Empowering Ethernet

Ethernet technology is widely used in pro AV for carrying audio and video, but increasingly it’s also being used to power devices at the end of the cable. Translation: If you haven’t already worked with Power-over-Ethernet (PoE) products, it’s probably time to learn how.

PoE’s growing popularity stems largely from its simplicity, which in turn reduces costs. Because PoE runs both power and AV traffic over the same cable – Cat3 or greater – there’s no need to bring in an electrician.

“The pros [advantages] are numerous on a custom installation job site,” says Dan Jackson, market and development engineer at Crestron. “The biggest is fewer number of cables and terminations to worry about because that creates so many benefits: shorter install time, lower cost, plenty of retrofit possibilities.”

One drawback is that hardware costs, particularly for head-end equipment, can be more expensive than non-PoE gear. But vendors and integrators agree that that premium is more than offset by savings on labour.

“In a public building like an airline terminal, the average electrical outlet costs $1,000 [€700] per installation,” says Keith Hopwood, vice president of marketing at Phihong, a U.S.-based company that makes PoE products. “Even at $20 [€14] per port [for PoE], you don’t need a calculator to do the payback analysis.”

For some installations, speed can be another advantage. For example, if the project involves installing PoE surveillance cameras in a department store or airport terminal, the client might require that the work be done outside of business hours. So by eliminating the step of pulling power lines, the integrator can finish the job faster and minimise overtime pay for working odd hours.

In other installations, PoE can address both safety issues and safety-related costs. For example, if parts of the installation are in areas that are wet, then PoE can save money if it means that the Cat5 cable can be safely run in an area where a conventional power line would be unsafe or at least require the additional expense of a conduit. Reduced safety concerns also can provide additional design flexibility. And if the device has to be moved later, doing so doesn’t require pulling a new power line, which also would be an additional expense.

Other benefits include aesthetics – no “wall wart” power supplies bulging out from the wall or ceiling near the device – and more power management options.

“PoE also allows the power for devices to be centrally managed, so you can enable and disable items remotely,” says Crestron’s Jackson. “It also makes UPS battery backup of devices much easier because you can drive just a single outlet and cover devices all over a building.”

Coming Up Short

PoE also has its share of caveats and disadvantages. Some are the fault not of the PoE standard but rather Ethernet itself, which was originally designed for IT rather than AV applications.

“The biggest con [drawback] to date is not PoE but the distance specification of Ethernet,” says Phihong’s Hopwood. “Ethernet is limited by spec to 100 meters. There are several extenders now appearing on the market to solve this problem.”

Ironically enough, power can be another shortcoming. Although the current PoE standard – IEEE 802.3af – supports 15.4W of DC power, some of that juice is lost along the way, so vendors build their products around the assumption that only 12.95W will be available to the device. But in complex installations, there often isn’t enough power to go around.

“PoE-enabled switches will not give out indefinite power, which will show [up] when you connect more power-consuming units than the switch can handle: The next unit connected will not get power,” says Søren Fleron, AV systems programmer at Dansk Data Display A/S.

Phihong recently singled out that factor as one of the biggest caveats about PoE.

“A 24-port PoE switch with power management typically has a 195W power supply,” the company wrote in “Myths about Power Over Ethernet.” “After the 40W needed to power the switch, you have approximately 155W remaining. If 12 of the 24 ports are used to connect end devices using 11.5W each, you would only have 17W remaining to provide power on the last 12 ports.”

Some PoE devices draw more power than others, a factor to keep in mind when designing an AV system. For example, pan-tilt-zoom (PTZ) surveillance cameras are some of the most power-hungry devices.

One obvious way to add power is to deploy more PoE switches. That works, but it isn’t necessarily cheap. Another option is to deploy one or more midspans – also known as power injectors – which take over the task of powering the AV devices, leaving the PoE switch to serve less-demanding devices such as voice over Internet Protocol (VoIP) phones. Midspans also can be used to provide power to PoE devices that are connected to a non-PoE switch.

VoIP also highlights another key consideration: the client’s IT infrastructure. Just because PoE switches are already in place to support VoIP phones doesn’t necessarily mean that the integrator can simply hang some AV gear on them and call it done.

