Fundamentals of PoE

Aug. 1, 2023
By running data and power over the same cable, PoE reduces the need for additional wiring, saving on costs and redundant cabling. Today, PoE allows for centralization of building control including power and the Internet of Things.

PoE Basics

With PoE, electrical current goes into the data cable at the power supply end (PSE – power sourcing equipment – switch) and then comes out at the device end (PD – powered device). This current is obviously separate from the data signal – neither interferes with the other. PSE feeds power into the cable, either through a switch, also called an end span, or through a midspan, if it resides anywhere other than at the end of the cable. On the other side, PDs (powered devices) will consume power.

Network cables such as Cat 5e, Cat 6, Cat 6A etc comprise eight wires arranged as 4 twisted pairs. With 10 and 100Base-T Ethernet, 2 of these pairs are used for sending information, and these are known as the data pairs. The other 2 pairs are unused and are referred to as the spare pairs (Gigabit Ethernet uses all four pairs). Because electrical currents flow in a loop, two conductors are required to deliver power over a cable. PoE treats each pair as a single conductor and can use either the 2 data pairs or the 2 spare pairs to carry electrical current. It can also use the 4 pairs simultaneously.

Power over Ethernet is injected into the cable at a voltage between 44 and 57 volts DC - typically 48 volts. This voltage allows efficient power transfer along the cable, while still being low enough to be regarded as safe for users. However, it could still potentially damage equipment that has not been designed to receive PoE. Therefore, before a PoE switch or midspan can enable power to PoE equipment, it must perform a signature detection process.

Signature detection uses a lower voltage to detect a characteristic signature of IEEE-compatible PDs (a 25kOhm resistance). Before ports turn on PoE devices, the PSE and PD are required to complete a “handshake procedure” (detection, classification, and inrush) to ensure a valid PD is connected.

The PSE starts this procedure by sending a detection current signal and calculates the detection resistance based on measured voltage. The detection resistance is considered valid if it falls in the range of 23.75kΩ to 26.25kΩ.

After successful detection, the PSE starts classification by sending a voltage signal and at the same time, the PD removes the detection resistor on the port and presents the class resistor. If the PD determines the classification signal is valid, the PD will turn on the internal LDO which applies 2.5V across the class resistor. The PSE measures the current going through the class resistor and recognizes the class level of the PD.

Finally, after a valid classification result (meaning there is no overcurrent or class mismatch), the PSE will turn on the port and start monitoring inrush current. At the same time, the PD takes control to limit the inrush current.

Even after power is supplied, the PSE continues to monitor the current. If the power becomes excessive, if there is a short circuit, or if a PD is unplugged, it will shut off power.

This means there is no issue with connecting non-compliant devices such as printers, fax machines or PCs - because these devices do not carry the IEEE 802.3af signature, the ports will automatically disable power. However, if you are using integrated PoE on a 24-port networking blade, and many of the ports are supporting non-powered devices, it might be more prudent to use a Midspan, and only connect powered ports to the devices that need it. This will lower the overall cost of installation.

PoE Standards Overview

Both IEEE 802.3af as well as 802.3at delivered power over two of the four twisted pairs of the Ethernet cable. The latter was backward compatible with the 802.3af.

Subsequently, a couple of non-standard solutions have been developed. HD Base-T Alliance introduced Power over HD Base-T (“PoH”) in 2010. This delivers video, audio, control, 1000Mbps Ethernet and power up to HD Base-T 4K.

The various PoE system classes range from 1 to 8. Type 1 and 2 (IEEE 803.2af and IEEE 803.2at) include Class 1 through Class 4. Type 3 (IEEE 802.3bt) includes Class 5 up to 40W and Class 6 up to 51W at the PD. Type 4 (also IEEE 802.3bt) includes Class 7 up to 62W and Class 8 up to 73W at the PD. Note there are different power levels at the PSE and the PD. Some power loss is budgeted for as a result of heat dissipation.

The International Organization for Standardization (ISO) and Telecommunications Industry Association (TIA) are working toward updating Standards to support 4-pair PoE in agreement with IEEE 802.3bt, with their respective Technical Services Bulletin, TSB-184-A and ISO’s Technical Report – ISO/IEC TR 29125. These provide cabling guidelines for transmission of 802.3bt PoE over 4-pairs along with other applications.

These International Standards provide information on installation conditions, guidelines on cable bundle sizes for different classification of 4-pair Ethernet cables and maximum power permissible on the cables. They also describe methods to help manage temperature rise within cable bundles.

In 2015 UL conducted a study to study the effects of higher current levels on communication cables and the safety of using such cables. One key observation was that even very small increases in current resulted in significant increases in the measured temperature. Another observation was that changes in cable construction had a very large impact on measured temperatures.

Thereafter UL introduced the Limited Power (LP) Certification to indicate that the cable has been evaluated to transmit the marked current under recommended installation conditions, without affecting the temperature rating of the cable. This marking is informative only, and optional. The LP cable marking is just an indication that the cable has been evaluated by UL to carry the marked current without exceeding the temperature grade of the cable.

PoE Myths and Misconceptions

PoE has compatibility issues – In the early days of PoE, many home-brewed and proprietary schemes were employed to get power over network cables. However, as PoE’s popularity has spread the IEEE 803.3af Standard has gained universal adoption, meaning compatibility between all modern PoE equipment is assured.

PoE requires electrical knowledge - IEEE 802.3af PoE is designed to ensure reliable operation in any configuration that would be possible with regular Ethernet. All the user has to do is wire up the network as normal and the equipment will take care of power delivery.

PoE requires special wiring - Not at all, the same cabling – Cat 5e, Cat 6 and Cat 6A etc – and “RJ45” style connectors are used for both regular and PoE-enabled local area networks.

PoE is forced into devices - This misconception is surprisingly common, however, as per the “handshake” procedure described above, the PSE will only supply the amount of power required by the PD. If the PD stops working for any reason, the PSE will switch off the power. It is important to remember that power ratings quoted by manufacturers are upper limits. 

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