What to consider when using

Ethernet for Power distribution

Power over EthernetInside or Outside the Switch?

IEEE802.3af

Independent View

PowerDsine invented and owns thepatents for the intellectual property forPower Over Ethernet (PoE). PowerDsinesupplies its PoE chipsets to most of theswitch manufacturers in the VoIP,Wireless, IP security and RFID marketplace. PowerDsine also manufacturesthe widest range of power injectors calledMidspans. Whichever way you decide toimplement PoE, you will most likelybecome a user of PowerDsine products.This puts us in a unique position toprovide an objective guide for when touse PoE integrated into your Switch orexternally via a power injector called aPoE Midspan.

New IEEE Standard Powers the Future

You have no need for concern.PowerDsine invented PoE in 1997 andthe first power injector ( Midspan) wasinstalled in 1998. Many manufacturespartnered with PowerDsine to make useof this innovation. The Institute ofElectrical and Electronic Engineers (IEEE)was approached to form an internationalstandard to facilitate wide spreaddeployment of the technology.

In June of 2003 the PoE specificationbecame the IEEE 802.3af standarddefining the delivery of safe power overstandard Ethernet cables, Cat5 and

Cat5E. PowerDsine assisted the industryfurther by providing a service to Ethernetdevice manufacturers to testconformance with the new standard.

To date, over three hundred terminaldevices have been compliance testedby PowerDsine, including most of theleading voice over IP telephones, wirelessaccess points and IP security cameras.Today the University of New Hampshire,being vendor independent, is the officialconformance testing body appointed bythe IEEE.

Typical Network Installation

Power over Ethernet integrates data and operating power (15W, 48V) into a single Ethernetcable. The technology allows IP telephones, Wireless LAN Access Points, Network Cameras andother enterprise terminals, to safely receive power over standard Category 5 and Category 5Ecabling, without any modification to existing infrastructure.

Ethernet Switch

PowerDsinePower over EthernetMidspan

NetworkCamera

BluetoothAccess Point

VoIP Phone Wireless LANAccess Point

IEEE 802.11a/b

Why Power over Ethernet (PoE)?

Manage the RiskTo understand the risk here are somestats reported for North America but aregenerally applicable to any location.The average number of power outagessufficient to cause IT system malfunctionper year at a typical site is 15.

• 90% of the outages are less than fiveminutes in duration.

• 99% of the outages are less thanone hour in duration.

• Total cumulative outage duration isapproximately 100 minutes per year.

Based on the theoretical availability andthe power protection strategy to achievefive nines or higher, Power over Ethernetsources must be backed up by a UPS.system with a minimum of one hourbattery life. This will protect the vastmajority of systems..

Whether you consider running IP telephonyacross your existing data network, orextending the reach of the network withstrategically placed wireless accesspoints, or just adding a physical securitylayer to your premises with IP cameras,the risk of possible failure is increasedby having remote devices that need ACpower connections. Losing data during apower outage is one thing, but losingdata, voice and the company security issomething else entirely.

A common goal is to consistently achieve99.999% availability with 5.3 minutes ofdowntime per year or less. By connectinga UPS to a PoE source in thecommunications room, the entire network,including the remotely located resources,is capable of continuous operation duringa power outage. Most UPS’s also providefor surge protection which is normallyunavailable for remotely connecteddevices, the disruption created by powersurges is more common than that createdby power outages.

For IP telephony, scalability is important;for wireless and security systems, thelocation is crucial. Understanding oneexample of the installation costs willhighlight the benefits to be gained fromPoE in each scenario.

If you are installing a Wireless Accesspoint, you tend to put it in ceiling or highup on a wall out of reach of the generalpublic. This area is usually void of ACoutlets, therefore to power the unitstraditionally an AC contractor is neededto install an outlet. In a private area,average costs are in the region of $200dependent on country. In public areas,

there is a health and safety issueassociated with AC power provisioning,which usually means that the power mustbe channelled into the wall or suppliedthrough metal conduit. This can increasethe costs by a factor of 10. The alternativeis to run an Ethernet cable from theWireless Access point to the Switch orRouter. It is very simple, at this location,to connect to a PoE port on the Switchor to use a one port power injector. Eitherway no AC contactor is needed and worstcase scenario you are looking at $50 forthe PoE port.

You save money, installation is quickerand easier and there is no disruptionwhile holes are cut into walls. Is theresomething useful you can do with themoney you saved on the installation?Buy a UPS.

How’s the Economics?

1. Understand the Options for Delivering PoE

IEEE802.3af allows for switches toprovide power on the Data Pair orthe Spare Pair. Most switches selectthe Data Pair. If you are consideringseparate Power Injectors calledMidspans, the standard states theymust use the spare pairs. Thereforeyou should check your cablinginstallation to see if you have allpairs or only data pairs. Note thatmost Midspans cannot be used ona Gigabit connection because thereare no Spare Pairs in a Gigabitinstallation. This is why PowerDsinehas the 6000G range to supportGigabit connections.

