Power over Ethernet: A Brief Overview
Power over Ethernet (PoE) is a technology for wired Ethernet LANs (local area networks) that allows the electrical current necessary for the operation of specific devices to be carried over the data cables, thus negating the need to have power at the end point of installation.
The concept behind this is to minimise the number of wires required for an installation. From a cabling perspective at least, the end result is lower cost, easier maintenance, and greater installation flexibility than with traditional wiring methods.
For PoE to work, the electrical current must go into the data cable at the power-supply end, and come out at the device end, in such a way that the current is kept separate from the data signal so that neither interferes with the other.
The current enters the cable by means of a component called an injector. If the device at the other end of the cable is PoE compatible, then that device will function properly without modification. If the device is not PoE compatible, then a component called a picker or tap must be installed to remove the current from the cable before being connected to the device. This "picked-off" current can then be routed to the power jack.
The IEEE standard for PoE requires category 5 cable or higher for high power levels. Power is supplied in common mode over two or more of the differential pairs of wires found in the Ethernet cables and comes from a power supply within a PoE-enabled networking device such as an Ethernet switch or can be injected into a cable run with a mid-span power supply.
The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA to each device.
The updated IEEE 802.3at-2009 PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power (minimum 44 V DC and 575 mA. The 2009 standard prohibits a powered device from using all four pairs for power.
Devices Typically Used With PoE
PoE provides both data and power connections in one cable, so equipment doesn't require a separate cable for each need. For equipment that does not already have a power or data connection, PoE can be attractive when the power demand is modest. For example, PoE is useful for IP telephones, wireless access points, cameras with pan tilt and zoom (PTZ), and remote Ethernet switches.
Downside of PoE With Biometrics
However, it is important to note that there are some significant potential downsides to PoE installations, most specifically related to biometric installations.
Principles of Access Control
Since most biometric readers are used to manage access in secure areas, it is considered good practice to ensure that all readers are powered from separate battery backup units. This way, in the event of a power failure of some kind, only 1 or perhaps 2 devices are affected, rather than the entire setup.
With PoE, if the one central switch goes down, it can take down not only the communications, but the function of the access control units, across an entire wing or even an entire building.
Another major disadvantage of biometric installations is that they tend to be connected to other devices such magnetic locks, striker locks and similar.
These devices tend to draw far more current than the biometric reader does, and most often, put significant strain onto the maximum allowed amperage being supplied by the device.
Experience has shown that whilst certain custom-made midspan devices can supply this current, their longevity is extremely poor, probably given that they do not follow the standards laid down for the maximum supply of power over Ethernet cables.
Lightning and Current Surges
In addition to the above, devices connected directly to the power supply are more susceptible to over voltage damage than those devices connected via a battery backup power supply, as the chances of the cable transmitting an overvoltage event such as a lightning strike are exponentially increased as the cable length increases.
Using PoE with iPulse Biometrics
The iPulse iPBX range of biometric products does NOT natively support PoE, primarily due to the disadvantages explained above, and due to the costs that this adds to the devices.
However, in environments where the biometric reader is being used in isolation, such as a time and attendance reader NOT connected to any other devices, the use of PoE is possible, and extremely simple, if required.
Cabling an iPBx reader to work with PoE
All the iPBX readers (with the exception of the iPB1 and iPB8 devices) are powered via the IntelliRelay. This device traditionally receives 13,8 V DC from a battery backup, and translates this power into that required for the reader.
In order to connect an iPBX reader to a PoE Ethernet cable, the best option is to purchase a splitter (also known as a “picker” or “tap”) which can be attached to the end of the PoE Ethernet cable sitting in the ceiling.
From this, you can then send the power to the IntelliRelay, and the network directly down to the reader, allowing your system to be powered from the central PoE switch.