Power and Cooling Implications of Power-over-Ethernet on Infrastructure Design
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.ethernet switch design 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.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.