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Power over ethernet is going to fundamentally change the way LANs are used in the next three to five years by substantially lowering WLAN installation costs and removing one of the key barriers to VOIP implementations.
The recent ratification of the POE standard will unleash a slew of new devices on IT managers. Accommodating these devices and applications will require strategic planning, but the benefits will likely be far-reaching.
POE, known more formally as the IEEEs 802.3af specification, defines a standard way for data terminal equipment to be powered over Media Dependent Interface, or MDI. The POE standard was published—the IEEEs equivalent of bestowing sainthood—in July, but a trickle of 802.3af devices started showing up well before then.
POE will have the most direct and immediate benefit in the areas of wireless LANs and voice over IP. The spec makes it possible for IT managers to confidently install WLAN APs (access points) and IP telephone handsets without regard to the manufacturer of the PSE (Power Sourcing Equipment). Of course, IP telephone handsets are still functionally tied to specific vendors. However, any midspan power injector will work, as we saw in eWEEK Labs interoperability tests with Alcatel Internetworking Inc.s E-reflex model 4210 and PowerDsine Ltd.s 6012.
The 802.3af spec can be used in both 10/100M-bps copper networks and over Gigabit Ethernet copper connections. (There is no way to conduct electricity through optic cables.) Power is supplied in one of two ways: over the spare wire pair (numbers 4, 5, 7 and 8) or “floated” over the data, with data and power transmitted on the same pair. Because PDs (powered devices) are required to accept power using either method, network managers likely wont need to be concerned with which method is used.
The exception is with Gigabit Ethernet, which makes use of the spare pair. However, this shouldnt be a concern for years to come because devices that generate gigabits of traffic will likely need far more power than can be supplied by 802.3af PSE.
POE Potential
POE Potential
Based on eWEEK labs work with currently available 10/ 100M-bps Ethernet-based devices—including VOIP handsets, network cameras and WLAN APs—we recommend that IT managers work POE into their two- to five-year plans for enhancing productivity.
During tests, we used WLAN APs from Hewlett-Packard Co., IP telephone handsets from Alcatel and network cameras from Axis Communications to test interoperability and to get hands-on experience using 802.3af-standard-compliant equipment.
We easily installed the devices by connecting a standard Ethernet patch cord to each. We installed an HP ProCurve 420 WLAN AP in just minutes, for example.
The cost advantages of POE became clear when we were able to place the HP AP in the most effective broadcast location—near the ceiling. We couldnt have done that if the AP had required AC power; we would have had to make a facility request to have an electrician rewire parts of our building to run power to the AP—a costly and time-consuming process.
Worries about the safety of POE appear unwarranted. We experienced no problems in our tests, and power has been running over Ethernet for some time (albeit not within the 802.3af standard).
One issue that has been brought up is the effect of POE on equipment that was never designed to accept electric power over the Ethernet interface. However, our tests showed that if a cable connected to the PSE is connected to a device that does not accept power, then the higher voltage associated with POE is not sent to the port. POE devices do send very low-voltage power to sense PDs, but this power is specifically set too low to harm or interfere with legacy devices.
Further regarding safety and equipment protection, the 802.3af spec calls for the voltage on the line to return to the sensing (or much lower-voltage) level within 400 milliseconds of a PD being disconnected from the PSE. After a disconnect, even if the RJ-45 connector is immediately replaced in the PD, the discovery and power classification negotiation must take place before power is resupplied to the port.
When a PD is connected to the PSE, a discovery and power classification negotiation occurs to provision power. This process, which takes 1 second or less, first discovers if the device is POE-capable. If a subsequent, optional negotiation takes place, the device can indicate one of four power classifications. Power classifications range from 0 (default, full 15.4 watts of power) to 4 (reserved for future use).
POE Planning
POE Planning
IT managers in or-ganizations large and small should start planning now to accommodate the changes POE will foment, both in the number of devices that will come on the data network and in the amount of electrical energy that LAN infrastructure equipment will consume.
Power reliability in outlying wiring closets is going to move from a nice-to-have checklist item to an infrastructure requirement similar to the resilient power required in data centers today. This will likely mean an extensive review of the UPSes (uninterruptible power supplies) currently installed in wiring closets. In some cases, these UPSes will need to increase in capacity and reliability (which usually means buying premium systems) to support POE.
Most Ethernet cable already installed in businesses around the country should support POE with no modification.
However, IT managers should start figuring out now how to provision wall power to POE PSE. One complicating factor is that not all devices—and especially not all VOIP telephone handsets—draw the same amount of power during normal use. For example, basic Cisco Systems Inc. VOIP telephone handsets typically use less than 2 watts of power while idle; they use the most electricity for the short period of time that they ring; and they use a more moderate amount of power during talk time. It is also likely that badge readers and fire sensors will use far less power than, for example, WLAN APs and network cameras.
These wide-ranging power needs will likely play a big role in determining how to provision PSE in the wiring closet. Extreme Networks Inc., Cisco and other equipment vendors have indicated that their chassis-based systems will allow a mix-and-match arrangement of powered and unpowered line cards. This is a good thing because the typical LAN is certain to have a mix of PDs and PCs, which will always need a separate AC power supply that cant be met by POE.
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Our testing and research showed a wide range of management features in PSE. Vendors are clearly thinking about managing power usage so that just enough power is reserved to support PDs.
However, there is still a long way to go in terms of managing PSE during a power outage.
None of the vendors we interviewed is currently working with UPS makers to handle selective shutdown of devices. Wed like to see much more policy-driven control of how PSE will behave in a power outage. For example, wed like to be able to specify that essential services, such as the phone at the front desk, are powered even if that means shutting down the wireless APs in all the conference rooms.
For now, IT managers will have to learn the quirks of each PSE. For example, PowerDsines 6012 prioritizes the first 10 ports over the next 10, and so on. All the vendors with which we spoke indicated that, with experience, more management capability would be added.
Senior Analyst Cameron Sturdevant can be contacted at [email protected].