Obviously, this current is separate from the data signal: neither interferes with the other. The PSE introduces the current into the cable, either through a switch, also called an end span, or through a midspan, if it resides somewhere other than the end of the cable. On the other end, the PDs (powered devices) will consume power.
Network cables such as Cat 5e, Cat 6, Cat 6A, etc., consist of eight wires arranged in four twisted pairs. With 10 and 100Base-T Ethernet, two of these pairs are used to transmit data and are known as data pairs. The other two pairs are unused and are called spare pairs (Gigabit Ethernet uses all four pairs). Since electrical currents flow in a loop, two conductors are needed to supply power through a cable. PoE treats each pair as a single conductor and can use either the two data pairs or the two spare pairs to carry electrical current. It can also use all four pairs simultaneously.
Power over Ethernet (PoE) is injected into the cable at a voltage between 44 and 57 volts DC, typically 48 volts. This voltage allows for efficient power transfer along the cable while remaining low enough to be considered safe for users. However, it could damage equipment not designed to receive PoE. Therefore, before a PoE switch or midspan can supply power to PoE equipment, it must perform a signature detection process.
Signature detection uses a lower voltage to detect a characteristic signature of IEEE-compliant PDs (a 25 kΩ resistance). Before the ports power on PoE devices, the PSE and PD must complete a "handshake procedure" (detection, classification, and breakout) to ensure a valid PD is connected.
The PSE initiates this procedure by sending a detection current signal and calculates the detection resistance based on the measured voltage. The detection resistance is considered valid if it falls within the range of 23.75 kΩ to 26.25 kΩ.
After successful detection, the PSE initiates classification by sending a voltage signal. Simultaneously, the PD removes the detection resistor from the port and presents the class resistor. If the PD determines that the classification signal is valid, it activates the internal LDO, which applies 2.5V across the class resistor. The PSE measures the current through the class resistor and recognizes the PD's class level.
Finally, after a valid classification result (meaning there is no overcurrent or class mismatch), the PSE will power on the port and begin monitoring the inrush current. Simultaneously, the DP takes control to limit the inrush current.
Even after power is supplied, the PSE continues to monitor the current. If the current is excessive, if a short circuit occurs, or if a PD is unplugged, it will cut off the power.
This means there's no problem connecting non-compliant devices, such as printers, fax machines, or PCs: since these devices don't carry the IEEE 802.3af signature, the ports will automatically disable power. However, if you're using integrated PoE on a 24-port network blade, and many of the ports support unpowered devices, it might be wiser to use a midspan and only connect powered ports to the devices that require them. This will reduce the overall installation cost.
Overview of PoE standards
Both IEEE 802.3af and 802.3at supply power through two of the four twisted pairs of the Ethernet cable. The latter was backward compatible with 802.3af.
Subsequently, a couple of non-standard solutions have been developed. The HD Base-T Alliance introduced Power over HD Base-T ("PoH") in 2010. This provides video, audio, control, Ethernet at 1000 Mbps, and power up to HD Base-T 4K.
The different PoE system classes range from 1 to 8. Types 1 and 2 (IEEE 803.2af and IEEE 803.2at) include classes 1 through 4. Type 3 (IEEE 802.3bt) includes Class 5 up to 40 W and Class 6 up to 51 W at the PD. Type 4 (also IEEE 802.3bt) includes Class 7 up to 62 W and Class 8 up to 73 W at the PD. Note that there are different power levels at the PSE and the PD. Some power loss is expected due to heat dissipation.
The International Organization for Standardization (ISO) and the Telecommunications Industry Association (TIA) are working on updating standards to support 4-pair PoE in accordance with IEEE 802.3bt, with their respective Technical Services Bulletin, TSB-184-A, and ISO Technical Report - ISO/IEC TR 29125. These provide cabling guidelines for 802.3bt PoE transmission over 4 pairs, along with other applications.
These international standards provide information on installation conditions, guidelines on cable bundle sizes for different 4-pair Ethernet cable classifications, and the maximum permissible power in the cables. They also describe methods to help manage temperature rise within cable bundles.
In 2015, UL conducted a study to analyze the effects of higher current levels in communication cables and their safety. A key finding was that even very small increases in current resulted in significant increases in measured temperature. Another finding was that changes in cable construction had a major impact on the measured temperatures.
From then on, UL introduced the Limited Power (LP) Certification to indicate that the cable has been evaluated to carry the marked current under the recommended installation conditions, without affecting the cable's temperature rating. This marking is purely informational and optional. The LP cable marking is only an indication that the cable has been evaluated by UL to carry the marked current without exceeding the cable's temperature rating.
Myths and misconceptions about PoE
PoE has compatibility issues – In the early days of PoE, many DIY and home-based schemes were used to obtain power over network cables. However, as PoE's popularity has grown, the IEEE 803.3af standard has achieved universal adoption, meaning that compatibility between all modern PoE equipment is guaranteed.
PoE requires electrical knowledge: IEEE 802.3af PoE is designed to ensure reliable operation in any configuration possible with standard Ethernet. All the user needs to do is wire the network as usual, and the equipment will handle the power supply.
