Power over Ethernet (PoE) – Demystifying Mode A and Mode B

PoE Mode A and B

Power over Ethernet (PoE) – Demystifying Mode A and Mode B

Power over Ethernet (PoE) can transmit power up to 100 meters. These long cables are easier to deploy than electrical cabling in locations where power is absent. Because PoE relays relatively low amounts of power, the technology presents low risks of electrical hazards.

When installing a Powered Device (PD) with a PoE-enabled Power Sourcing Equipment (PSE), it is important to take note that the devices use compatible modes to supply power across pins.

Mode A vs. Mode B

PoE relays power using two different modes known as Mode A and Mode B in 10BaseT and 100Base-Tx.

Mode A utilizes data pairs 1-2 and 3-6 to deliver power. Data signals cannot interfere with power signals since both signals are at opposite ends of the electromagnetic spectrum.

802.3af Mode A

Mode B on the other hand, utilizes its “spare pairs” to supply power. The power sourcing equipment applies a positive voltage to pins 4- 5 and a negative voltage to pins 7-8.

802.3at Mode B

With 1000Base-TX, power is sent through the spare pairs when operating in Mode B and power is sent through the data pairs when operating in Mode A.

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Ultra PoE

Injectors that deliver 60 watts, double the capacity of PoE+, utilize both spare pairs and data pairs, Mode A and Mode B,  to supply power. 

POE-173 from Planetech USA

PDs are compatible with both modes and will normally adjust themselves according to the mode used by the PSE.

PSE’s that relay power through their spare pairs (Mode B) are also known as midspan devices while those that relay power through their data pairs (Mode A) are referred to as endspan.  Endspan devices tend to describe switches with PoE capabilities. Midspan units on the other hand, describe stand alone units such as injectors that will usually accompany switches with non-PoE capabilities.


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Comments (2)

  • Gregg Hill Reply

    Why did you change the cable termination images from T568A to T568B when describing PoE Mode A and PoE Mode B? The PoE modes have NOTHING to do with the COLORS of the wiring. Regardless of whether the cable itself is T568A or T568B, PoE Mode A will always use 1-2 and 3-6 pairs, and PoE Mode B will always use 4-5 and 7-8 pairs.

    Using two different cabling standards in your explanation only confuses people MORE if they are not familiar with the wiring standards.

    **Electrically**, the ONLY difference between a T568A cable and a T568B cable is the different number of twists in the 1-2 and 3-6 pairs. Both of them will still have the same pinout of 1-2, 3-6, 4-5, 7-8, regardless of color. I could bleach the jackets of all the wire pairs, then terminate them, and still end up with either a T568A or T568B cable.


    February 18, 2019 at 11:14 am
    • Planet Technology USA Reply

      Thanks for pointing this out, Gregg. We made the changes.

      February 18, 2019 at 12:30 pm

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