Power-over-Ethernet (PoE) enables the use of a single Ethernet cable for simultaneous delivery of power and data, eliminating a significant number of electrical receptacles and circuits. The benefits include simplified infrastructure management, reduced power consumption and operational costs, improved safety, greater flexibility regarding placement of devices, and higher reliability (with less infrastructure there is less opportunity for error). However, a lack of understanding of how and where power is sourced and used in a PoE environment may result in unanticipated downtime. This can be caused by insufficient available power or inefficiency due to improperly sized cooling in the equipment room.
Understanding the basics of PoE and the implications of its use promotes favorable decisions concerning electrical and mechanical system design for spaces where this equipment may be installed.
PoE Components, Terminology, and Capabilities
Before delving too deeply into specific power and cooling needs for PoE applications, one must first understand the basic required components for PoE applications and the typical types of devices that take advantage of its benefits.
Figure 1 illustrates the components and basic connectivity found in every PoE application.
In the broadest sense, there are two primary types of PoE devices: those that provide and integrate power onto the data cabling, and the end devices that use this supplied power, as summarized in Table 1.
A PoE injector is the Power Sourcing Equipment (PSE) that supplies power into the powered device (PD). PSEs generally fall into two categories: mid-span and end-span. A mid-span PSE is a unit that injects power downstream, after the network switch. It is typically connected in-line, between the network switch and the patch panel, adding power to the line. The other type of PSE is the end-span, which injects the power, along with the data stream, at the network switch itself. Although this document specifically addresses the power and cooling requirements for PoE switches (end-span PSEs), the same concepts and results apply equally where mid-span PSEs are utilized.
Figure 2 ? Typical Mid-span PoE injectors Originally, typical applications for PoE included IP phones and wireless access points since the IEEE PoE Standard (IEEE Std. 802.3af) permitted a maximum of 15.4 Watts of power delivered over the twisted-pair data cabling. However, the drive to achieve greater energy efficiency, integrate building systems, and reduce costs created demand for devices that require more power. In October 2009, the IEEE approved the ?PoE+? Standard (IEEE Std. 802.3at). This allows up to 30 Watts of power delivered over four pairs of data cabling (category 5 or better). PoE+ supports IP-based security camera and card access readers, radio-frequency identification (RFID) tag readers, bar code scanners, print servers, and Building Automation System (BAS) devices. With the onset of PoE+, a whole spectrum of additional, higher-powered devices become possible ? some still emerging.
Collectively, these standards define four ?classes? of devices based on their specific power needs. The following table shows the various classes with maximum power output at the PSE and maximum power input at the PD:
Although the specific capabilities vary, for the purposes of this document ?PoE? and ?PoE+? are used synonymously.
PoE Power Requirements and Heat Dissipation
For most types of electrical and IT equipment and traditional data network switches, all of the ?work? produced by the equipment takes place at the device itself. Since the electronic circuits within the equipment do not operate at the same voltage as the power being supplied, the device?s power supply converts the incoming power from AC to DC, then filters and transforms it to the voltages necessary for the function of the devices. Some of the energy is lost at the power supply (typically 10-15% of the incoming power). The remainder is used by the internal circuitry of the device. The most common way this energy is expended is in the form of heat. It may also be realized in the form of light (as in a CRT or LCD display) or motion (like in a spinning hard drive platter) ? although this is typically just a small fraction of the total energy consumed. This is why it is reasonable to determine the equipment room cooling requirement based on the total power consumption of the equipment within the room.
Such is NOT the case for PoE applications. Assuming the use of PoE switches, power supplied to the switch is not only used for the operation of the switch itself, but it is also used as a source of power to the end PoE device. This device is typically located well outside the equipment room. See Figure 4.
Now, working from the end PoE device back to the equipment room, we will examine where and how power, and therefore heat, is used for PoE applications.
Power at the End PoE Device or PD
The actual power used by any given PoE device can be roughly determined by the class of the device and whether it can operate using standard PoE or requires PoE+ power levels. Power requirements can also be determined from manufacturer product datasheets. Where specific devices have not yet been chosen, values based on the device type or worst case values (15.4 or 30 Watts, depending on whether the device uses PoE or PoE+) may be used for the sourced power. Table 3 ? Individual Powered Device (PD) Power Demands.
Power in the Network Cable
Note that in table 2, the power delivered to the PoE device is less than the power sourced at the PSE. As current flows in the cable, a voltage drop occurs due to its resistance. The result is a loss of energy, in the form of heat, over the length of the data cable.
The IEEE 802.3af standard takes this voltage drop into account. As a result, powered devices (PDs) have a wider operating voltage range (36-57 V DC) than the PSE supply range (44-57 V DC). In a worst-case scenario, when the PSE is at its minimum voltage of 44 V, up to 7 V can be dropped along the length of the cable before the voltage at the PD is below its minimum operating range.
Assuming a network cable is at its maximum length of 100 m (328 feet), and it has a worst-case resistance of 20 Ohms, then the voltage drop along the cable would be 7 V and the associated power loss in that cable would be 2.45 W (see Figure 5).
Power at the PSE or PoE switch
A power supply is required within the PSE to rectify, transform, and condition the incoming utility power to voltages defined in the PoE standards. This power supply is certainly not 100% efficient, so some energy is lost as heat at the PoE power supply. There is also a nominal amount of additional internal circuitry within the PoE switch associated with superimposing the power onto the appropriate data cable pairs. As a result, typically only about 10-15% of the total additional power required to support PoE devices is dissipated as heat at the PSE. The remainder is dissipated in transporting the power to the end device and at the PoE device itself.
Some data network equipment vendors, such as Cisco Systems, provide excellent web-based power and heat calculator tools available to assist the designer with properly determining actual requirements for power (circuits, phasing, sizing) and heat (BTU/hr) based on actual system configuration and the PoE devices to be used. At a minimum, the results tend to represent worst case operating conditions, which is reasonable for system design purposes. Where such tools are not available, a conservative estimate would be to assume that not more than 15 ? 20% of the PSE power is dissipated as heat within the equipment room.
Because it is not easy to make changes to electrical and mechanical infrastructure after initial installation, it is recommended that reasonable worst-case information be used when sizing these systems. This requires participation and input from the end user to ensure that reasonable assumptions are made and infrastructure systems are sized properly.
The benefits of PoE ? cost savings, flexibility, and simplicity ? are compelling. Some types of PoE devices, such as IP phones and wireless access points, are becoming ubiquitous in enterprise and commercial office environments. The variety and types of devices is continually increasing, particularly since the adoption of the PoE+ standard by the IEEE. Nevertheless, it cannot be denied that PoE applications increase the demand for power and cooling in the IT equipment rooms. The calculations for the additional power are relatively straightforward, particularly where vendor provided tools are available to assist the designer. However, the vast majority of the heat associated with this additional power draw is not dissipated in the IT equipment room where the power is sourced, but at the end PoE device and the network cabling.
If available, the data from the vendor-provided-tools that include information for heat dissipation should be used. Otherwise, the equipment room cooling requirement to support PoE can be conservatively estimated using worst-case quantities for network and PoE devices (assuming 15-20% of the additional power for PoE is dissipated as heat). This requires collaboration between the designer and end user to ensure reasonable values are used. Following these guidelines will result in mechanical and electrical infrastructure capable of satisfying day one and future needs without additional, unnecessary, and often cascading expenses due to unintentionally overdesigned infrastructure systems.
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