Analysis of the key technologies of Power over Ethernet (POE)

Power over Ethernet (POE) Overview

POE (Power Over Ethernet) refers to the technology that can provide DC power to some IP-based terminals (such as IP phones, wireless LAN access points AP, network cameras, etc.) while transmitting data signals without making any changes to the existing Ethernet Cat.5 cabling infrastructure. POE technology can ensure the normal operation of the existing network while ensuring the safety of the existing structured cabling, minimizing costs.

POE is also known as the power supply system based on the local area network (POL, Power over LAN) or Active Ethernet (Active Ethernet), and is sometimes referred to as Ethernet power supply. This is the latest standard specification for transmitting data and electrical power simultaneously using existing standard Ethernet transmission cables, and maintains compatibility with existing Ethernet systems and users. The IEEE 802.3af standard is a new standard based on the Ethernet power supply system POE. It adds relevant standards for direct power supply through network cables on the basis of IEEE 802.3. It is an extension of the existing Ethernet standard and the first international standard for power distribution.

IEEE started to develop the standard in 1999, and the earliest participating manufacturers were 3Com, Intel, PowerDsine, Nortel, Mitel and National Semiconductor. However, the shortcomings of the standard have always restricted the expansion of the market. It was not until June 2003 that IEEE approved the 802.3af standard, which clearly defines the power detection and control matters in remote systems, and stipulates the way routers, switches and hubs supply power to devices such as IP phones, security systems and wireless LAN access points through Ethernet cables. The development of IEEE 802.3af includes the efforts of many company experts, which also allows the standard to be tested in various aspects.

A typical Ethernet power supply system is shown in Figure 1. Keep the Ethernet switch equipment in the wiring cabinet, and use a powered hub (Midspan HUB) to provide power to the twisted pair of the LAN. At the end of the twisted pair, the power is used to drive phones, wireless access points, cameras and other devices. To avoid power outages, a UPS can be selected.

Key technologies of POE

System composition and power supply characteristic parameters of POE

A complete POE system consists of two parts: power supply equipment (PSE) and power receiving equipment (PD). PSE equipment is a device that supplies power to Ethernet client devices and is also the manager of the entire POE Ethernet power supply process. The PD device is the PSE load that receives power, that is, the client device of the POE system, such as IP phones, network security cameras, APs, PDAs or mobile phone chargers and many other Ethernet devices (in fact, any device with a power of no more than 13W can obtain the corresponding power from the RJ45 socket). The two establish information about the connection status, device type, power consumption level, etc. of the power receiving device PD based on the IEEE 802.3af standard, and based on this, the PSE supplies power to the PD through Ethernet.

The main power supply characteristic parameters of the POE standard power supply system are:

◆The voltage is between 44 and 57V, and the typical value is 48V.
◆The maximum allowable current is 550mA and the maximum starting current is 500mA.
◆The typical working current is 10～350mA, and the overload detection current is 350～500mA.
◆Under no-load conditions, the maximum required current is 5mA.
◆Provide PD devices with five levels of power requests from 3.84 to 12.95W, with a maximum of no more than 13W. Working process of POE power supply When a PSE power supply device is deployed in a network, the working process of POE Ethernet power supply is as follows. ◆Detection: At the beginning, the PSE device outputs a very small voltage at the port until it detects that the connection at the cable terminal is a powered device that supports the IEEE 802.3af standard. ◆PD device classification: After detecting the powered device PD, the PSE device may classify the PD device and evaluate the power loss required by this PD device. ◆Start power supply: During a configurable startup period (generally less than 15μs), the PSE device starts to supply power to the PD device from a low voltage until a 48V DC power supply is provided. ◆ Power supply: Provide stable and reliable 48V DC power to PD devices, so that the power consumption of PD devices does not exceed 15.4W. ◆Power off: If the PD device is disconnected from the network, the PSE will quickly (generally within 300-400ms) stop powering the PD device and repeat the detection process to detect whether the terminal of the cable is connected to the PD device. When connecting any network device to the PSE, the PSE must first detect whether the device is a PD to ensure that current is not provided to Ethernet devices that do not meet the POE standard, as this may cause damage. This check is achieved by providing a small current-limited voltage to the cable to check whether the remote end has a characteristic resistance that meets the requirements. Only when the resistance is detected will the full 48V voltage be provided, but the current is still limited to prevent the terminal device from being in an incorrect state. As an extension of the discovery process, the PD can also classify the power supply method required by the PSE, which helps the PSE provide power in an efficient manner. Once the PSE starts to provide power, it will continuously monitor the PD current input. When the PD current consumption drops below the minimum value, such as when the device is unplugged or the PD device power consumption is overloaded, short-circuited, or exceeds the power supply load of the PSE, the PSE will disconnect the power supply and restart the detection process. The power supply device can also be provided with a system management capability, such as the application of Simple Network Management Protocol (SNMP). This function can provide functions such as night shutdown and remote restart. Studying the POE power supply method, it can be seen that there are two key issues to consider in the power supply process, one is the identification of the PD device, and the other is the capacity of the UPS in the system. The principle of POE power supply through cable The standard Category 5 network cable has four pairs of twisted pairs, but only two pairs are used in l0M BASE-T and 100M BASE-T. IEEE80 2.3af allows two usages as shown in Figures 2 and 3.

When the idle pin is used for power supply, the 4th and 5th pins are connected as positive poles, and the 7th and 8th pins are connected as negative poles.

When the data pin is used for power supply, the DC power supply is added to the midpoint of the transmission transformer, which does not affect the data transmission. In this way, the line pairs 1 and 2 and the line pairs 3 and 6 can be of any polarity.

The standard does not allow the above two situations to be applied at the same time. The power supply device PSE can only provide one usage, but the power application device PD must be able to adapt to both situations at the same time. The standard stipulates that the power supply is usually 48V and 13W. It is easier for the PD device to provide 48V to low voltage conversion, but at the same time, there should be an insulation safety voltage of 1500V.

The POE standard also specifies the unshielded twisted pair cable that should be used to transmit electrical power, that is, 3, 5, 5e or 6 category cables. It is clear that the existing cable facilities that work with it do not need any changes, including 3, 5, 5e or 6 category cables, various short-circuit wires and wiring boards, power socket leads and connected hardware. The POE standard is compatible with the IEEE 802.3 standard series.