▾Network ethernet cables are typically used for data transmission. However, the IEEE 802.3af and 802.3at standards also define the function of power over a network cable. This way, you can power small network devices without the need for an additional power cable. Let's focus on the advantages and disadvantages of PoE (Power over Ethernet) and its specifics.
The motivation for the creation of PoE was the boom of internet telephony
At the turn of the millennium, internet telephony technology, known as VoIP, became widely popular. One of the disadvantages of IP phones was the need for a separate power cable connected to a power source. The elimination of an additional power source was the main motivation for the creation of PoE. This function enabled IP phones to be powered and data to be transmitted through a single cable, which made device connections easier and saved on cables.
Two types of power sources for PoE
For power to be transmitted through a data cable, power must be injected into them. This is done in two ways, either using a PoE-supporting switch (usually active) or using special PoE injectors (passive).
Network switches with PoE capability
Some data switches directly support the function of active or passive PoE. All ports may have support for power over data cables. In such cases, it is important to note that the switch's power source is not designed for simultaneous powering on all ports. To avoid overloading, do not connect more than half the devices over PoE compared to the total number of ports. For example, a switch with 10 ports is unlikely to power more than 5 devices simultaneously, often fewer. There are also network switches that have clearly marked PoE-supported ports. With these, full power output is guaranteed, but you always need to verify which ports are data-only and which are powered.
PoE injectors
PoE injectors operate on the principle of passive PoE, where the required voltage is simply brought to the unused wires in the network cable. These are added to individual ethernet cables and serve to power a single device. The use of injectors is a simple and cheap solution but brings several disadvantages and risks. Using PoE injectors can result in significant voltage drops on long cables. Incorrect use of cabling or incompatible devices may even risk equipment damage. If a cable with passive Power over Ethernet power is connected to a standard interface that uses all wire pairs in the cable for bidirectional data transmission, a short circuit and burnout can occur.
For what devices is PoE power used
Besides the mentioned IP phones, PoE power is mainly used for web cameras. This eliminates the concern of drawing long power cables for individual IP cameras, with data transmission and power solved by a single ethernet cable. PoE power is also supported by some types of access points (Wi-Fi Access Points) to extend the range of the wireless signal.
If you decide to power some of your devices through an ethernet cable, choose the most suitable voltage injection method. While using PoE injectors is easy and cheap, it brings several risks that need to be considered.
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