PoE Promises Simplified Infrastructure

Looking for centralized, redundant power for your wireless APs, access-card readers and IP telephone? Then you're ready for Power over Ethernet.

April 23, 2004

9 Min Read
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Deploying 802.3af PoE on a small scale can be as simple as dropping in a new line card or fixed form-factor switch. Most equipment vendors are planning to ship PoE-enabled line cards and fixed LAN switches, if they haven't done so already. But to deploy PoE enterprisewide you must upgrade your network infrastructure, and you may have to change your physical power infrastructure.

What 802.3af PoE Does

802.3af defines two main elements, the PSE (power sourcing equipment) and the PD (powered device). PSEs, such as Ethernet LAN switches and midspan power injectors, contribute power to the network. PDs, on the other hand, which include wireless access points and video cameras, receive power from it. PDs aren't just networking devices: Any appliance with an RJ-45 connector could conceivably draw power from the network, including a cell phone or a PDA.

Today's 802.3af-based devices offer interoperability and can draw up to 15 watts of power from the network. They have built-in safety mechanisms to protect nonpowered devices like PCs, laptops and printers from stray voltage from an inline power injector.

An 802.3af-compliant PSE typically puts 48 volts on the line. Some of that voltage gets lost along the way because of the resistance of the thin copper wires in your Ethernet cabling plant, resulting in 13.95 watts of usable power at the end device.How 802.3af PoE Works

Before 802.3af, some LAN equipment vendors used proprietary power injectors to insert power into an Ethernet cable. Although this approach worked, some injectors posed a real hazard to wired equipment. For example, one product we tested in our NETWORK COMPUTING Green Bay, Wis., Real-World Labs came with large yellow warning stickers for the powered Ethernet cable. If you plugged that "hot" Ethernet cable into a PC or laptop, the network interface could suffer electrical damage, or even worse, the hot cable could fry the entire machine or start a fire.

Luckily, 802.3af remedied this safety issue. 802.3af detects PoE-capable devices and puts power on the cable only if there's a compatible device on the other end of the line. If the device on the opposite end is not PoE-capable, the port behaves just like a normal LAN switch port. No sparks or smoke.

Power it UpClick to Enlarge

Power can be delivered to an 802.3af-compliant PD over active data lines or, in the case of 100-Mbps Ethernet, over the two unused pairs in the cable. This method requires new 802.3af-compliant Ethernet switching hardware that can put power on the line. Alternatively, power may be supplied to the PD using midspan power injection, whereby powered patch panels inject power onto the unused pairs in a 10/100 cable. "Power It Up," left, illustrates these three methods.

Before 802.3af, many vendors recommended midspan devices because they provide PoE without requiring changes to the LAN switching equipment. The downside is that midspan devices can add power only to unused cable pairs. In a network with all four pairs in use, you can't use a midspan injector.In fact, only an Ethernet 10/100 network will support midspan power injection. So if you're using midspan technology and want to deploy Gigabit Ethernet to the desktop, you'll need to run a separate gigabit infrastructure for your data devices.Contrary to popular belief, the hundreds of 802.3af ports in your wiring closet won't generate enough heat to burn down the door. Most heat is generated at the PD, where the power is used. The remaining power is converted to heat resulting from resistance in the Ethernet cabling, and this small loss gets dissipated across the wire.

PoE can raise the temperature a few degrees because power loss occurs each time electricity is converted from one form to another. Power supplies convert, or "rectify," standard AC wall power into DC power the switch needs to operate. There's always some inefficiency in this process, which produces heat in the wiring closet. Your wiring closet shouldn't start to feel like a chicken roaster, but if it does, take a close look at the efficiency of your vendor's power supply.

PoE-enabled switches require more power than non-PoE equipment because each PoE port must be able to support 15 watts of power. Keep in mind that only a fraction of that power will be disappated in the wiring closet. this fraction is based on the efficiency of the power supplies in the PSE.

Managing the available power in your wiring closet will be more of a challenge than the heat. Each energy-generating port adds 15.4 watts to the total power required, known as the power budget. While this may seem trivial, it adds up quickly. A modest wiring closet supporting 100 PoE users must deliver up to 1,540 watts of power simultaneously to those users. Add this to the 1,000 watts the average switch requires, then do the math: Power (watts) = Current (amps) x Voltage (volts). A switch requiring 2,540 watts of power will need 23-amp service.