An Ethernet cable connected to aPoE source will not carry power if noend device is connected. The IEEE802.3af standard requires that thesource first tests the connecteddevice for compliance to the standardbefore enabling power. After passingthe signature test for compliance, asecond test is carried out. This testis to determine the amount of powerrequired at the remote location. Thisinformation is recorded by the powersource as the Class of PoE.

Pre-standard Cisco powering methodis different from IEEE802.3af in thepolarity of the power on the connectorand the signature of the end device. So if these devices have to besupported, you will need to knowhow the cable connection can rectifythe polarity of the power and howthe power source, Midspan or Switch,can recognize the Cisco signature.

Pre-Standard capacitive test. Thereare still legacy devices in the marketthat were early adopters of PoE andimplemented a pre-standardsignature based on capacitivesignature, which is different obviouslyfrom today’s resistive test.

Devices that do not conform to thePoE specifications can still benefitwith the use of a Splitter. The Splitteris mounted next to the remote deviceand receives the PoE Ethernet cableac input. It then splits the out putinto a standard Ethernet dataconnection and a separate DC powerconnection.

2. Understand the Power Requirements ofAttached Devices

IP Camera

10-12 watts

Wireless LANAccess Point

8-12 watts

Video IP Phone

10-12 watts

IP Phone

3-5 watts

Class PSE (W)

0 15.41 42 73 15.44 As Class 0

According to the IEEE 802.3af standard, the amount ofpower available after 100 meters of Cat5 or cat 5E cable,is up to 12.95 watts. See below the typical powerrequirements of the most common IP devices. The powerconsumption of each IP device can be found at thetechnical specifications of the manufacturer’s data sheets.IEEE802.3af power is 15.4 watts at the power sourceand 12.95watts at the Powered Device. However part ofthe standard’s specification involves testing the class ofpower, which refers to the specific power requirement ofthe end device.As an example, a 7 watt device is Class 2. Manufacturersmarketing departments are using this as a tool insteadof saying their Switch only supplies 7 watts they say theirSwitch is a Class 2 PoE conformant device. This is aproblem if Wireless AP, Video IP phones or many securitycameras should be powered, as these devices requiremore power than 7W (Class 2).The standard mechanism when a device requires morepower than can be delivered is to disable the power. Thesolution from the Switch manufacturers is to provideadditional external power supplies.

Note: when installing PoE switches with class 2 PoE, youshould check the cost and sizing of adding external powersupplies before deciding whether to have internal orexternal PoE capabilities.

You do not have to worry about powering non compliantdevices ,such as Printers, Fax machines or PC’s becausethese devices do not carry the IEEE802.3af signature,thus the ports will automatically disable power. Howeverif you are using integrated PoE on a 24 port networkingblade and many of the ports are supporting non powereddevices, it might be more prudent to use a Midspan andonly connect powered ports to the devices that need it.This will lower the overall cost of the installation.

3. Caution regarding Full Power & Forced Power

The question of using managed poweror full power has to be considered.Full power is where the power deliveredby all ports simultaneously is 15.4watts. This looks like an attractiveoption, however the issue to consideris the actual power requirements inan organization. If the installationmainly consists of IP phones withpower consumption of 3-5 watts perphone, the overall power consumptionwill be in the range of 120W. Thus,putting a 24 port PoE injector in thecomputer room delivering 400 wattsis false economy. All systemsinstallation teams have to calculatethe BTU’s and Airflow requirementsin the computer room to ensure theair conditioning systems can performadequately. A better solution is to usepower injectors that use the Class ofPower information to manage thepower delivery to the ports that requirehigher levels. Power managementallows the automation of efficientpower distribution from a powerinjectors with smaller power supplies.

Some PoE injector manufacturers areusing the term Forced Power. This iswhere they leave the power oncontinuously. This is outside thestandard. The IEEE802.3af requiresthat power is removed within 47 milli-seconds of disconnect. The reason isthat an engineer might be swappingcables at a ‘cross connect panel’ andby accident connect a powered portinto a PC connection. Today, PC’scannot be powered using PoE thus48volts can burn out the Ethernet portat the computer side and it also runsthe risk of damaging Switch ports.

In order to avoid EMI from noisegenerated by the power source, it isimportant to conform to the cablespecifications of Cat5 and Cat 5E.Among the crucial tests is crosstalk.Devices that do not conform to thesespecifications will generate excessiveerrors into the data path. The worstcase of cross talk we have measuredto date from an uncertified powerinjector is taking a 10mbps link anddropping it to 1mbps.

