takes the power it needs via an RJ45 connector without a building-in powersupply. PoE provides a flexible and convenient means of powering devices that are located in out-of-the-waylocations, and saved costs by eliminating the expense and difficulty of direct electrical wiring. A challenge withPoE is reaching remote locations outside the distance limitation of UTP cabling. The maximum length for a cablesegment is 100 meters, while PoE power injectors or midspans do not increase the distance of the data link.

PoE Media Converters

Fiber optics is the best solutions to get beyond the 100-meter limit, fiber optic cabling is an effective method toovercome the distance and bandwidth limitation of UTP. To integrate copper network equipment with fiber optic,the PoE media converter is needed, which is a good solution to extend network distance via fiber to remote PDdevices such as the original VoIP phones and wireless Access Points, security cameras, Point-of-sale (POS)terminals, temperature control systems, and even in-flight entertainment systems. A PoE media converter convertsthe fiber to copper, and sends DC power to the PDs over the UTP cabling.

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Picture show How PoE media converter works

With a standard PoE media converter, the copper UTP cabling is converted to fiber. The fiber is run a longdistance to the PoE media converter located near a convenient AC or DC power source, where it converts the fiberback to copper UTP. The PoE media converter also functions as a power-sourcing mini switch, and injects PoE(DC power) over the UTP cable. At the other end of the UTP cable is the PD devices like a IP Cameras, located upto 100 meters away from the PoE media converter.

PoE media converter are available with 10/100/1000Base-T(X) to 1000Base-FX Gigabit Ethernet and10/100Base-T(X) to 100Base-FX Fast Ethernet PoE Converter Fast Ethernet data rates and can support PoE (15.4watts) or PoE+ (25.5 watts). The PoE media converters function like PoE mini-switches, and are available in avariety of multi-port configurations including dual RJ-45 and dual fiber ports. They can support fixed fiberconnectors or small form pluggable (SFP) transceivers.

Multi-port PoE media converters makes the network design flexible. They typically feature single or dual RJ-45ports to power one or two PDs like IP cameras per converter, and single or dual fiber ports.

Single fiber PoE media converters are deployed in a point-to-point, network start topology. In this network, thePDs e.g. IP cameras are installed throughout a facility fiber. Fiber is distributed from a fiber switch, a copperswitch or media converters. The fiber links connect to distant locations where PoE media converters are installednear AC or DC power sources. The PoE media converters convert the Fiber to copper and inject PoE over the UTPcabling to the IP cameras.

PoE media converters with dual fiber ports enable linking multiple media converters in a linear daisy chainconfiguration. This architecture can be used in a variety of outdoor applications where PDs such as IP camerasand/or wireless access points are installed along railroad and subway lines, highways, pipelines, mine shafts,perimeter fences, etc.

Dual fiber ports also link multiple media converters in a ring architecture. The fiber switch shown in this diagramsupports spanning tree to enable a redundant ring architecture. In the event of a fiber failure in the ring, the fiberswitch would reroute the traffic in the opposite direction.

Dual fiber ports can be used to deploy redundant fiber links for mission-critical applications requiring fiber

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facility protection. There is an active fiber port, and a protection fiber port, that can support a fiber failureswitchover of less than 50 milliseconds.

PoE media converters combine the benefits of PoE or PoE+ and fiber optic cabling in a compact, reliable andcost-effective device. Classified as Power Sourcing Equipment (PSE), the PoE media converters provide power toPowered Devices (PDs) using the same UTP cables that carry the Ethernet data. This enables powering devicesin hard-to-reach locations where there is limited access to AC power outlets, or locations where AC power createssafety issues.

FiberStore is one of the main suppliers of PoE media converters on the market to support the high-power PoE+standard. Models are available in Gigabit 1000BASE-X fiber to 10/100/1000 UTP and Fast Ethernet100BASE-FX fiber to 10/100 UTP.

A variety of port configurations are available, including single or dual SFP and single or dual powered UTP ports.Models with dual SFP ports support critical applications that require redundancy and sub 50ms switch over in theevent of a fiber failure. The product is DC powered and available with an optional external 100 - 240VACuniversal power adapter.

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