PoE and PoE+, an article to understand Power over Ethernet

Jacktang

PoE and PoE+, an article to understand Power over Ethernet [Copy link]

By combining power delivery and communications over a single Cat3 or Cat5 cable, engineers can quickly build Ethernet networks at a lower cost and with less maintenance than would be possible with separate systems. Not surprisingly, the technology has been quickly adopted and formalized under Institute of Electrical and Electronics Engineers (IEEE) standards. Known as Power over Ethernet (PoE), the technology's main advantages are simplicity and the availability of power wherever there is a data outlet.

This article introduces PoE and the higher-power PoE+, outlining the standards, describing the components, powered devices, power supply equipment, "midspan" and "endspan" Ethernet switches and splitters, and describing a simple system.

Early PoE

PoE was originally developed to solve the problem of powering Voice over Internet Protocol (VoIP) phones. Traditional phones are powered directly from the copper wires that carry voice calls. However, the increasingly popular VoIP phones do not connect to these conventional circuits, but instead make calls over Ethernet cables on a company's local area network (LAN). Ethernet cables do not carry power, so VoIP phones must be plugged into a power source via an adapter. It was an imperfect solution, and if there was a power outage in the building, the phones would not work.

In 2000, telecommunications equipment supplier Cisco became the first company to emulate traditional phone systems by introducing proprietary technology that enabled Ethernet cables to provide 48 VDC power to VoIP phones (Figure 1). However, the real explosion of PoE came in 2001 and 2002, when other manufacturers, particularly wireless access point manufacturers, adopted the technology.

Figure 1: Cisco VoIP Phone with PoE (Image courtesy of Cisco).

The technology eventually caught the attention of the IEEE, which had been tasked with developing the “Ethernet Standard” (IEEE 802.3) as early as 1983. The organization felt it was important to create a standard version of PoE so that any manufacturer could make their products “PoE-enabled.” The work was assigned to a working subcommittee within the IEEE 802.3 Ethernet committee and was named “802.3af.” By June 2003, the working subcommittee approved the IEEE 802.3af PoE standard. In 2009, a second standard, IEEE 802.3at, was approved, which defined a similar technology but could handle higher power.

What’s Defined in the Standard?

IEEE 802.3af details a PoE technology designed to deliver up to 15.4 W DC power (minimum 44 VDC and 350 mA) to each device. (Due to losses in the cable, only 12.95 W is guaranteed to be delivered to the powered device.)
The technology uses a single standard RJ45 connector and Cat5 (or even Cat3) cable to handle tens of watts of power. After an Ethernet network is installed for communications, it can also be used to deliver power, saving materials, labor, installation time, and ongoing maintenance costs.

Because a typical 10 to 100 Mbps physical layer requires only two of the four twisted pairs in a Cat5 cable, power can be delivered over the unused conductors of the Ethernet cable (this technique is called "Option B" in the IEEE standard). Power can also be delivered over the data conductors of the cable by applying a common-mode voltage to each twisted pair. Because Ethernet uses differential signaling, this does not interfere with the data transmission of the cable ("Option A" in the standard).

IEEE 802.3af defines two types of PoE equipment: power sourcing equipment (PSE) and powered devices (PD). The PSE draws power from its own regular power source and then manages the power delivered to the PD over the Ethernet cabling network, which can draw the power it needs through the RJ45 connector, eliminating the need for an internal power supply. PoE is capable of powering PDs over typical Ethernet cable runs up to 100 meters in length. PD devices include original VoIP phones and wireless access points, security cameras, cash register (PoS) terminals, temperature control systems, and even in-flight entertainment systems.

In addition to standardizing the existing alternate pair and common mode data pair power delivery practices, the IEEE PoE standard also specifies signaling between the PSE and PD. This signaling allows the PSE to detect compliant devices and avoid damage to non-PoE devices connected to the network. The PSE and PD "negotiate" the required or available power. To detect a PD, the PSE applies a DC voltage of 2.8 to 10 V across the wire. The PSE then determines if there is a connected PD by measuring the loop current. The PD should present a resistive load of 19 to 27 kΩ and be marked with a shunt capacitor of no more than 120 nF.

Figure 2 shows the schematic of a PSE powering a PD.

Figure 2: Typical PoE application (Image courtesy of Texas Instruments).

Enhanced Standards

Although PoE can deliver about 13 W of power to a PD, some devices may benefit from more power (for example, cameras with pan, tilt, and zoom [PTZ] capabilities). To meet the needs of these products, a second standard, IEEE 802.3at, was introduced in 2009. Also known as "PoE+," this technology can deliver up to 25.5 W of DC power to a PD. The PSE provides 50 to 57 VDC, while PoE technology provides 44 to 57 VDC. PoE+ increases the current to 600 mA, compared to 350 mA for earlier technologies.

