Network technology is becoming increasingly prevalent in the security space. In the past few years alone weve seen advanced systems configurations evolve that range from hybrid networked camera multiplexers to integrated IP-based surveillance, access control, intercom and security systems. ©iStockphoto.com/ryasick
Network technology is becoming increasingly prevalent in the security space. In the past few years alone we've seen advanced systems configurations evolve that range from hybrid networked camera multiplexers to integrated IP-based surveillance, access control, intercom and security systems.
Today's combination of new technologies and affordably priced IP cameras and video management systems (VMS) has further accelerated the migration to networked systems with even greater performance and functionality. As a result, more and more system designers and installers are realizing the need and overall efficiency of power over Ethernet (PoE).
PoE is a means of introducing power safely through Cat-5 or higher cable along with network data, thus allowing devices to be powered and communicate over the same Ethernet cable. This technology was first used for VoIP phones and has since been applied to an increasing number of devices covering virtually every category of security products.
PoE has also evolved during the past several years. The original standard IEEE 802.3af was limited to 15.4 watts. The latest standard, IEEE 802.3at PoE+, has a capacity of 25.5 watts. Additionally, a new PoE standard is in development with a capacity of up to 70 watts.
These PoE standards have helped to redefine the way we think about power supplies for security devices. Rather than powering IP cameras from standard type power supplies, integrators are using PoE switches or midspans. The benefits of new PoE midspans have become increasingly clear versus PoE switches.
While both midspans and switches will inject managed power onto the Cat-5 cable, there are pros and cons to each. In a typical IP surveillance network, a camera communicates with a video IP server through a network switch. A midspan is placed between the switch and the camera, passing through its video data and injecting power; whereas an endspan incorporates the switch in the same enclosure. Let's investigate the benefits offered by new PoE midspans versus PoE switches.
PoE Switches Vs. Midspans
One of the primary issues in developing new PoE standards was to minimize the risk of damaging non-compliant PoE devices if accidentally connected to a PoE source. The PoE bus is nominally 48VDC; however, the PoE controls the delivery of this power to a device.
First the PoE power supply attempts to verify that the device, such as a video camera, is PoE compliant by sending low voltage and low current test signals to the device. If the device does not respond with the authenticating signature, the PoE power supply will remain in the test mode. Upon authentication, the PoE will supply proper bus voltage. However, it will restrict the power based on the authentication verification of the PoE device.
This is restricted to a maximum power of approximately 13 or 25 watts for devices compliant to the IEEE 802.3af or IEEE 802.3at standards, respectively. The PoE remains vigilant and any deviation from its normalcy will cause removal of its bus voltage. Even an open circuit will cause the same bus voltage removal.
With so many options available, selecting the right PoE switch for the job can be challenging. For example, PoE switches that were first developed for VoIP don't provide enough current for video surveillance and access control devices. Pan/tilt/zoom (p/t/z) and infrared (IR) cameras require more power than conventional IP cameras. The correct PoE solution will provide security professionals with the flexibility to specify different cameras for specific surveillance applications within the same system. When choosing a PoE solution, it's important to consider the following:
- Is the power supply in the switch made for constant and consistent current draw?
- Does the switch have PoE on all ports?
- Is there enough current to supply every device?
- Will it power 25.5-watt devices?
- Is port management desired?
- Does the unit provide over-current protection for each port?
- Does the security of the switch management software need to be upgraded?
- Is the PoE switch UL Listed for IT and UL294 for access control?
Conversely, there are very few questions to consider with PoE midspans:
- How many devices do you need to power and what is your total device current draw?
- Can you power 25.5-watt devices?
- Is your PoE midspan managed?
- What agency approvals does the product carry?