Power-over-Ethernet (PoE) technology offers many benefits for security applications. It enables network cameras to be powered over existing network cabling within a unified power and data infrastructure, and then monitored and managed remotely. Cameras can be deployed anywhere without having to install costly AC outlets or send electricians to pull cables, modify building plans or get safety approvals. To maximize PoE efficiency, security professionals should take advantage of PoE technology's comprehensive power-management capabilities.
PoE power management is an integral part of today's midspans, which are the solution of choice when companies wish to add PoE capability without having to upgrade their existing switch infrastructure. Midspans are simply installed between the switch and powered devices (PDs), and are a green alternative to PoE-capable switches because they deliver only the necessary power for a given application.
IEEE802.3af standards make it easy to add new PDs to a network, and the latest IEEE802.3at-draft4.2 standards extend that simplicity to a broader range of terminals with higher power requirements, including pan-tilt-zoom (PTZ) cameras. Power allocation and delivery is simple, and there is no need to worry about whether PDs are clearly labeled with their power classification, or how to allocate power per port or across the network. In fact, while some suggest that switches and/or midspans with a power supply smaller than 15.4 W per port should use power allocation based on classification, this is not the best approach. Power allocation is be seamlessly handled by today's midspan systems in a very intelligent manner using dynamic power management.
The IEEE 802.3af standards mandate a number of specifications related to PoE power delivery and management, including PD PoE compliance detection for safe powering, and safe PD disconnection in overload, short circuit or under-load conditions. The IEEE standard also supports the use of classification to determine the maximum power that can be allocated to a PD. Classification can be used as an alternative to the previously described disconnection methods. The IEEE 802.3af standard defines four classes of power, including Class 0, which is "non-defined" status. However, most PDs do not support classification, and are therefore interpreted as class 0, which means that full 15.4 W power is allocated from the PSE to the port. Cable losses can also impact PSE and PD class limits.
The IEEE classification feature can also be used for power management. However, it only supports static power management, in which the PD uses classification to establish maximum power prior to applying power. The PSE checks if power is available and then either denies or delivers power. The problem with this method, as mentioned earlier, is that most PDs don't support classification and so, by default, deliver full power whether it's needed or not. This quickly consumes available power budget. Further, the classifications are so broad that, when a PSE detects a higher class, it delivers the full incremental additional power, rather than just what's needed by the device.
A better approach is to leverage midspans' dynamic power management capabilities, which enable real-time measurement and management of power on each port. With this approach, there is no need for fixed power allocations, because only the necessary power is delivered to each port. The system monitors the actual power consumption on each port, including that being consumed by class 0 PDs (which would otherwise always be delivered full power). The midspan provides the host CPU with frequent port status updates.
The benefits of this approach in terms of improved power efficiency are significant. Consider a typical 1U 24-port midspan, for which there will be 24 x 15.4 W, or 370 W of power to manage. The real-time power needs of the various PDs on the network might only be just over half that requirement. By using dynamic power allocation and a distributed power architecture, the midspan might require only a 200 W supply to support real-time network power demand, and could potentially (depending on features of each midspan, these are determined by the manufacturer) use external power supplies for incremental additional power as needed.