Power over Ethernet

Topics: Ethernet, Category 5 cable, Network switch Pages: 7 (2203 words) Published: November 13, 2013
PoE: A Success Story
In the process control industry, two sets of wires – one for communications and the other for power- were utilized. This, however, is quite costly and impractical. It didn't take long enough for engineers, being ingenious species1, to find a way to alleviate the growing need; thus, challenging them to find an answer to how structured cabling could be more economical and still efficient at the same time. And so the concept of Power over Ethernet (PoE) was born. This paper provides a research of PoE and its advantages, and the fundamental changes brought about by this technology to commercial and industrial applications. An overview of the advancements on the technology is provided as well. The Story behind the success

Power over Ethernet or PoE technology describes a system to safely transfer electrical power, along with data, to remote devices over standard data cables in an Ethernet network (Cat 3/Cat5/Cat5e/Cat6). Long before its discovery, structured cabling has been a puzzle for most integrators. Sometimes, they think that they have found a “perfect location” for a device, only to find out that it would be too costly or too burdensome to add a power outlet. At other instances, wires are needed to be extended because of the impossibility of the location of field devices. So, it has always been and will always be a problem if not for the breakthrough of the Power over Ethernet technology. Whether installation of instruments is considered, or extending the reach of the network with strategically placed wireless access points, the risk of possible failure is increased by having remote devices that need AC power connections. Losing data during a power outage is one thing, but losing data and the process safety is something else entirely. Therefore, PoE became a must. PoE is Standardized

PoE was invented by PowerDsine back in 1997 and the first power injector (Midspan) was installed in 1998. To save costs in the planning, wiring and installation of networks, many manufacturers entered into partnerships to make use of the PoE innovation. The Institute of Electrical and Electronics Engineers (IEEE) was approached to form an international standard to facilitate wide spread deployment of the technology. In June of 2003, the PoE specification became the IEEE 802.3af standard, also called Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI). It is the first international standard to define the transmission of Power over Ethernet infrastructure.

The original IEEE 802.3af-2003 standard version of PoE supplies up to 15.4 W of DC power (minimum 44 VDC and 350 mA) to each device. Only 12.95 W is assured to be available at the powered device (PD) as some power is dissipated in the cable.

PoE Technology

MDI Settings
The Medium-Dependant Interface (MDI or RJ-45) serves as the data/power interface between Ethernet elements. As such, it has two optional connection methods (feeding methods) to carry the power namely the Phantom Power method and the Spare-Pair Method.

Phantom power Method (Alternative A)
In the phantom power the voltage is coupled to the wire pairs 1/2 (-) and 3/6 (+). This method can be used in networks with four-wire or eight-wire wiring.

Spare-pair power Method (Alternative B)
The spare-pair power uses the free wire pairs. The voltage is fed directly to the free wire pairs 4/5 (+) and 7/8 (-). This method can be applied exclusively in networks with eight wire wiring. This does not apply for Gigabit Ethernet because here all eight wires are used for signal transmission and no spare-pairs are available.

Power Sourcing Equipment (PSE)
A typical PoE system consists of a PSE and a PD. The IEEE 802.3af specification defines PSE (Power Sourcing Equipment) as the element responsible for inserting power onto the Ethernet cable. The PSE may be located at the switch (Endspan configuration), or it may be a separate device located...
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