That spells a problem for many wiring closets. In most buildings, electricity is rated for only 15- or 20-amp service. Modern LAN switches support 240 to 360 ports in a single chassis, requiring 35- to 50-amp service to support PoE fully on each port. Although some wiring closets may have multiple 15- to 20-amp circuits, these circuits typically are deployed to deliver fault-tolerant power.With a large 802.af installation, your power budget is a big concern. Existing buildings offer few wiring choices. You can run new electrical lines to your wiring closets, but this is expensive and often unfeasible. Another method is to collapse your wiring closets back to the data center, but Ethernet's distance limitations may prevent this.

Alternatively, you can try to manage your power budget by using devices that support 802.3af's optional "power classification" feature. Many 802.3af phones, for example, support this feature. The last remaining option is to deploy PoE in conjunction with standard wall power at the PD, powering only those devices where AC power isn't conveniently available.

If you're constructing a facility with PoE, make sure multiple 20-amp circuits and/or 220-volt service are available in every wiring closet. In addition, consider deploying high-capacity DC power, which has several advantages over standard AC wall power. It can be converted more efficiently into usable power for your infrastructure, which means better capacity and less heat. DC power also can be set up for centralized fault tolerance, using large banks of DC batteries to store electricity. This eliminates the need for UPSs in data wiring closets, which cuts down on heat and saves on per-closet infrastructure costs.

Power classification is an 802.3af option that gives standards-compliant PDs the opportunity to tell the network how much power they need. This optional part of the standard defines four classes of power that the device can request. Class Zero, the default class, requests full power from the PSE. Class 1 requests no more than 4 watts from the PSE. Class 2 requests no more than 7 watts. Lastly, Class 3 requests full power and at least 7 watts. A fifth class is reserved for future use.

Here's an idea of how much power different devices require: A CDMA2000 cell phone uses 3.6 watts, an 802.11b AP needs 6 watts, a trimode AP needs 12 watts and an IP telephone needs 7 to 10 watts.Power-classification technology can help stretch your wiring closet power budget by leaving more power in the switch for additional devices. A switch is not required to power up all ports simultaneously. If it runs out of power budget, it can refuse to power up a device.

Devices that support power classification let the switch dole out power more granularly. A device without power classification must be supplied with a full 15.4 watts, whereas one with power classification can request a lesser amount.

Cisco takes power classification a step further, using CDP (Cisco Discovery Protocol) for its devices to specify their exact power requirements after initial power has been placed on the line. Although its approach is proprietary, Cisco can power more devices with the same power budget--a big plus if you have limited power in your wiring closet.

Whether you go with 802.af's approach or a proprietary one like Cisco's, power classification lets you support more devices on the same switch simultaneously without adding more power in the wiring closet. The catch: Not all PSEs and PDs support power classification.

Warts and AllPoE promises a simplified infrastructure for your WLAN, VoIP and building security. It also lets you collapse your power network to the data center so you can centralize your power distribution and add redundancy. That's crucial for ensuring that your mission-critical voice system and building security, for instance, are always available.

But 802.af may entail more than buying new switches. It can mean making a strategic investment in an electrical power infrastructure. When planning new construction, provision for PoE by investing in DC power or high-capacity AC infrastructure (such as 220-volt, high-current service) for your wiring closets. If you're retrofitting an existing building with PoE, be sure you understand the features and limitations of your infrastructure provider's PoE solution. And if retooling your existing power infrastructure isn't feasible, decide which end stations really need PoE and which can still get by with an ugly wall wart.

Joel Conover, a former senior technology editor of NETWORK COMPUTING is principal analyst for enterprise infrastructure at competitive intelligence firm Current Analysis. Write to him at [email protected]. Some savvy questions to ask your 802.3af vendor:

  • Does your PSE support power classification?

  • Do your PDs support power classification?

  • What's the maximum wattage the system can deliver to a set of PDs?

  • Can the PSE power all ports simultaneously, or is the PSE limited by its power supply?

  • How much power will the PSE draw from the wall?

  • Does the PSE support a DC power option?

  • Does the PSE deal gracefully with a loss of power resulting from a failed power supply?

  • Does the PSE use internal power supplies or external power shelves to supply power to the line cards?

  • How efficient are the AC power supplies in converting power to DC?

  • Will you need special power provisions in your wiring closet to support the PoE switch port density you require?

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