Editor’s note: I am pleased to have Victor Chertakovsky back on Planet Analog. Below, he is sharing a good tutorial and refresher on Power over Ethernet (PoE). I think this is a good topic of discussion especially in light of the many industry demands for the standardization of higher power PoE.
What is Power over Ethernet?
Power over Ethernet (PoE) is a technology that allows network cables to carry electrical power. With PoE, power can be supplied either by a PoE switch or from a PoE injector (a small power supply placed between a switch and a powered PoE device).
For example, a digital security camera or an indoor/outdoor home security health care device normally requires two connections to be made when they are installed:
- A network connection, in order to be able to communicate with video recording and alarm signal acquisition systems
- A power connection, to deliver the electrical power the camera or other security devices need to operate.
However, if this equipment is PoE-enabled, only the network connection needs to be made, as it will receive its electrical power from this cable as well.
Why use POE?
Specifying Power over Ethernet brings many advantages to an installation:
- Time and cost savings are achieved by reducing the time and expense of having electrical power cabling installed. Network cables do not require a qualified electrician to fit them, and can be located almost anywhere.
- Flexibility : Without being tethered to an electrical outlet, devices such as IP cameras, wireless access points and indoor/outdoor home security & health care devices 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, under-powering, or incorrect installation.
- 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.
Devices that use Power over Ethernet
POE has many applications, but the three key areas are:
- VoIP phones : This is the original PoE application. Using PoE means phones have a single connection to a wall socket, and can be remotely powered down, just like with the older analog systems.
- IP cameras : PoE is now ubiquitous on networked surveillance cameras, where it enables fast deployment and easy repositioning.
- Wireless WiFi and Bluetooth APs and RFID readers are commonly PoE-compatible, to allow remote location away from AC outlets, and relocation following site surveys.
The main advantage of using PoE for security device powering is for powering of Home Security & Health Care Devices. Usually, inside and outside the house, Home Security & Health Care Devices are powered from batteries. The ability to replace batteries by PoE makes it possible for the user to save a great deal of cost on Alkaline Batteries and prevent the waste of sending dozens of used batteries into our environment during the year. The PoE replacement will avoid the pollution caused by discarded Alkaline Batteries.
Besides the simplicity of reduced cabling and the convenience of eliminating the need for direct power in difficult locations, PoE offers some other benefits. Almost any device can occasionally require a restart to restore functionality. How many times have you had to climb a ladder or a flight of stairs to power-cycle a device? You’re not alone. With a full-featured PoE switch, an administrator can power-cycle a device from the switch’s built-in management interface.
It is cheaper and simpler to deploy Ethernet cable data points than to mount additional AC outlets, since a typical electrical installation of a single device is $200-$750 on average.
How to upgrade to PoE for Home Security Devices
Adding PoE to your network is straightforward, and there are two routes you can choose:
- A PoE switch is a network switch that has Power over Ethernet injection built-in. Simply connect other network devices to the switch as normal and the switch will detect whether they are PoE-compatible and enable power automatically.
- PoE switches are available to suit all applications, from low-cost unmanaged edge switches with a few ports, up to complex devices. One of the possible applications is the usage of PoE to power Home Security & Health Care Devices which are mounted inside and outside the house.
A Typical PoE Connection.
A midspan (or PoE injector) is used to add PoE capability to regular non-PoE network links. Midspans can be used to upgrade existing LAN installations to PoE, and provide a versatile solution where fewer PoE ports are required. Upgrading each network connection to PoE is as simple as patching it through the midspan, and as with PoE switches, power injection is controlled and automatic.
Midspans are available as multi-port rack-mounted units or low-cost single-port injectors like the home security equipment for indoor and outdoor applications.
It is also possible to upgrade powered devices, such as IP cameras, indoor and outdoor security devices to PoE by using a PoE splitter . The PoE splitter is patched into the network connection, and taps off the PoE power, which it converts into a lower voltage suitable for the camera and security device.
Demystifying PoE: Myths and Misconceptions
PoE is a fairly 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 and discrepancies:
- PoE has compatibility problems . Not so. It is true that in 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. <
- PoE requires electrical knowledge . Again, early Ethernet device 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 has to do is wire up the network as normal, and the equipment will take care of power delivery.
- 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.
- 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 or security/healthcare device into a 15 Watt injector does not result in 10 Watts of power being lost somewhere; the camera or other home security device will simply draw as much electrical power as it needs.
How does POE 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. 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.
With respect to power consumption, Home Security and Health Care Devices belong to the PoE Power Class 1 and 2.
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.
Practical Advice for PoE Upgrades.
PoE network is possible for distances more than 100m. It is possible to use simple routers instead of PoE adaptors.
Power over Ethernet technology permits the powering of network equipment through network cable that prevents user from mounting AC outlets in places that mounting of them is not convenient, as well as preventing the usage of bulky power supplies. Nominal voltage is 48V.
Some more myths about PoE usage:
- 220VAC does not influence PoE with regard to noise; there will be no interference and network noise.
- Fluorescent lamps switching also do not add noise to the PoE network.
- PoE networks can be near land line telephones, fire prevention systems and telemetric power lines.
- Power over Ethernet can be designed into a new network or as a retrofit to upgrade existing infrastructure. PoE is flexible, safe and reliable, and offers installers and customers a wide range of benefits. With all the exciting applications, PoE helps you address customers’ needs, while delivering a better, more manageable, and more flexible network.
What are your thoughts and comments about PoE? Please share your ideas with our readers for discussion.
Please be sure to visit the Power Electronics News Special Report on Power over Ethernet here.