white paper Power Over Ethernet: Final approval of the Power over Ethernet (PoE) standard in June, 2003 removed a significant barrier to widespread adoption of IP telephony. The standard, IEEE 802.3af, specifies how to supply power to compliant devices over Ethernet LAN cabling. Armed with uninterruptible power supplies (UPS), IT staff can now deploy Voice over IP (VoIP) telephones without worrying about them not functioning during a power outage. This, of course, is a well-tested idea. Public telephone companies have been using a similar strategy for over a century to ensure reliable service to their customers. The benefits of this enabling technology are not limited to IP telephony, however. Many applications that deploy equipment with low power requirements in areas that aren’t already wired for power will benefit from reduced cabling costs. A single standard Ethernet cable will now be able to provide both power and communication services to wireless network access points, IP cameras, security badge readers, and many other devices. A list of the advantages PoE confers includes: • Reduced cabling costs. One set of wires provides both power and data. • Reliability. A UPS at the power distribution source can guarantee power to all connected devices. • A worldwide power standard—the first. This means that PoE devices will plug in anywhere. • SNMP network management. This system can be used to monitor and control connected devices, including resetting them and shutting them down remotely. • Simplification of wireless LAN installation. Access points can be moved and wired in easily. Technology Overview The 802.3af standard defines two new hardware components. The first, power sourcing equipment (PSE), is responsible for supplying up to 15.4 watts of power (at 48 volts) to the second, called the powered device or PD. The two types of PSEs, DTE and midspan PSEs (also called midspan controllers), are discussed below. New Life for VoIP 2 DTE PSEs DTE PSEs are PoE-compliant Ethernet switches and are sometimes called end-span PSEs. They use pins 1/2 and 3/6 of the eight-pin Ethernet cable for both data and power. This powering scheme is sometimes called Alternative A (Figure 1). Note that pairs 1/2 and 3/6 can be of either polarity. Figure 1. Alternative A Midspan Controllers Midspan controllers, like the ADC Midspan PoE Controller Panel (see page 4), are used on LANs that don’t have PoE-compliant switches. Sometimes called midspan injectors, these devices stand between a legacy Ethernet switch and a PD and supply the PD with power (Figure 2). These devices use a powering scheme called Alternative B, which means they use pairs 4/5 and 7/8 (Figure 3). Again pairs 4/5 and 7/8 can be of either polarity. Midspan controllers cannot be used on 1000Base-T LANs. Figure 2. Midspan Controller Configuration UPS Midspan Power over Ethernet Controller Ethernet Switch VoIP Phone Wireless LAN Access Point Network Camera 3 Figure 3. Alternative B Powered Equipment (PD) Because DTE PSEs and Midspan PSEs place power on different pins, the 802.3af standard requires that powered equipment be engineered to accept power from either alternative. To power non-compliant equipment, devices called splitters can be installed. Splitters separate the DC voltage and data into different outputs that the non-compliant device can use. Detection of Compliant Devices To avoid damaging devices that aren’t PoE compliant, the standard specifies a method of detecting PoE compliance before significant voltage is placed on the cable. It works like this: The PSE applies a small current-limited voltage to check for the presence of a 25k ohm impedance in the PD. Only if the PSE detects this impedance is the full 48V applied. Limitations The standard specifies that the current delivered to each PD be limited to 350 milliamps and the total continuous power to 12.95 watts—factoring in power loss over the cable. This is insufficient for power-hungry equipment, such as servers, printers, and routers. Buyer Beware Prior to the finalization of 802.3af, aggressive vendors were developing PoE products based on where they thought the standard was heading. Because of some last minute changes to the specification, these products may not be in compliance. Buyers will have to make their own judgments regarding these premature offerings. Products developed after the June 2003 approval of 802.3af, however, are likely to comply. ADC Midspan Controller Panel Figure 4. ADC Midspan Controller Panel The ADC Midspan Controller Panel is a rack mounted, fully 802.3af compliant midspan PSE that can power between 1 and 24 PoE devices. It offers the flexibility of universal input power (115/230VAC, 50/60Hz) and multiple configurations. Configuration Optional SNMP Module 24-Port Modules Yes 16-Port Modules Yes 8-Port Module Yes Table 1: Midspan Controller Panel Configurations The Midspan Controller Panel’s powerful management features include: • Front panel LEDs that display information on individual port detection and powering • An optional CPU module that enables SNMP management and complies with RFC3621 • Management control of up to five chassis using a single IP address ADC Telecommunications, Inc., P.O. Box 1101, Minneapolis, Minnesota USA 55440-1101 Specifications published here are current as of the date of publication of this document. Because we are continuously improving our products, ADC reserves the right to change specifications without prior notice. At any time, you may verify product specifications by contacting our headquarters office in Minneapolis. ADC Telecommunications, Inc. views its patent portfolio as an important corporate asset and vigorously enforces its patents. Products or features contained herein may be covered by one or more U.S. or foreign patents. An Equal Opportunity Employer 1290249 4/04 Original © 2004 ADC Telecommunications, Inc. All Rights Reserved Web Site: www.adc.com From North America, Call Toll Free: 1-800-366-3891 • Outside of North America: +1-952-938-8080 Fax: +1-952-917-3237 For a listing of ADC’s global sales office locations, please refer to our web site. . midspan PSEs (also called midspan controllers), are discussed below. New Life for VoIP 2 DTE PSEs DTE PSEs are PoE-compliant Ethernet switches and are. Overview The 802.3af standard defines two new hardware components. The first, power sourcing equipment (PSE), is responsible for supplying up to 15.4 watts of