Does PoE require special cabling ? - Not at all, the same cabling - Cat 5e, Cat 6 and Cat 6A, etc. - and "RJ45" style connectors are used for both normal local area networks and PoE-enabled ones.
Forced PoE on Devices – This error is surprisingly common; however, according to 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 disconnect the power. It is important to remember that the power ratings specified by manufacturers are maximum limits.
Types and classes of POE. RP categories
Power over Ethernet can bring many advantages to an installation, especially with Type 4/Class 8, which provides up to 90W. At these higher power levels, the planning and installation requirements are more demanding, complex, and potentially confusing.
PoE Classes
Power is injected into an Ethernet cable at a voltage between 44 and 57 volts DC. This relatively high voltage allows for efficient power transfer along the cable while remaining low enough to be considered safe for users. However, it can damage equipment not designed to receive PoE. Therefore, before the PSE supplies power to a PD, a power negotiation/classification sequence takes place.
There are 8 PoE Classes, plus Class 0 (unclassified). The Class reflects the lowest power level that both the PSE and PD can support, and is determined during the classification handshake procedure.
The goal of PoE negotiation, and the entire class system, is to ensure that the amount of power delivered is the required amount. It's a common misconception in PoE that power is "pushed" through the cable regardless of the device or devices at the other end. In reality, a lower-class device—or even an unpowered device—can be connected to a Power Supply Equipment (PSE) with higher power capacity without risk of overpowering. Each subsequent PoE version incorporates the previous classes, so while IEEE 802.3af incorporates only Classes 1-3, IEEE 802.3bt (Type 4) incorporates all eight.
Types of PoE
PoE types are based on the standards that describe them, the number of wire pairs that receive power, and the voltage.
Type 1 PoE, standardized in IEEE 803.2af, supplies power over two pairs of wires to provide a maximum current of 350mA.
Type 2, IEEE 803.2at, also uses two pairs of wires and provides up to 600 mA / 30 W.
IEEE 802.3bt, which includes types 3 and 4, allows power to be transmitted over all four pairs of twisted-pair cable, making it possible to deliver up to 90 W to the PSE (and 71 W to the PD). Type 3 delivers 600 mA and up to 60 W to the PSE, while Type 4 delivers 960 mA and up to 90 W to the PSE, depending on the PoE class.
As mentioned earlier, PoE standards are backward compatible, so Type 1 or Type 2 devices can connect to a Type 4 PoE network without issue. However, a Type 4 device is unlikely to draw enough power from a Type 1 or 2 network to operate.
Remote Feeding (RP) Categories
PoE types and classes exist to ensure that components in a PoE network can negotiate, supply, and receive the correct amount of power. In contrast, remote power (RP) categories are designed to address installation practices.
The three RP categories were defined in 2020 by international and European standards (ISO/IEC 14763-2 and EN 50174-2). The RP categories are defined based on the average current served by the distribution point. RP1 covers the lowest current applications, where the average current (mean ic) on all conductors served by the distribution point does not exceed 212 mA; RP2 covers from 212 mA to 500 mA; and finally, for category RP3, the connection of a PSE or PD to a distribution point is not restricted by the Ic limit of < 500 mA. However, thorough pre-installation planning, along with documentation and administrative controls, is still required during subsequent cabling expansions. Therefore, all Type 4 PoE installations must adhere to RP3 by default.
The purpose of the RP categories is to ensure that heat buildup from the power supply is properly managed in situations where it could be a problem. In lower-power installations (RP1), the risk of heat buildup is minimal, so the installation and planning requirements are minimal. Conversely, higher-power (RP3) networks carry a much greater risk of heat buildup, which must be taken into account when designing and implementing the network.
In the case of lower-power networks, it can be argued that working with RP1 or RP2 will save planning and documentation time. However, while this would simplify network installation, it would entail additional administration and documentation work throughout the network's lifespan. Every time a change or addition is required, it will be necessary to demonstrate that the average current will not exceed permissible levels. With RP3, planning is integrated into the initial network design, resulting in a much easier-to-manage architecture in the future. Connecting remote power equipment to a distribution panel is unrestricted, provided the current limit of ic < 500 mA is respected.
An RP category applies to an entire cable bundle. For example, if you mix cables supplying 15W Type 1 and 55W Type 3 PoE in the same bundle, the average current of the entire bundle would determine the minimum RP category.
An RP category applies to an entire cable bundle. For example, if you mix cables supplying 15W Type 1 and 55W Type 3 PoE in the same bundle, the average current of the entire bundle would determine the minimum RP category.
Therefore, even if the initial installation was only intended to supply low-power PoE, type 1 or 2, it may be more prudent to work towards compliance with the RP3 standard. This will depend on the long-term goals of the network, and as always, those goals should be clearly defined in advance.
It is also recommended to apply a consistent category to all distributors where remote feeding is applied.
Likewise, when the installation specification describes an extension of the existing cabling, the RP category of the existing installation must be specified.
Finally, specific labeling is necessary: it is recommended to have labels on both the cabinet and the distribution boards (or connection panels).
Getting it right on the ground
