PoE Switches vs PoE Injectors – What To Use When

PoE

Power over Ethernet (PoE) delivers power and data to PoE-enabled devices over the same ethernet cable.  Ethernet cables contain 4 pairs of wire, but most networks only use 2 of the pairs for data transmission.  PoE uses the 2 unused pairs of wire to provide DC power to the device.  This allows the deployment of PoE compatible devices in locations that lack electricity.   Since this eliminates the need to run separate power lines, PoE saves money in materials and installation time.  The flexibility this provides has led to an increase in PoE’s use with the only limitation being the amount of power that can be supplied.  The PoE standard, IEEE 802.3af, was developed in 2003 to support wireless Access Points with up to 15.4 Watts of DC power.  The updated version IEEE 802.3at, released in 2009, allows up to 30 Watts of DC power for VoIP phones and IP cameras.  The latest revision, IEEE 802.3bt, released in 2018, allows for up to 60 / 100 Watts of DC power at the source by supplying power over four pairs of wires versus the previous standards of supplying power over just two pairs of wires.  This increase in power provides support for a greater range of devices and applications such as Point of Sale (POS) systems and flat panel displays.

The following table summarizes the IEEE standards.

The PoE system is composed of two parts – the Power Sourcing Equipment (PSE) and the Powered Device (PD).  PDs are devices on the network that receive power such as wireless Access Points, VoIP phones and IP cameras.  PSEs are devices that source the power, and the most common PSEs are PoE network switches (endspans) and PoE injectors (midspans).

PoE Switch

A PoE switch is a network switch designed to provide power as well as the network connection to a PoE device through the same ethernet cable.  PoE switches are either active or passive.  Active PoE switches, known as standard PoE, are compliant to the IEEE 802.3af, IEEE 802.3at or IEEE 802.3bt standards.  They have auto-sensing ports that automatically detect whether the connected device is PoE-compatible and negotiate the proper voltage between the PSE and the PD.  If the PD is not compatible with the switch, the switch will not deliver power, preventing any potential damage to the non-PoE device. 

Passive PoE switches, known as non-standard PoE, are not compliant to any IEEE standard.  They provide power to the PD over the Ethernet cable without the negotiation or communication process.  The power on the port is set to a certain voltage so care must be taken to ensure that every device connected to the switch is a PoE compatible device and rated for that voltage level. 

Having this power to the port functionality centralized in the switch improves reliability using centralized, redundant, and uninterruptable power supplies to guarantee the functionality of the PDs.  It offers flexibility for installing PDs in locations with no electrical wiring present, which improves the scalability of the network and reduces the cost of deployment.  PDs are hot-swappable or plug-and-play so anyone can install it. 

PoE switches can support from 4 to 64 ports, so they are ideal for large deployments of PDs such as an office or campus setting with many IP devices such as IP phones and cameras. 

PoE Injector

A PoE injector “injects” DC Voltage on one or both unused pairs of wire in the ethernet cable, depending on the IEEE standard implemented.  This allows support of PoE enabled devices on a network without a PoE switch.  A PoE injector typically has three ports – data input, power input, and a data/power output port. DC power is provided by an external power supply and injected on to the ethernet cable plugged in to the input data port to provide data and power on the output port. 

PoE injectors fall into the same two categories as PoE switches – active and passive.  Active PoE Injectors follow the IEEE standards and will not inject power if the PD is not PoE compatible.  Passive PoE injectors always inject power so care must be taken to ensure the PD is compatible with the voltage applied.  Active PoE injectors apply 44 to 57 V DC while passive PoE injectors apply 18 to 48 V DC.

Deploying the right device for your network

The decision to deploy a PoE switch vs a PoE injector comes down to these factors:

•          Number of PDs
•          Flexibility
•          Manageability
•          Cost

If the network switch does not support PoE, using a PoE injector is the simplest and quickest way to support a PoE-enabled device on the network.  But a PoE injector has only one port and can only power one device at a time.  A PoE switch supports between 4 and 64 ports and can power multiple devices at a time.  So, if many PDs need to be powered, initially or over time, the PoE switch is a more economical choice.   

The PoE injector features a compact and light-weight design which is easy for wall-mounting with some units available in a weatherized IP67 rated enclosure.  This allows flexibility in the installation, taking advantage of existing electrical outlets indoor or outdoor.  PoE switches are typically rack mounted in an IT closet which requires routing power and network cables to a central location.   

The biggest difference between the PoE switch and PoE injector is in their manageability.  A PoE injector is essentially a small unmanaged PoE switch, featuring plug-and-play capability without complicated software configuration.  The PoE switch can be divided into the managed and unmanaged switch with the managed PoE switch providing full management capacities with network security, control, and management.  

The PoE injector is more cost effective than the PoE switch on a per-port basis.  If the network switch does not support PoE, using a PoE injector is cost effective way to support a few PoE devices without changes to the switch or electrical wiring.  For networks with many PoE-enabled devices, the PoE switch is a more cost-effective choice to set up a large and complicated network with centralized monitoring management.

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