Some switches not only transmit data over a connected UTP cable but use that cable to provide power to an attached device. For example, say that you want to mount a wireless access point (AP) on the ceiling. Although no electrical outlet is available near the AP's location, you can, as an example, run a Cat 6 UTP plenum cable above the drop ceiling and connect it to the AP. Some APs allow the switch at the other end of the UTP cable to provide power over the same wires that carry data. Examples of other devices that might benefit from receiving power from an Ethernet switch include security cameras and IP phones.
The switch feature that gives power to attached devices is called Power over Ethernet (PoE), and it is described by the IEEE 802.3af standard. The PoE feature of a switch checks for 25,000 ohms of resistance in the attached device. To check the resistance, the switch applies as much as 10V of direct current (DC) across specific pairs of wires (that is, pins 1 and 2 combine to form one side of the circuit, and pins 3 and 6 combine to form the other side of the circuit) connecting back to the attached device and checks to see how much current flows over those wires. For example, if the switch applies 10V DC across those wires and notices 0.4 mA (milliamps) of current, the switch concludes that the attached device has 25,000 ohms of resistance across those wires (based on the formula E=IR, where E represents voltage, I represents current, and R represents resistance). The switch could then apply power across those wires.
Next, the switch must determine how much power the attached device needs. The switch makes this determination by applying 15.5V - 20,5V DC (making sure that the current never exceeds 100 mA) to the attached device for a brief period of time (less than one-tenth of a second). The amount of current flowing to the attached device tells the switch the power class of the attached device. The switch then knows how much power should be make available on the port connecting to the device requiring power, and it begins supplying an appropriate amount of voltage (in the range 44V - 57V) to the attached device.
The IEEE 802.3af standard can supply a maximum of 15.4W (watts) of power. However, the later standard IEEE 802.3at offers as much as 32.4W of power, enabling PoE to support a wider range of devices, such as power-hungry IP video cameras. This newer standard for PoE is often referred to as Power over Ethernet Plus (PoE+).
About The Author
Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, StormWind.com. Anthony is also a VMware Certified Professional.
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