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In today’s environments a large number of companies are designing their networks with Power over Ethernet in mind. The reason for this is the implementation of Voice over IP networks, IP security cameras and wireless connectivity.

What is Power over Ethernet (PoE) technology?
PoE is a technology for one of the most widely installed networks in use today, the wired Ethernet network. PoE allows the electrical power necessary for the operation of an end device to be carried by data cables along with the data rather than using a separate power cord for that end device.

In June of 2003 the IEEE committee ratified the standard for 802.3af, the means of providing up to 15.4watts of power over a Category 5 or better Ethernet cable.

With a PoE Ethernet switch, the power that is transmitted to the end devices is done over the data pairs (pins 1,2 & 3,6), this is sometimes referred to as “phantom” power. It is also possible to supply power over the unused pairs (pins 4,5 and 7,8).

What is Legacy Power over Ethernet?
Before the IEEE 802.3af standard was adopted by all companies for providing power over the Ethernet cable, there were a few companies that were using proprietary methods for the means of transmitting power over the data network. Cisco Systems was early to create such a solution back in 2000. This Legacy Power is also commonly referred to as ‘Inline Power.’ The majority of switches offered today that support 803.af PoE, including Transition Networks’ switches, will support this Legacy Power option. However, it should be known that when using 803.af Ethernet switches in the Legacy Power mode you will need to have a cable that is pinned for reverse polarity. See Fig.1 below:

Alternatively, some PoE devices have the capability to support the Legacy power option via a dispatch or jumper setting on the devices to reverse the polarity.

Devices and Benefits
Two devices are identified when discussing PoE, the power source equipment (PSE) and the powered device (PD). The PSE provides the power and the PD accepts the power. An example of power sourcing equipment is an Ethernet Switch that supports 803.af PoE. Examples of a powered device could be a VOIP phone, a security camera, or a wireless access point.

For PoE to work, the electrical current must travel on a standard CAT5 Ethernet cable or better. A CAT5 cable has four twisted pairs, but only two of the pairs are used for data. A PSE provides a maximum of 15.4 watts of power at -48VDC and a PD can have a maximum input power requirement of 12.95 watts. This accounts for degradation of power over the cable. Power over Ethernet allows network devices to be powered in remote locations where access to power is unavailable or difficult to install. For applications that are within 100 meters of a data closet, users can install one of the Power over Ethernet switches that are available on the market or use a standard switch with a mid span injector. A mid span injector combines the data and power and transmits over the data cable. A network manager needs to be cautious when using mid span injectors because unlike 802.3af compliant devices, mid span injectors do not verify the end device they are powering is a PD. A discovery protocol was implemented in the 802.3af standard that requires a PSE to probe end device and determine if the end device is a PD and if so, its capabilities. This prevents damage from occurring to non-PD devices.

Here are just some of the benefits to utilizing Power over Ethernet Technology;

• Only one set of wires to bring to your device which simplifies the installation process and saves space
• The PD devices can be easily moved, to wherever you can run an Ethernet cable or have already ran one which minimizes disruption to the workplace
• Safer - no high voltage main wires in remote locations.
• Reliability - connecting a central UPS ensures power to the appliance during mains power failure.
• Easily Managable - you can use SNMP network management to monitor and control the powered devices, and even shut down or reset them remotely.
• Flexibility - In wireless LAN systems it simplifies the RF survey task, as the access point can easily be moved and wired in. No need to have to an AC outlet nearby now.

Keywords: 802.3af, Connectivity, Equipment, Ethernet, Ethernet Switches, IEEE, IP Networks, LAN, Legacy, Network Manager, Networks, PoE, Power over Ethernet, Power-over-Ethernet, protocol, SC, SNMP, ST, Systems, Transition Networks, Transmit

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