“Most PoE switches on the market today were developed for IP phone systems,” says Phihong’s Hopwood. “Most IP phones are less than 8W, and it is not possible they are all off hook making a call at the same time. Typical switches can only offer between 50 percent and 65 percent of total possible power. I hear all the time of installers plugging in cameras, and all of a sudden they won’t power up. Or even worse, they have cameras randomly dropping out. Totally unacceptable for a security system.”

Power Struggles

That scenario goes back to the power issue. Each PoE device can tell the PoE switch how much power it needs, a negotiation that’s spelled out in the 802.3af standard. But sometimes the device doesn’t tell the switch what it needs, a potential problem to look for when designing or troubleshooting an installation

“Most PoE switches will assign maximum power to a connected unit if no negotiation takes place,” says Dansk’s Fleron. “For instance, an IP telephone will publish a need of 6W to the switch, [and] the switch will then only allocate that amount of power to the port, leaving more power to other ports. Problems has been seen where a switch will assign maximum power to half the ports and leave the rest of the ports without power.”

Even when the negotiations work perfectly, the PoE switch still has a finite amount of power available. It doles out that juice based on a hierarchy that’s established when the switch is installed and configured. If the hierarchy can’t be changed to give, for example, the surveillance cameras priority, then the solution is to provide them with a separate power source, such as another PoE switch or a midspan/injector.

“We would typically only use injectors when connecting to a small number of devices, when it cannot be cost-justified to use a more expensive PoE switch or some other power sourcing equipment,” says Blaine Brown, director of technology at Sensory Technologies, a U.S.-based integrator.

One way to decide whether to add a injector or a PoE switch is to look at the number of connected devices. That decision includes taking into account future growth, such as whether the system one day will need to scale up with more attached devices.

“If they only have one device that needs to be powered via PoE, an injector is more cost-effective,” says Crestron’s Jackson. “But as soon as more than one PoE device is used, a PoE switch will often be easier to manage. There are small, four-port PoE switches available at not much cost.”

Know When to Ask for Help

Some integrators consult vendors to determine how best to power the AV devices.

“I rely on the manufacturer to tell me if a power injector is needed,” says Darren Cheshier, an engineer at SKC, a U.S. integrator. “Most of the time for AV it is. AV still is worried – and with good reason – to step on any IT toes, and therefore they like to provide a separate power path for those devices.”

PoE also is yet another example of the importance of knowing IT basics – not just to be able to work with IT equipment such as switches, but also to be able to communicate with the client’s IT department.

“Whenever designing an AV system that will work on the client’s network, always consult the client’s IT department,” Cheshier says. “They will help in design, along with future growth estimates.”

Some other considerations:

  • “Many [PoE switches] are very loud because of their high-speed fans, so they can’t be used in rooms with the end users,” says Crestron’s Jackson.
  • “Check the power supply mean time between failure (MTBF) rating,” Jackson says. “A lot of your critical equipment will be relying on this one power supply, so you want to make sure you have quality equipment used in it.”
  • “Most PoE switches have ports that come out the front of the rack because they are designed for IT closets,” Jackson says. “If the rack is in a user space, that could be an issue.”

What’s Ahead: More Power, More Devices

Like any other standard, PoE is evolving. The latest version is IEEE 802.3at – also known as PoE+ – whose features include twice as much power for each device: 26W. The extra juice is a plus for power-hungry devices such as PTZ cameras, infrared illuminators and cameras installed outdoors in climates cold enough to warrant a heater.

The new standard also could make it easier for AV integrators to offer a wider range of security solutions beyond video surveillance.

“The higher power is also now enabling many access control and biometric devices that could not previously get enough power,” says Hopwood, whose company is a voting member on the PoE standards body. “The new standard is truly enabling a total IP security system that can be powered from a single, backed-up source.”

The new standard requires at least one change.

“The current IEEE 802.3af PoE spec will work with Cat3 cable and up,” says Shaun Robinson, director of product management at AMX. “However, the new IEEE 802.3at spec will require Cat5 cable as a minimum.”

Some vendors believe that 802.3at will expand the selection of PoE AV devices.

“PoE supplies have limited power output, and therefore there is still a limited amount of PoE enabled end devices in use,” Robinson says. “However, this will change in the future with the ratification of 802.3at, which will allow for the increase in power output for PoE injectors.”

The selection of PoE AV devices also should get a boost from the ongoing trend toward AV-IT convergence.

“In my opinion PoE is not really made [it] into the AV world,” says SKC’s Cheshier. “However, I think it is coming as AV and IT share more resources in the foreseeable future.”

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