PoE+ uses only Cat5 cables (which have eight wires inside compared to four wires for Cat3), which reduces possible impedance opportunities and reduces power dissipation. PoE+ also provides more capabilities for network administrators, such as providing new remote power diagnostics, status reporting, and PD power management capabilities (including remote power cycling for embedded devices).

Finally, PoE+ provides dynamic power allocation, optimized power distribution, and good power utilization, thereby improving system efficiency and reducing costs.

Table 1 compares PoE ( IEEE 802.3af ) and PoE+ ( IEEE 802.3at ).

[td] property IEEE 802.3af IEEE 802.3at Type 2 PD Available Power 12.95 in 25.50 W Maximum power provided by PSE 15.40 W 34.20 W Voltage range (PSE side) 44.0-57.0 V 50.0-57.0 V Voltage range (PD side) 37.0-57.0 V 42.5-57.0 V Maximum current 350 mA 600 mA Power Management Three power levels Four power levels Supported cables Category 3 and Category 5 Category 5 and above Supported modes Options A and B Options A and B

Table 1: Comparison between PoE and PoE+.

Endspan and Midspan

PSEs can be implemented as endspans (a PoE-enabled Ethernet switch) or as midspans (a powered hub that works in conjunction with a non-powered Ethernet switch already installed on the network). PDs can receive power equally effectively whether they are endspans or midspans.

Endspans directly power the device. Depending on the specification, endspans can use spare or data pairs in the cable, which can also be used for Gigabit Ethernet transmission. Endspans require a PoE-enabled switch and are therefore usually specified for new installations that require brand new equipment. Midspans use an

intermediate powered patch panel or "injector" that is placed between the existing Ethernet switch and the PD. Midspans are usually located adjacent to the switch and are therefore considered a PSE, allowing the cable to be run unimpeded to the remote device. The specification only allows midspans to use spare pairs in the cable; therefore, they cannot be used to deliver power over data lines, such as Gigabit Ethernet connections.

There are many midspan injectors available that can be added to traditional Ethernet networks. Laird Technologies offers the POE-48I power supply for this application. The power supply automatically ranges at the input to provide a regulated output. The device can be used with any device that complies with the IEEE 802.3af standard. The POE-48I has one port that can provide 48 V at 500 mA, with a maximum power of 24 W.

Microsemi Analog Mixed Signal Group offers a single-port midspan power supply that complies with the high-power IEEE 802.3at standard, the PowerDsine 9001GR. The 9001GR produces up to 30 W at 55 V, providing remote power for a range of new applications including PTZ cameras and videophones. The device is backward compatible with IEEE 802.3af and can power existing 10/100Base-T network equipment and emerging wireless 1000Base-T equipment such as WiMAX and IEEE 802.11n wireless access points. Figure 3 illustrates a typical application.

Figure 3: Application example of the PowerDsine 9001GR midspan Ethernet switch.

There are also a number of multi-port midspan devices on the market. Phihong offers 8, 16, and 24-port midspans. The POE370U is a 24-port midspan injector that complies with the IEEE802.3af standard. Each port can provide 15.4 W of power without the need for additional power management. The device provides detection, disconnection, and overload protection, and is available with a 1 U rack-mount kit.

In addition, midspan vendors can also provide "splitters." These devices use a PoE input that is split into two outputs: data and power. The power can be redirected to the end device through more traditional means such as DC cables. The splitter acts as an intermediary device between the compliant PSE and the non-compliant PD.

Laird Technology offers the POE-12S-AFI active PoE splitter, which can accept PoE power from any IEEE 802.3af router or power supply. The device has overload and short-circuit protection, and immediately shuts off the power when a short circuit is detected without causing any damage to the PoE system.

Choosing an Endspan or Midspan PoE Network

Endspans are more expensive to implement and only support PDs defined by the IEEE standard, but there are some good reasons to choose this technology. For example, if an engineer is replacing all old switches, or if it is a new installation, endspan switches are preferred.

Another reason to use endspans is to avoid the extra patch cords and extra work that comes with connecting a regular Ethernet switch to a midspan PoE hub. In addition, replacing one device with two doubles the potential points of failure and makes the administrator's job more complicated for companies that insist on using separate IP addresses for each device on the network.

If an engineer chooses endspan switches, they should be aware that many endspan devices can only provide a maximum of 200 W, so a 24-port switch can only provide a maximum of 8.3 W per port, which is less than the 15.4 W maximum power specified in the PoE standard.

