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Power over Ethernet (PoE)


Power over Ethernet (PoE) is a mechanism for supplying power to network devices over the same cabling used to carry network traffic.

PoE allows devices that require power, called Powered Devices (PDs), such as IP telephones, wireless Local Area Network (LAN) access points, and network cameras to receive both power and data over existing infrastructure without. No infrastructure upgrade is necessary.

This feature can simplify network installation and maintenance by using the switch as a central power source for other network devices. The challenge during installation is to calculate the total power consumption required so it is less than the power budget of the switch.


Calculating the total power needed


There are currently two standards for PoE: 802.3af allows for a maximum of 15.4 W per channel, whereas PoE 802.3at doubles the available power to 25 W.

The total power consumption requirement of all equipment that will be connected to a specific switch on a network needs to be calculated to ensure sufficient power is available per switch. This total wattage requirement must be less than a switch’s PoE power budget – total PoE power per switch and per port.

The following chart shows the power consumption at both the PSE and the PD.

Class Usage Power Level Output at the Power Sourcing Equipment (PSE) Maximum Power Levels at the Powered Device (PD)
0 Default 15.4 W 0.44 - 12.95 W
1 Optional 4.0 W 0.44 - 3.84 W
2 Optional 7.0 W 3.84 - 6.49 W
3 Optional 15.4 W 6.49 - 12.95 W
4 Valid for 802.3at High PoE 30 W 12.95 - 25.5 W

Figure 2. Values to be used when calculating the power budget of a system.


Example of power requirements in a PoE system





Figure 3. Example of power requirements in a PoE system.

As illustrated in Figure 3, six cameras, all PoE Class 2, are connected to one switch. Since a Class 2 device draws 7 W maximum from the switch, we can calculate the power requirements for a total of 6 cameras X 7 W = 42 W.
This will be the PoE power budget. Therefore, we need a switch with at least 42 W available for PoE.


Examples of xPoE and powering calculation with Axis cameras


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


High PoE with AXIS Q6032-E


The AXIS Q6032-E power input is specified in the datasheet as max. 60 W and in the Installation Guide (IG) it is specified as 50 W (max.). However, the midspan AXIS T8124 input is specified as max. 74 W.

Why is the input 50 W/60 W for the camera, but 74 W for the midspan?

The background for this is that the midspan itself consumes power and there is loss of power in the RJ45 cable from the midspan to the camera. Therefore, to ensure proper power to the camera, the midspan needs input and output power that is higher than the camera needs.
To conclude: input to the midspan is 74 W, and output from the midspan is 60 W, while input to the camera is 50 W.


PoE with P13xx-E


Some cameras are specified with two different classes of PoE. This is because products can require different wattages, depending on whether they are used with or without extra equipment, such as heating or cooling. The first PoE number specifies the wattage for the product itself, whereas the second number specifies the wattage needed for the product, including 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".


Using Direct Current (DC) Midspan


DC may be used for certain applications, such as solar panels, and AXIS has the T81B22 30W DC midspan for just this purpose. T81B22 is specified as “51 V DC at: 12 DC IN (max. 30 W) or 24 V DC IN (max. 15 W)”.


PoE switch with P3384-VE


AXIS P3384-VE is specified as "Power over Ethernet IEEE 802.3af Class 3; max 12.1 W".

How can you find out what switch to use?

Usually the provider of the PoE switch describes three parameters that should be taken into account when deciding upon what switch to use. For example, the three parameters could be as follows:

  • Supplies power to PD: up to 15,4 W
    This value is the maximum PoE power the switch can deliver per port, and is not related to total PoE budget. It is important to remember that it says "up to".
  • Total PoE budget:
    The total PoE budget is what the switch can deliver in total PoE power on all ports. High value and few ports means a higher value of W per port. Low value and many ports mean a lower value of W per port.
  • Average PoE W / port: 13
    Example: 50 W is the total PoE and the switch has 4 ports => 52 W / 4 = 13 W
    This value is basically what the switch per port can handle if all PoE ports are being used. It is important to have a margin here to be on the safe side to know my device.

Basically, in this example, an 802.3af PoE switch would be suitable for the camera, and can be used to connect four AXIS P3384-VE’s.


Ensure the right PoE for environmental conditions


The PoE powering of a device becomes more critical depending on temperature. Many devices can function at different low temperature levels based on the amount of power available. It is imperative to verify the correct midspan is used for exterior cameras. As seen in the specification sheet below, the AXIS P1344-E can operate down to -40°C when using high PoE.

Power AXIS P1343/P1344/P1346/P1347: 8-20 V DC or Power over Ethernet (PoE) IEEE 802.3af
AXIS P1343/P1344: max 6.4 W, PoE Class 2
AXIS P1346: max 9.6 W, PoE Class 3
AXIS P1347: max 9.0 W, PoE Class 3
AXIS P1343-E/P1344-E/P1346-E/P1347-E: PoE IEEE 802.3af max 12.95 W or High PoE max 25.5 W
Connectors RJ-45 10BASE-T/100BASE-TX PoE; 3.5 mm mic/line in, 3.5 mm line out; terminal blocks for power, 1 alarm input and 1 output
Edge storage SD/SDHC memory card slot (card not included)
Support for recording to network share (network-attached storage or file server) available in firmware version 5.40 and up
Operating conditions AXIS P1343/P1344/P1346/P1347: Humidity 20-80% RH (non-condensing); 0°C to 50°C (32°F to 122°F)
AXIS P1343-E/P1344-E/P1346-E/P1347-E: -30°C to 50°C (-22°F to 122°F) with PoE; down to -40°C (-40°F) with High PoE


Figure 4. The table shows that when high PoE is used, the AXIS P1344-E can operate at temperatures down to -40°C.





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