Power over Ethernet (PoE) is an important consideration in IP video security infrastructures, and many people believe Institute of Electrical and Electronics Engineers (IEEE) standards are the last word. However, when it comes to PoE, there is a wide range of both voltages and wattages that can qualify as being within an individual IEEE power class. This applies to both the device being powered and the source of the PoE. A video surveillance camera may require the highest source voltage of 48 volts and highest source wattage of 15.4 watts to be considered as IEEE 802.3af, Class 3. However, a PoE source output of only 37 volts and 7 watts can also be considered as Class 3-compliant under the standard.
Surge factor in PoE-powered devices
The surge factor is another consideration with regard to PoE-powered devices. In any consideration of power, it always takes more power to turn a device on than to maintain its operation. For video security cameras, this requirement applies both to startup and also to when we activate accessory functions such as day/night operation, LEDs, and auto back-focus to name just a few. If the surge applied when turning on these functions exceeds the ability of the source to provide it, or rises for more than approximately 40 milliseconds, the PoE source will stop transmitting power based on the safety built into the 802.3 PoE standards. It remains in that state until the port is reset either by turning power to the port off and on, or by disconnecting and reconnecting the Ethernet cable.
A customer might interpret this type of problem to be a camera failure – it was working during the day and failed at night. He might return the camera to the manufacturer, only to be told there is nothing wrong. The solution is simple and two-fold:
- First, even when a manufacturer states a specific PoE power for a camera, always count on the maximum class power source. For example, if a manufacturer states a camera requires 10 watts and is 802.3af Class 3, you should always have a PoE source of 15.4 watts.
- Next, estimate any surge that might be present as requiring at least 20 percent more power. So in the case of the 10-watt security camera, that would really need about 12 watts, in this case putting it within the limits of a full Class 3 source of 15.4 watts. The problem will usually occur when the surge exceeds a particular class, especially at the transition from Class 3 to Class 4.
|The considerations regarding bandwidth and PoE are the same as with the camera or remote device|
Network PoE switch
Once we have determined the power requirements of the connected or remote site device, we need to turn our attention to the source that will provide PoE power and receive the signal. Often this is a network PoE switch.
The considerations regarding bandwidth and PoE are the same as with the camera or remote device. The concerns here again are with the lack of standards and how specifications are determined. There are several factors to take into consideration.
First, networking equipment in general is tested and specifications are determined using a packet size of 64 bytes. However, even the smallest one- to two-megapixel cameras will approach the limits of packet size testing at around 1,538 bytes, while cameras with two megapixels and above are even higher. In short, networking products are not tested using packet sizes comparable to those required by video cameras.
Once we have determined the power requirements of the connected or remote site device, we need to turn our attention to the source that will provide PoE power and receive the signal
While many switch specifications will state they handle Jumbo frames, that ability is restricted to port programming at 1 Gbps speeds. In networking, the 100 Mbps output of a video camera must match the bandwidth input speed of the switch port.
The second switch consideration is called the switch fabric, which connects all the switch ports. Its bandwidth must be at least two times greater than the sum of the highest bandwidth of all the ports. There is no standard for this, and the actual bandwidth is a reflection of switcher cost. With regard to PoE, be careful not to mistake the total power supply for the PoE budget for the power allocated for camera PoE power. They are different. If the total power were allocated to PoE, there would be no power left for switch functions. In addition, if all ports required PoE, the switch could easily overheat and fail. There should be anywhere from a 10 to 25 percent separation between the total switch power supply and the PoE budget. Finally, there are no standards for the methods used to allocate PoE power within a switch. It could be equally divided as more ports are connected. It could be programmed, or it could be fixed to each port.