Too Many Choices! Picking the Right PoE for Your Application

Power over Ethernet (PoE) is extremely useful in many networking applications, as it means that only a single Ethernet cable needs to be run to a powered device (PD) such as a wireless access point, IP camera, VoIP phone, and other devices.

PoE, however, comes in many shapes and sizes, and it can often be difficult to figure out which PoE source should be used in which scenario. PoE can, however, be broken down into a few different categories.

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PoE injector vs. PoE switch

Power is delivered via Poe from power sourcing equipment (PSE), of which there are two types:

Midspan: These are PoE injectors, which are placed in-line in the Ethernet connection and inject data on the appropriate pairs in the Ethernet cable. PoE injectors have two ports, one for “data” to connect to the network, and one for “data+power” to connect to the powered device. PoE injectors have a built-in AC to DC transformer, so they can be plugged into a regular power outlet. They are useful when powering only one or two devices at a location. PoE injectors are generally passive, meaning that they will always supply power when plugged in. This can potentially damage equipment that either is not designed to handle PoE or that is expecting PoE at a different voltage level. The most common wiring mistake is to reverse the “data” and “data+power” cables. For outdoor use or in otherwise harsh environments, PoE injectors generally need to be placed in some type of NEMA enclosure to protect the cable connections from the elements.

Endspan: These are PoE switches that are network switches, which also supply PoE power on particular ports. PoE switches are generally active, so they will only supply power if it detects 25 kΩ resistance across the powered pairs when it is connected. This prevents power from being supplied to network devices like PC’s that are not utilizing PoE. PoE switches are generally useful for larger deployments where more than two devices need to be interconnected. There are industrial switches that are designed for outdoor use or harsh industrial environments, though placing the switch inside a NEMA enclosure is generally recommended to protect the cable connections from the elements. Indoor switches may either be designed for desktop or rack-mount use and are intended to be installed in climate-controlled environments such as data closets.

Standard vs. non-standard PoE

IEEE has established a series of standards for PoE (see figure below). The most common standards are IEEE 802.3af (48 VDC nominal, up to 15.4W) and IEEE 802.3at (48 VDC nominal, up to 30W). These standards guarantee power up to 100 m / 328 ft of CAT5 (or better) Ethernet cable.

To keep costs down any outdoor access points and other equipment may use different voltage levels for PoE. In Wi-Fi, using 24V is common for outdoor point-to-point / point-to-multipoint equipment, especially for surveillance and small-to-medium business (SMB) applications. Such access points generally come with 24V PoE injectors, and most vendors sell PoE converters, which are in-line devices that take in standard IEEE 802.3af/at PoE from a PoE switch and deliver 24V passive PoE out. Some vendors also manufacture PoE switches capable of providing 24V on specific ports.

The power being supplied by PoE has been increasing over the last few years due to the higher power demands of some applications. For Wi-Fi access points, the multiple radios required for dual-band APs and multiple spatial streams with IEEE 802.11n, IEEE 802.11ac, and IEEE 802.11ax requires more power to operate. For cameras, the motors needed to drive pan-tilt-zoom (PTZ) cameras quickly consume a lot of power when in motion. By comparison, a non-PTZ camera normally only consumes about 3-5 W. Recently, several companies have started deploying LED lighting powered by PoE, which in general is driving the IEEE 802.3bt specification.

Fast Ethernet vs. Gigabit Ethernet

Most network devices today utilize either Fast Ethernet (10/100 Mbps, or 100Base-TX) or Gigabit Ethernet (10/100/1000 Mbps, or 1000Base-T), though there are devices that are starting to utilize 2.5 Gbps Ethernet or even 10 Gbps Ethernet. Typically, modern Wi-Fi access points are utilizing Gigabit Ethernet (or faster), whereas most other network devices, including phones, cameras, and LED lights, will generally only utilize Fast Ethernet.

When a device is connected to a switch, the device and the switch automatically negotiate the fastest mutually available speed for communication. A PoE injector, however, does no such negotiation. When using PoE injectors, therefore, it is important to match the capacity of the PoE injector to the capacity of the connected powered device. One common mistake is deploying a gigabit switch and a gigabit client device with a 10/100 PoE injector, as the switch and the device will negotiate to use gigabit speeds, but the in-line PoE injector will not handle that much traffic, and data will get lost in transmission.

Takeaway messages

When selecting a PoE power source for a powered device, it is critical to understand the input voltage, maximum power, and maximum data rate that the powered device is expecting. Using PoE switches are generally advisable, though using PoE injectors are useful when only one or two devices are being deployed. If using PoE injectors, be careful to avoid the more common deployment issues, such as reversing the data and data+power cables and mismatching the speed of the PoE injector with the speed of the powered device.