ACHIEVING COMPATIBLE CONNECTIONS
When a PoE-compatible device is plugged into a PoE-capable network, the device doesn’t simply start using the power it needs. A “handshake” needs to occur.

When the device is first connected the PoE supply node powers up the new equipment at the highest class (more on classes soon). As the edge device comes online it sends out a message (along with other network announcement traffic) that describes how much power it needs and how much, maximum, it can use. The supplying node responds with the maximum amount of power it is allowed to supply on that port. If all matches up, the device continues to operate.

The rules of PoE basically state that a device will never use more power than it has asked for, nor will it ask for more power than its class allows it to. PoE devices are assigned to classes based on how much power (maximum) they can draw. This is important to understand as some supply devices can only provide a certain level of power, so they can only operate with certain classes of devices.

In the table above, you will notice there are technically five levels, or classes, in the standard. Only three, however, are used with regularity. They are classified by the wattage available at each level.

This is where it becomes important to know what classes your devices are compatible with, both on the switch side as well as the edge device (camera, access controller, IP phone, etc.).

A switch data sheet can indicate it’s PoE-capable, and it can even note that it can supply PoE on all ports. But what classes is it compatible with? That is the important question. If you plug in several cameras, all certified as Class 3, they may quickly overwhelm the available power of a lower class switch. How many times have you hooked cameras up to a small PoE switch and had no problems with the first three cameras, but when you add No. 4 everything dies? These kinds of problems can cause numerous troubleshooting headaches.

Next time you see symptoms like this, check the data sheets of both the edge device and the switch. See if the classes line up.

DESIGN IT RIGHT THE FIRST TIME
As we’ve mentioned many times in this column, doing homework up front can greatly reduce the time it takes to install and get a system up and running. PoE problems aren’t always obvious. Knowing that the first rule of troubleshooting is to check the physical layer (cabling), it may be a while before you check the power, especially if the problem is intermittent. There are lots of other things that can cause similar issues.

One important item to understand is that a PoE-compatible edge device like a camera can also plug into a non-PoE switch and work fine, provided power is supplied externally. It is perfectly safe to plug a PoE device into a non-PoE switch (although it won’t work) and it is perfectly safe to plug a non-PoE device into a PoE switch. The negotiation phase of the device startup will figure itself out pretty quickly.

Make sure you know how much power each device needs, what class it belongs to, and make sure your switch is compatible with the device’s class and can supply sufficient power on as many ports as you need.

Being able to pull one cable to a network end point is a huge blessing for installers. No longer do you need to pull several multi-conductor cables to an access control door. No longer do you need to pull bulky RJ-11 cable and an 18/2 for power to get your camera up and running.

Now you simply pull a standard Cat-5, -5E, -6 cable or better to your location from the nearest IDF closet. Hopefully that makes life a little easier.

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