THE POWER BEHIND THE TECHNOLOGY

• By Glenn Meeks
• 12/01/05

As more and more systems move toward using IP-based devices connected to the local building data network, the owner has to determine how those IP-based devices will obtain the DC power required for them to operate. To make sure all of us are on the same page; I am specifically referring to Wireless Access Points, Voice Over IP (VoIP) phones and IP security cameras. Door access control units and their related locking mechanisms operate on DC power, but the wide variety of current draw requirements, dependent upon the locking device, may require placing them on a separate power source.

You have three choices regarding DC power for IP devices. You can use a data switch or patchbay with POE (Power Over Ethernet) built in, an AC-to-DC power supply located in a wiring closet or an AC-to-DC hardwired transformer located in an electrical box near the device. Some solutions are superior to others so let’s review all three options.

Sorry about getting geeky, but we need to. IEEE (Institute of Electrical and Electronic Engineers) is the international organization that issues consensus-based standards for the electrical and data/cabling industries.“IEEE-802.3af” is the standard for distribution of DC voltage across data switches, patch bays and UTP Cat5e/6 cabling to power IP devices and is called POE (Power Over Ethernet). I personally think using POE on a data switch is an extremely limiting way to distribute DC power, because Gigabit Ethernet and POE are inherently incompatible with each other. You cannot use POE with a copper-based Gigabit Ethernet circuit. Copper-based data connections are DC voltage based binary systems, meaning there is DC voltage on the cable with one level of voltage representing“0” and a higher voltage representing “1.” Fast Ethernet (Base100T) runs at 100Mbps and uses two of the four pairs of a UTP cable. Gigabit Ethernet (Base1000T) runs at 1000Mbps and uses all four pairs of a UTP cable. Guess what? The POE standard uses the fourth pair for distributing a constant DC voltage to the IP device.

Wait a minute! If I have a constant DC voltage going down the UTP cable’s fourth pair, how can I see the DC voltage differences for my “0s” and “1s”? You can’t! Oooopppsss!

When I think about the life-cycle of phones and cameras in a school, versus data switches, I can see some real problems for schools down the road. Due to manufacturing cost reductions, five years from now your computers will come with Gigabit Ethernet built in and you will be required to upgrade your data switches. But when you do that, your new switches will not have POE capability and your Wireless Access Points, VoIP phones and IP cameras will not work.

Installing a POE (IEEE-802.3af) patchbay in the wiring closet is a better solution, but you should also know that there are manufacturers with individual POE inserters for locations with few IP devices connected to a wiring closet. For all POE solutions, you will need to calculate the actual maximum current draw for all IP devices to ensure the load is within the capability of the device.

A second, but more expensive, alternative is to install a security industry, rack-mounted 18-24VDC power supply in the wiring closet and a one pair 18awg cable, which is daisy chained to each IP device. A third and most expensive solution requires locating an electrical junction box near the IP device and installing a hardwired AC/DC transformer in the box. However, locating power for Wireless Access Points cannot occur ahead of the RF site survey, which establishes the optimum location for those devices.