Power over Ethernet (PoE) is a method for supplying power to network devices over the same cabling used for data traffic. PoE allows Powered Devices (PDs), such as IP telephones, wireless access points, and network cameras to receive both power and data over the existing infrastructure, with no upgrade necessary.

This feature can simplify network installation and maintenance, by using the switch as a central power source for other network devices.

There are currently two standards for PoE:

• IEEE 802.3af provides a maximum of 15.4 W per port.
• IEEE 802.3at provides a maximum of 25.5 W per port. This is known as High PoE.

    In PoE IEEE 802.3af, although Power Sourcing Equipment (PSE) provides a maximum output of 15.4 W per port, some of this power will be lost on a twisted pair cable. Consequently, only 12.95 W is guaranteed to be available for the powered device.

The IEEE 802.3af standard also specifies various power levels for PoE devices, so-called power classes. If the PSE supports this power classification, then the amount of power supplied to each port is automatically adapted to the power class (1-4) of the connecting PD. Class 0 is the default and provides a maximum of 15.4 W. Classes 1-3 provide less power than the default, and Class 4 provides more power, although this is only available to PDs conforming to the IEEE 802.3at standard.

If the connecting PD does not support power classification, the PSE will default to providing power at Class 0 (15.4 W).

The following table lists the power levels at both the PSE and the PD.

A PSE will normally supply a certain total maximum power, typically 300-500 W – this is known as the power budget. If the connecting PDs do not support power classification, the full 15.4 W (the default Class 0) must be reserved for each PoE port, meaning that a 300 W switch will only be able to supply power to 20 of the 48 ports. However, if all of the PDs communicate to the switch that they are Class 1 devices (4 W), then 300 W will be enough to supply power to all 48 ports.

The total power consumption of all the equipment to be connected to a specific network switch must be calculated, to ensure that it does not exceed the switch’s power budget.

Examples of PoE and power specifications for Axis cameras

The examples below present the concept behind the PoE and power calculations for an Axis camera. The exact figures and products used in the examples may change over time.

In the figure above, six Class 2 cameras are connected to a single PoE switch. As a Class 2 device requires 7 W maximum from the switch, we calculate the total power requirements as 6x7 W = 42 W. This will be the PoE power budget, meaning that we need a switch with at least 42 W available for PoE.

High PoE with AXIS Q6044-E

The AXIS Q6044-E power input is specified in the datasheet as max. 60 W and the midspan AXIS T8124 power requirement is specified as max. 74 W.
This is because the midspan itself also consumes power and there is a loss of power in the RJ-45 cable from the midspan to the camera. To ensure proper power to the camera, the midspan requires more input/output power than the total requirements of the cameras.

PoE with P13xx-E

  Some cameras are specified with two different PoE classes. This is because different wattages may be required depending on whether or not they are used with extra equipment, such as heating or cooling. The first specification is the wattage for the product itself, and the second is the wattage required for the product plus extra equipment. The AXIS P13xx-E is an enclosed product, and is specified as “PoE IEEE 802.3af  max. 12.95 W or High PoE max 25.5 W”.

How to determine the switch to use?

PoE switch manufacturers often specify three parameters that can be taken into account when deciding which switch to use. For example, these three parameters could be as follows:

• Supplied power to PD: up to 15.4 W
  This value is the maximum PoE power that the switch can deliver per port, but this is not related to the total PoE budget. It is important to remember that the value is specified as “up to”.
• PoE budget: 
  The PoE budget is the total PoE power that the switch can deliver for all ports. A high PoE budget and only a few ports means a higher W-per-port value. A low PoE budget and many ports means a lower W-per-port value.
• Average PoE W / port: 13
  Example: 50 W is the PoE budget, and the switch has 4 ports: 52 W / 4 = 13 W
  This value is what the switch can provide per port when all PoE ports are supplied with power. It is important to have a safety margin here. 

The PoE powering of a device becomes more critical depending on the ambient temperature. Many devices can function at different temperature levels based on the amount of power available. It is imperative to verify that the correct midspan is used for outdoor cameras. As seen in the specification sheet below, the AXIS P1353-E can operate in temperatures as low as -40°C when using High PoE.