Power-over-Ethernet

Power over Ethernet, or PoE, is a technology that allows you to transfer electrical energy to a remote device along with data through a standard twisted pair in an Ethernet network. This technology is intended for IP telephony, wireless access points, web cameras, network hubs and other devices to which it is undesirable or impossible to draw a separate electrical cable.

Power over Ethernet is standardized according to the IEEE 802.3af-2003 standard. There are several variants of this technology prior to this standard, but they are not widespread.

According to the IEEE 802.3af-2003 standard (see 802.3-2005), a DC current with a rated voltage of 48 V (min = 36 V, max = 57 V) is provided through two pairs of conductors in a four-pair cable, with a maximum current of 400 mA to provide a maximum power of 12.95 W. The standard defines 5 classes of devices powered by PoE: 0-4. Each class has its own power and current parameters. The most common is 1st grade. For it, the input current is 120 mA, and the power can vary from 0.44 to 3.84 watts. Class 4 is not used and is reserved for the future.

The technology uses phantom power to transfer power. Data in Ethernet standards is transmitted as a potential difference between conductors in the same pair (for example, between wires 1 and 2 or 3 and 6). Supply voltage is supplied as potential difference between pairs of conductors (for example, between pairs 1-2 and 3-6)

The standard does not specify which pairs should be powered as well as its polarity, so devices designed to receive PoE power should be designed to use pairs 12-36 or 45-78 to obtain, and the correct polarity should be obtained using a diode bridge. This technology works with the existing cable system, including Category 5 cables, without the need for any modifications to the existing SCS.

History

2023: Internet cable introduced with record data rate of 5Gbps

On December 4, 2023, the South Korean industrial corporation LS Cable & System announced the development of a new Power over Ethernet (PoE) cable with the highest bandwidth in the world, reaching 5 Gbps. A product called SimpleWide 2.0 can be used to connect CCTV cameras, audiovisual equipment, digital signage, etc. Read more here.

PoE equipment and operating principle

PoE technology does not affect the quality of data transmission. To implement it, the properties of the Ethernet physical layer are used:

• Use of high frequency transformers at both ends of the line with a central tap from the windings. The constant supply voltage is supplied to the central taps of the secondary windings of these transformers, and is also removed from the central taps on the receiving side. The use of central taps of signal transformers makes it possible, without mutual influence, to transmit high-frequency data and constant supply voltage over one pair of wires.

• Use free pairs to connect power. Modern Ethernet cable networks that comply with the 100Base-TX standard consist of four pairs, two of which are not involved.
  

Accordingly, there are two options for powering the PSE device:

• A - to twisted pairs on which data
  * B goes
  - to unused pairs in the cable
  

PSE devices (injectors) differ, depending on variants A or B, while splitters, that is, PD devices, are universal. The PD device must be able to receive power in any version, including when the polarity changes (for example, when a computer-to-computer crossover cable is used). It is important that the PSE only supplies power to the cable if the device to be connected is of the PD type. Thus, equipment that does not support the PoE standard and is accidentally connected to the PSE device will not be disabled. In fact, no standard Ethernet equipment, starting with the 10Base-T specification, can be disabled by a certain difference in potentials between pairs, since the standard provides for mandatory galvanic isolation. The procedure for supplying and disconnecting power to the cable consists of several stages.

Connection Determination Step

The connection determining step is to determine whether the device connected at the opposite end of the cable is a PD type device. In this step, the PSE applies a voltage of 2.8 to 10 B to the cable and determines the input resistance parameters of the connected device. For a device, PD is a resistance of 19 to 26.5 kOhm with a capacitor of 0 to 150 nF connected in parallel. Only after checking the correspondence of the input resistance parameters for the PD device, the PSE device proceeds to the next step - the classification step, otherwise the PSE again, after a time of at least 2 ms, proceeds to the "connection determination" step.

Classification stage

After the connection determination step, the PSE may further perform the classification step. The classifying step serves to determine the power range that the PD device can consume to then monitor that power. Each PD device, depending on the declared power consumption, will be assigned a class from 0 to 4. The maximum power range is class 0. Class 4 is reserved as a standard for further development. The PSE device can de-energize the cable if the PD device began to consume more power than what it announced during the classification step. Classification is carried out by introducing a voltage from 14.5 V to 20.5 V into the cable by the PSE device and measuring the current in the line.

Full Voltage Step

After the determination and classification steps, the PSE supplies the 48V voltage to the cable with a rising edge no faster than 400 ms. After full voltage is applied to the PD device, the PSE device monitors its operation in two ways.

1) If the PD device consumes less than 5 mA for 400 ms, the PSE device removes power from the cable.

2) The PSE supplies a voltage of 500 Hz with an amplitude of 1.9-5.0 V to the cable and calculates the input resistance. If this resistance is greater than 1980 kOhm for 400 ms, the PSE takes power off the cable.

In addition, the PSE continuously monitors the overload current. If the PD device consumes more than 400 mA of current for 75 ms, the PSE device will de-energize the cable.

Disconnection stage

When the PSE detects that the PD has been disconnected from the cable or the PD has overloaded its current consumption, the cable is de-energized for at least 500 ms.

Benefits of PoE Technology

Ethernet power technology is an attractive alternative way to power network devices. Moreover, its use is possible both in the organization of new networks and in the modernization of existing ones. Most often, when upgrading the network, it is necessary to install active equipment exactly where there is no nearby power source and electrical outlets. Thanks to the IEEE 802.3af standard, it becomes possible to install equipment in the most suitable places for this, despite the lack of wiring. For example, you can install a Wi-Fi access point at the best signal location, even if there are no electrical outlets, or place an IP camera in a view-friendly location. PoE not only significantly saves on the cost of power cables and other components, but also reduces the time it takes to install Ethernet equipment.