Power Over Ethernet
Published on : Tuesday 04-08-2020
Abhijit Mandal clears some of the most common misconceptions associated with PoE – a recently adopted technology – that is keeping many people away from it.
With development in various technologies, the energy footprint of many electronic equipment has reduced drastically. Illumination devices are one bunch where there has been a significant drop in energy footprint over the years, starting from incandescent lamps to fluorescent lamps to compact fluorescent lamp to the present LED lamps. A lot of small charging devices also have become transformer-less now a days resulting in a very small energy footprint. And with this the feasibility of powering equipment over Ethernet has increased and opened new horizons. This list will be endless, and we are looking at devices with a power consumption of typically below 60 Watts.
What is Power over Ethernet?
Power over Ethernet (PoE) is a technology that lets network cables carry electrical power. Let me quickly explain this using an example of a CCTV camera. As we can see that a normal IP camera would have a network cable and a separate power cable, however a PoE IP camera uses the pairs in the network cable to draw the power instead of having a separate power cable
How does it work?
Network cables, such as Cat 5e and Cat 6, comprise eight wires arranged as four twisted pairs. In 10 and 100BASE-T Ethernet, two of these pairs are used for sending information, and these are known as the data pairs. The other two pairs are unused and are referred to as the spare pairs (Gigabit Ethernet uses all four pairs). Because electrical currents flow in a loop, two conductors are required to deliver power over a cable. PoE treats each pair as a single conductor and can use either the two data pairs or the two spare pairs to carry electrical current. Power over Ethernet is injected onto the cable at a voltage between 44- and 57-volts DC, and typically 48 volts is used. This relatively high voltage allows efficient power transfer along the cable, while still being low enough to be regarded as safe.
This voltage is safe for users, but it can still damage equipment that has not been designed to receive PoE. Therefore, before a PoE switch or midspan (known as a PSE, for power sourcing equipment) can enable power to a connected IP camera or other equipment (known as a PD, for powered device), it must perform a signature detection process.
Signature detection uses a lower voltage to detect a characteristic signature of IEEE-compatible PDs (a 25kOhm resistance). Once this signature has been detected, the PSE knows that higher voltages can be safely applied. Classification follows the signature detection stage and is an optional process. If a PD displays a classification signature, it lets the PSE know how much power it requires to operate, as one of three power classes. This means that PSEs with a limited total power budget can allocate it effectively. Some of the PoE power classes are as follows:
The differences between power delivered by the PSE and power received by the PD account for power that is lost as heat in the cable. If a PD does not display a signature, it is class 0 and must be allocated the maximum 12.95 Watts. PoE Plus equipment has a power class of 4. If a regular 802.3af PoE source detects this class it will simply enable power as if it was a class 0 device. However, an 802.3at PSE will not only recognise the PD as a PoE Plus device, it will also repeat the classification stage, as a signal to the PD that is connected to a power source with full PoE Plus power available. (In theory the PD should also be able to request the extra power by communicating across the network link.) PoE Plus PSEs can supply up to 30 Watts and available device power is 25.5 Watts.
The final stage after detection and classification of a newly connected device is to enable power: the 48V supply is connected to the cable by the PSE so the PD can operate. Once enabled, the PSE continues to monitor how much electrical current it is delivering to the PD and will cut the power to the cable if too much, or not enough, power is drawn. This protects the PSE against overload and ensures that PoE is disconnected from the cable if the PD is unplugged. The powering capabilities over Ethernet has increased over time since 2002 and so have the standards. This increased capacity has also increased the horizon of equipment that can be powered over Ethernet and has been shown graphically in the illustration.
Why Use PoE?
Time and cost savings
-No need of having electrical power cabling.
-Network cables do not require a qualified electrician to fit them and can be located anywhere.
Flexibility
-Without being tethered to an electrical outlet, devices such as IP cameras and Wi Fi points can be located wherever they are needed most and repositioned easily if required.
Safety
-PoE delivery is intelligent, and designed to protect network equipment from overload, underpowering, or incorrect installation.
-The voltages involved are low and so is the probability of an electrical shock.
Reliability
-PoE power comes from a central and universally compatible source, rather than a collection of distributed wall adapters.
-It can be backed-up by an uninterruptible power supply or controlled to easily disable or reset devices.
Scalability
-Having power available on the network means that installation and distribution of network connections is simple and effective.
Myths and misconceptions
PoE is a recently developed technology, and many people are put off adopting it by the raft of conflicting or out-of-date information that is available on the subject. Here are the most common misconceptions:
1. PoE has compatibility problems. Not so. It is true that the early days of PoE, many home-brewed and proprietary schemes were employed to get power over network cables. However, the IEEE 802.3af standard has gained universal adoption as PoE's popularity has spread, meaning that compatibility between all modern PoE equipment is assured.
2. PoE requires electrical knowledge. Again, early ad-hoc implementations may have required careful design, but IEEE 802.3af PoE is designed to ensure reliable operation in any configuration that would be possible with regular Ethernet. All the user must do is wire up the network as normal, and the equipment will take care of power delivery.
3. PoE requires special wiring. Not at all, the same cabling – Cat 5e, Cat 6, etc., – and ‘RJ45’ –style connectors are used for both regular and PoE-enabled local area networks.
4. Power is forced into devices. This misconception is surprisingly common; however, it is important to remember that power ratings quoted by manufacturers are upper limits and are not fixed. Plugging a 5-Watt camera into a 15-Watt injector does not result in 10 Watts of power being lost somewhere; the camera will simply draw as much electrical power as it needs.
Conclusion
With artificial intelligence and IoT being integrated into day to day systems, PoE would be the logical progression in the development towards systems with smarter and integrated controls. The adoption of power and data over Ethernet cable is inevitable and is going to only increase in various horizons for times to come.
Abhijit Mandal has masters in Power System from IIT, Roorkee. He has an industry experience of over 2 decades and his expertise is in the field of Engineering and Quality Assurance. He has been professionally associated with top conglomerates, like NTPC Ltd and Reliance Group. Currently, he is heading the Electrical Design Engineering & Quality Department of Reliance.
