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White Paper: Power Over Ethernet Technology


What is the advantage of a PoE device?

A Power over Ethernet (PoE) configuration can be found in common applications such as VoIP phones, IP cameras, POS, lighting controllers, industrial devices, wall clocks, physical security devices, and controllers, where minimizing the amount of cables while delivering power is necessary. PoE is commonly found in such applications because it eliminates the need for a data cable and a power cable, by combining them both into one cable.

What is an example of a PoE device?

A powered device (PD) such as an IP camera must receive power from a power sourcing equipment (PSE) in order to operate. IP cameras must also capture frames and send the data to a computer to process it. Two types of PoE configurations include a PD connected to an endpoint PSE or a midspan PSE. The RCO-3000/6000 rugged embedded PC, by the ODM/OEM hardware manufacturer Premio Inc., can be used as a midspan PSE; Premio also manufactures a 10.1” Panel PC that can be used as a PD.

Are there any standards for PoE devices?

The IEEE has developed techniques for transmitting power over Ethernet cabling – these are called “standards” and there are two of them:  Alternative A and Alternative B. When using 10Base-T and 100Base-TX cabling, only two of the four data/signal pairs in typical CAT-5 cable are used, whereas 1000BaseT uses all four pairs. Alternative A and Alternative B both use end point PSE. The only difference is that Alternative B power is supplied on the unused twisted pairs to form a LAN interface and the data signal has its own set of twisted pairs. The unused pairs terminate on pins 4, 5 and 7, 8 of the RJ45. On the other hand, Alternative A shares power with the data twisted pairs. These pairs terminate on pairs 1, 2 and 3, 6 of the RJ45.

How does a midspan PSE work?

Another configuration is that of a midspan PSE. Midspan Alternative B encompasses a midspan PSE in order to supply the power over the unused twisted pair. The only difference is that the midspan PSE sits in between the switch and the PD in order to provide an Ethernet connection to the internet but the switch does not provide power to the PD. Instead, a midspan PSE does the job. This method is also called an inline PSE, and a common application of this is when the switch cannot provide enough power to the PD.

It is entirely possible to have Alternative A and Alternative B PoE operational for the same device, with one acting as a backup for the other to provide seamless failover. It is also possible for this arrangement to cause problems with incompatible signaling. To aid in this power resilience, the PD must be capable of sustaining a voltage of up to 57V as well as be able to cope with polarity reversing on the LAN interface (e.g., the MDI/MDI-X scenario).

What kind of switches supports PoE?

There are two types of switches that can provide PoE: one is a ‘guarantee per port’ and the other operates with a ‘total power budget.’ A switch that guarantees certain wattage per port means that you can be sure that no matter how many Class 3 or Class 0 devices are plugged in the switch is able to power them.

A switch that has a total power budget can only power as many PoE devices as it can supply. For example, a 4-port switch that carries a total budget of 30 watts can power four Class 2 cameras (4 x devices x 7.5 watts = 30 watts), and even more with the RCO-3000-4P which includes an optional AC/DC power adaptor of 120 watts.

Allocating approximately 30 watts for the motherboard and installed components provides around 90 watts total power budget without taking into account operating temperature and power supply derating due to operating temperature. This is enough to power four Class 3 devices (4 x devices x 15.4 watts = 61.6 watts). Furthermore, this provides an allowance of 28.4 watts remaining in the system.

Because Premio’s RCO-3000-4P includes an optional AC/DC power adaptor of 60 watts, it can put aside the same 30 watts for the motherboard and components resulting in a total power budget of 30 watts. This is enough to power one class 4 device (1 device x 15.4 watts = 15.4 watts) or four class 2 devices (4 x devices x 7.5 watts = 30 watts).

The 802.3af/at IEEE standards

The IEEE 802.3af standard takes approximately 48VDC and up to 350mA so that power initiated from the PSE does not exceed 15.4 watts. The power that is delivered to the PD does not exceed 12.95 watts and is supported by category 3, 5, 5e, and 6 cables.There are currently two mainstream IEEE standards for PoE: 802.3af and 802.3at. There is a new upcoming standard (802.3bt) that is also covered later in this article. These IEEE standards provide a means to power a PD as well as to communicate with a LAN interface, receiving the power from a LAN switch (endpoint PSE) or an inline alternative (midspan PSE).

Today, PoE panel PCs or motorized pantiltzoom cameras require more than 12.95 watts. In September 2005, IEEE released 802.3at which allows a higher communication standard for PoE. The 802.3at standard is also backwards compatible with the legacy lower-powered PD.

There is a limit to how much power that can be drawn across the 24AWG wires before electrical damage occurs due to overheating within connectors and cable bundles, as well as signal interference issues. This means that multiple pairs may need to be used to deliver power. Presently, the 802.3at standard limits the number of pairs that can carry power to two.

The 802.3bt IEEE standard

With increasing technology and devices that require more power, IEEE is creating an 802.3bt standard. The main objective of the 802.3bt standard is to comply with the extra low-voltage safety requirements, per ISO/IEC 60950. This means that the power cannot exceed 100 watts per port.

Data can be delivered over four twisted pairs or two twisted pairs. The pairs terminate connections over wires 1, 2, 3 and 6, but it can also be delivered over wires 4, 5, 6, and 7 similar to 802.3 af/at.

The 802.3bt standard PSE identifies the PD type and sets the power. Both architectures can be supported as it allows PoE to power more applications. When possible, dual signature PD enables the load to work with two pair of PSEs. For a surveillance camera built with a dual signature PD, one pair can be connected to the camera and the other pair can be connected to the heater or PTZ mechanism. The 802.3bt standard also introduces classes 5–8 (see chart above).

With the current 802.3at standard, the port requires a minimum power signature of 10mA with a duty cycle of 20% to keep it alive (e.g., consumption/waste of 2 mA average). At a typical voltage of 50 V, this amounts to 100 mW; and when multiplied by the number of ports, it results in a significant waste of power.

The 802.3bt standard improved this parameter by reducing the duty cycle to 1.875%, which results in less than 10 mW consumed to keep the port alive. This is ten times better than existing solutions. The 802.3bt standard also has auto class, which allows the PSE to determine the actual maximum power drawn by the connected PD. The PSE implementing auto class measures the power consumption of the PD throughout a defined period of time. During this time, the PD consumes the maximum power it ever needs. PSE can set the maximum power output based on the power drawn during auto class plus a margin. Normally, there is some dissipation on the cables so power at the PSE is not available at the input of a PD.

The 802.3bt standard also uses extended power. Typically in PoE applications there are power losses and cable resistance due to the distance between the PSE and PD. The idea behind extended power follows this approach:  if a PSE is sourcing a 90 watt output, the PD should expect only 71 watts and then 19 watts are “saved” for dissipation on the cables in cases where the PD is deployed 100 m away from the PSE. If the PSE and PD are in close proximity, dissipation should be less.

The extended power feature allows the PD (or the PSE) to use the maximum available power based on the cable’s true total resistance. Once the PD measures the cable resistance, it can calculate the power that is lost on the cable and claim the worst-case reserve that the PD then uses. At worst, the power reserve is the same as in the current standard, but it can also be much lower. This feature is currently supported by the Power over HDBaseT (PoH) standard created by HDBaseT Alliance, which is also backwards compatible with PoE and extends the maximum power deliverable to 95 watts.

Furthermore, the 802.3bt standard is backwards compatible if the PSE is capable of supplying the PD.

To read the full white paper, click here!