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The standard, IEEE 802.3af, specifies how to supply power to compliant devices over Ethernet LAN cabling.. A single standard Ethernet cable will now be able to provide both power and com

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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

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

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

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

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.