There is a defined limit to the currentthat can be sent down a pair of twistedwires. Currently the ITA specificationis 175milliamps on one pair of cables.According to the IEEE802.3afspecification for delivering the requiredpower, it is 350 milliamps on two pairs.PoE is running the cable at its maximumallowed capacity. Therefore there is areal need for effective current protectionto prevent failing end devices fromdrawing too much current and causingcable faults.

4. Cable Specifications Often Missed

5. Evaluate Internal Power or Separate Midspan Units

Here are some of the questions toconsider when evaluating how to deployPoE:

How old are your Switches and do theyhave the features such as QOS that fityour requirements? Midspans weredesigned to be used with any switchand save on the expense and timeinvolved in doing an upgrade to theswitch to implement a PoE solution.Midspans can be installed while thenetwork is live with zero down time.

Will the proposed Switch have thecorrect power level to support alldevices, or are additional external powersupplies needed? What are the costsof these external PSU’s? How muchrack space do they take? Can they beredeployed if you change your supplier?

What is your history of upgrades?Companies keep their networkingswitches for 3-10 years.

Over the last 5 years how manyadaptors did you swap out of the switchfor new ones, most probably, with newfeatures? If you are doing 2–3upgrades, then you will buy power 2–3times if it is integral to the switch.Compare the purchase cost (over 5years) between integrated PoE andexternal Midspans which are designedto be part of the cabling scheme andlast for 10 years.

How much rack space does theSwitch’s external PSU take? Midspansconsume rack space. 48-port Midspanfrom PowerDsine takes 1U.

In some cases, integrated PoE will bea better strategy for powering terminalswith low power consumption.However, when an external powersupply is required for higher powerlevels, a Midspan solution might beyour cost effective choice.

SNMP remote network management tool

Enterprise users might need to account for power usage by the different departmentsinside the company. Therefore consideration should be given to the ability of the PoEsystem to monitor power consumption of the remote devices. Also on power failure,when the UPS kicks in, the management could use a priority scheme to ensure thatas the power outage continues, the power is directed to key resources. Thismanagement of failed power distribution could enable cost savings in the size of UPSneeded for any given scenario. This will extend the life of the UPS support and it isa non linear function. If you halve the load on a UPS, it doesn’t last twice as long,but more like 3-4 times as long. Better value and increased business continuity.

6. Managed or Unmanaged PoE?

PoE Midspans can be SNMP managedor unmanaged. The areas of relevanceto most enterprises are powerconsumption and the ability to remotelycontrol Power On – Power Off.Management data and controlfunctions need to be secured againstunauthorized personnel. A hackerintruding into your network could usethe power to disable all peripheral PoEdevices.

Wireless Access Points and SecurityCameras tend to be installed out ofreach of the public. This means theyare also not very accessible to

engineers. The ability to remotelyperform a power on reset could beextremely useful.

IPT requires that all IP phones bepowered. SNMP management mightbe useful as the controlling tool forapplications that can power telephoneson and off when the relevant staff haspassed security into the building andpowering the phones off when theyleave. It might be interesting for energyconservation to use batch files toenable telephones during work hoursand disabling telephones in theevenings, weekends and holidays.

PoE is a must. Savings in power outletinstallation costs, the speed ofinstallation and the lack of disruptionsto office environments are goodjustifications for PoE Midspan. Theuse of a central UPS’s in conjunctionwith a Midspan to deliver the UPSservice to remote locations is a betterjustification for PoE.

Use the Integrated PoE supplied withthe switch unless you need higherpower than its standard PSU candeliver. Use a Midspan for all additionalpower and high power requirements.

Choose units that conform to the IEEEstandard and legacy support whereneeded.

Ensure that they conform to EMI andXtalk specification and have currentprotection. The biggest singleinvestment in your networkinfrastructure is the cable plant so youshould protect it.

It is recommended to use powermanagement SNMP v3 for securecontrol, monitoring and managementof power distribution. It allows you tocontrol an orderly shut down ofconnected devices, and optimize thepower backup of the UPS in the system.

7. Conclusions

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The information contained herein is believed to be accurate and reliable at the time of printing. However, PowerDsine Ltd. cannot assume responsibility for inadvertenterrors, inaccuracies, omissions, or subsequent changes. In the interest of continuous product improvement, and in view of our commitment to enhance quality and reliability,PowerDsine Ltd. reserves the right to make changes to products and to their specifications at any time. The company is also committed to keeping the standards of theIEEE 802.3af. It is suggested that before embarking on any product design based on details in this document, the latest and most current information should be obtainedfrom PowerDsine. No rights to any PowerDsine Ltd. intellectual property are licensed to any third party either directly, by implication or by any other method. PowerDsineis a trademark of PowerDsine. All other trademarks are the property of their respective owners. EUG 05/06 © 2006 PowerDsine Ltd.