If an engineer has relatively new Ethernet switches, midspan switches are preferred because the cost of replacing switches just to gain PoE functionality is high. However, engineers must ensure that the switch manufacturer is happy for the network to support midspan products and that there is enough space to accommodate them.

From an electrical perspective, midspan switches are the preferred choice if network engineers want to use nonstandard voltages (such as 24, 12, and 5 V) for specific PDs and want to maximize the available power per port. Midspan switches are also a useful choice for networks that contain many legacy devices and PoE devices with their own power supplies. Midspan

switches have longer warranties than endspans (one year versus two years, respectively) and support both standard and proprietary applications.

Figure 4 shows a network built using endspan and midspan switches.

Figure 4: Schematic diagram of a network using endspan and midspan switches.

Traditionally, PSEs have been discrete circuits divided into the communication interface between the power source and the Ethernet network, and the power source itself. However, to make implementation easier, silicon vendors have introduced integrated PSE controllers to optimize the operation of PoE+ installations.

These controllers reduce the complexity and number of external components required for PoE and PoE+ devices by combining the interface circuitry with a linear regulator or switching power supply that converts the power voltage to 50 to 57 VDC suitable for Ethernet cables.

What's Next? - IEEE 802.3bt

In summary, PoE has been a popular technology since the early 2000s, especially for commercial and industrial applications. The technology is relatively simple to implement, especially for new installations, and the introduction of midspan switches has eased the challenges of adding PoE to legacy networks. The introduction of PoE+ (IEEE 802.3at) increased the power available to PDs, leading to the introduction of new applications that were previously too power-hungry to operate using older technologies.

Since IEEE 802.3at was ratified in 2009, PoE technology has continued to evolve as engineers are keen to add new PDs that require more power. To supplement IEEE 802.3at, many proprietary PoE protocols have emerged on the market with different names that can provide up to 60 W to 95 W of power, such as:

• UPoE (Cisco)
• LTPoE (Linear Tech/Analog Devices)
• PoH (MicroSemi/Microchip)
• PoE++ (Industrial)
• 4PPoE (Industry)

Note that these technologies are not compliant with current standards and are not interoperable, which could be harmful to incompatible hardware. However, they do set a precedent for higher power versions of the standard to be developed. In

late 2018, a new PoE standard, IEEE 802.3bt, was ratified, which allows 90 W of power to be delivered from the PSE, of which 71.3 W can be delivered to the PD.

Table 2 summarizes the PSE transmit power capabilities and PD receive power capabilities for all standards:

[td] standard PSE minimum output power Guaranteed minimum PD input power Ethernet Cable Categories Ethernet cable length Power over Ethernet cable pair IEEE 802.3af 15.4 In 12.95 in Cat3 100 m 2 pairs IEEE 802.3at 30 W 25.5 in Cat5 100 m 2 pairs IEEE 802.3bt 60 W 51 W - (60 W) Cat5e 100 m 2-4 pairs IEEE 802.3bt 90 W Cat5e 71.3 W - (90 W) 100 m 4 pairs

Table 2: PoE power supply capabilities of all standards (Image courtesy of Microchip)

Note 1: If the cable length is 2 to 5 meters, the extended power capability for PD input can reach 60 W and 90 W.

In addition to being able to transmit more power over Ethernet cables, the new PoE IEEE 802.3bt standard defines many other new features/improvements compared to the previous IEEE 802.3af and IEEE 802.3at standards, including:

• Supports two PD structures: single-signature PD and dual-signature PD
• Works over the four twisted pairs of Ethernet cable
• Automatic classification function
• Extended power capability for known cable lengths
• Low standby power support (short MPS)
• Support 2.5G-BaseT, 5G-BaseT, 10G-BaseT
• Backward compatible with IEEE 802.3at/af

达普POE交换机模组

Why are many POE switches on the market very cheap? Because it is a single-chip microcomputer. Single-chip microcomputer: integrated chip! Widely applicable. The single-chip microcomputer in the POE switch is a fake POE switch. There is no POE chip inside. The program is written to imitate the regular POE chip to run the protocol! There is no corresponding hardware support. It saves a lot of materials, so it is cheap. The following are the problems that single-chip microcomputers are prone to 1. The microcontroller is prone to crashing and burning MOS tubes! 2. The microcontroller often restarts itself during operation! 3. The microcontroller is not compatible! 4. The microcontroller has poor load capacity and cannot carry enough equipment! 5. The microcontroller cannot recognize the af/at protocol and can only provide a very poor power supply protection! Conventional detection of whether the POE switch is a microcontroller can be done with an oscilloscope or a multimeter!