Details of Power over Ethernet (PoE) and Mode A, Mode B

| 3/18/2017 12:05:59 PM


  • PoE
  • PSE
  • PD
  • Mid-span
  • End-span
  • Mode A, also referred to as Alternative A
  • Mode B, also referred to as Alternative B
Power over Ethernet or PoE describes a system whereby Power Source Equipment (PSE) provides power to a Powered Device (PD).

PoE is increasingly necessary due to locations that lack additional standard AC power sources, such as locations for PTZ cameras, VoIP phones and other peripherals. IEEE 802.3af and IEEE 802.3at standards define PoE power ranges, as well as sub-standards requirements.


PSE, Power Source Equipment—also referred to as power injectors—transfer electric power and data over a CAT5 or higher twisted pair cabling. This allows a single cable to provide both a data connection and electric power to devices, such as the peripherals previously listed. While the power and data are passed along simultaneously, the data’s throughput is not impacted, allowing for full bandwidth.
PSE devices are available as End-spans or Mid-spans. An End-span device is usually a network switch that can provide PoE power on each port. End-span PSEs typically are used in new installations where a new switch or router is required. Mid-span devices are often added to an existing network to add PoE capabilities.

How they carry the power falls into Mode A [Alternative A] and Mode B [Alternative B]. Only two of the four twisted pairs on an Ethernet cable are used for data in 10/100BASE-TX Ethernet; thus, power can be transmitted on the unused two pairs. In an IEEE sub-standard, this is referred to as Alternative B. It is a common PoE technique because it separates data and power conductors, making troubleshooting easier.

Power may also be transmitted on the data conductors by applying a common-mode voltage to each pair. (Remember: four pairs). Because twisted-pair Ethernet uses differential signaling, there is still no interference with data transmission. The common mode voltage is easily extracted using the center tap of the standard Ethernet pulse transformer. In the IEEE sub-standard, this is referred to as Alternative A. (Gigabit speed also uses Mode A).
The pins for Mode A and Mode B are as follows:

Mode A [End-span]
Pins 1, 2+
3, 6-

Mode B [Mid-span]
Pins 7, 8+
4, 5-


In addition to standardizing the practice for the spare-pair (Alternative B) and common-mode data pair power (Alternative A) transmission, the IEEE PoE standards provide for signaling between the Power PSE and PD. This signaling allows the presence of a PD to be detected by the power source, and allows the device and source to negotiate the amount of power required or available. PD supports both Mode A and Mode B.

As a result, the PD will only use the power it needs, and there will never be an “over power” condition. No damage will ever occur. IEEE 802.3af (PoE) and 802.3at (PoE+) are standards that define the amount of power that can be supplied. While PoE supports up to 15.4W, PoE+ provides up to 25.5W.


PSE provides power on the spare pairs of wire, and is known as Mode B or Mid-span. Mid-span indicates a PSE device that is connected in-line to each end device and adds power to the line. Most "injector" devices (as opposed to full switches) are Mode B. End-span is typically a network switch with multiple PSE ports, and typically supports Mode A.

PDs support both Mode A and Mode B. As a result, other than making sure the product meets the standard, it is important to understand how much power it must draw.

While the goal of Power over Ethernet seems simple, the fact is there are existing caveats. Without having all the information, equipment will appear to have compatibility issues; some products may not conform to the PoE standard. It is also about which Mode the product actually supports. This is why finding the specification about a given PoE product is so critical—make sure to read the entire spec, and if you cannot locate the information, call the vendor.


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