Eye on Video: Power over Ethernet for network video surveillance

Plus a look at the newly proposed 802.3at PoE standard


Power over Ethernet (PoE) can be an intelligent way to reduce installation time for network cameras. The advantage of PoE is that it gives you the option of using your data communication cable to supply power to your network video components instead of running a second cable to those devices. The design stems from analog telephony systems where the phone line not only provided a channel for audio communication, but also served as the power source for the handset. That same design was then carried over to IP telephony with Ethernet cabling delivering the same dual functionality.

Once the IEEE ratified the 802.3af standard for PoE in 2003, manufacturers began incorporating the feature into their product designs. For the network camera market, the adoption of the standard has been extremely beneficial in accelerating market acceptance of IP-based solutions. The two primary advantages for surveillance users are:

1. PoE cuts installation cost.
The significant cost savings inherent in PoE technology has certainly been the main impetus for its widespread adoption. Today, PoE is being used for everything from IP phones and wireless access points to network cameras. In the case of the latter, not having to install a power cable can save up to several hundred dollars per camera, depending on where the camera is deployed. A PoE configuration also makes it easier to move a camera to a new location, or add cameras to a video surveillance system, since only a network drop is needed - something readily available in today's buildings.

2. PoE can power a device even in the event of a power outage.
Enhanced security is another benefit driving widespread adoption of PoE. A video surveillance system installed with PoE can be powered from the server room, which is usually backed up with an uninterrupted power supply. This ensures that the video surveillance system remains operational even during a power outage when facility security is often most vulnerable.

The current 802.3af standards

The specifications for the current 802.3af standard include delivering 48 volts DC over unshielded twisted-pair (UTP) wiring at a maximum power of 15.4 watts per port (12.95 watts at the powered device after factoring in the normal power loss that occurs on a twisted pair cable). It works with standard CAT-5 or better twisted-pair cable with a maximum length restriction of 328 feet or 100 meters. Of the four pairs of twisted cables in typical CAT wiring, PoE can use either the two dormant twisted pairs or overlay the current on the two twisted pairs used for data transmission. As a rule, switches with built-in PoE supply electricity via the same twisted pairs used for data transmission, while midspans normally use the dormant twisted pairs. A powered device such as a network camera supports both options.

802.3af defines a method for automatically identifying if a device supports PoE, and only upon confirmation that it does will power be supplied to that device. For added safety, the Ethernet cable connected to a PoE switch will not supply any power if the switch is not connected to a PoE device. This eliminates the risk of getting an electrical shock when installing or rewiring a network.

A network switch with PoE support typically supplies between 300 and 500 watts. On a 48-port switch, that means 6-10 watts per port, if all the ports are connected to PoE-enabled devices. Unless those devices support power classification (see table below), a full 15.4 watts must be reserved for each port that uses PoE. Hence, a 300-watt switch could only supply power on 20 of the 48 ports. However, if all the devices are capable of letting the switch know they are Class 1 devices, for instance, then 300 watts would be sufficient to supply power to all 48 ports.

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