The Long and Short of PoE

Power where you need it


Powering security cameras through the same cable used to transmit data and video is one of the greatest benefits of IP cameras over network cabling. Ease of installation with one pull, one cable and also cost reduction by limiting AC power at the device end are the most obvious benefits. PoE also becomes the most reliable method of delivering power to an IP camera because all networked devices are connected to back-up uninterruptible power supply (UPS) equipment in the telecommunications room (TR). In the event of loss of power, the camera continues to send data and video.

There are three basic components needed to operate PoE: 1.) the powered device (such as the IP camera); 2.) the powered source equipment (PSE); and 3.) the media-either copper or fiber.

The powered device must meet the Ethernet protocol, either IEEE 802.3af that allows up to 15.4 watts of power for each port, or IEEE 802.3at PoE+ that allows up to 25.5 watts for each port. The higher wattage expands PoE to include some power-hungry applications such as Pan/Tilt/Zoom (PTZ) cameras.

The PoE or PoE+ power level supplied by the PSE will vary depending on the power requirement of the powered devices. PSE comes in two forms-a midspan or an endspan. A midspan is either port-to-port power injectors located on the device end or a powered patch panel that connects to the switch in the TR. The midspan injects DC power to the twisted pair cable and the data passes through the injector. An endspan is actually an Ethernet switch with PoE capabilities built in, so both data and power can be sent from the switch through passive patching fields.

Media selection: twisted pair or fiber

PoE is typically deployed along with data and video over unshielded twisted-pair (UTP) network cable where access to AC power is inconvenient, expensive or infeasible to supply. IP cameras are equipped with standard RJ-45 ports that connect to the four pairs in UTP cabling.

However, according to the ANSI/TIA/568-C standards for copper twisted pair cabling (either Category 5e or 6), the maximum length for a cable segment is 100 meters (328 feet). Most indoor cameras fall within that distance limitation from the TR. If the camera location is further, there needs to be an additional cross connection with switches and active components. But, in the structured cabling realm, no more than three cross-connections are allowed from the main cross connect (also known as MDF or equipment room), through the horizontal cross-connect. Therefore, extending the distance to remote locations, such as up a pole in a parking lot, along fences or even in transit routes, it is not feasible to use twisted pair cabling. Another challenge with twisted pair is the heat from power-hungry PoE+ devices, such as a PTZ camera, can lead to a rise of temperature within copper cable, resulting in increased insertion loss, which can affect transmission, or foster signal degradation.

Optical fiber is the choice to extend the distance. In addition, fiber is impervious to EMI, RFI and is a more stable mode of transmission in varying temperatures. But since the IP cameras and switches operate off of RJ-45 ports, media converters must be deployed at each end. PoE media converters can support PoE or PoE+ and convert the fiber to copper by injecting DC power over the UTP cable. Figure 1 shows the media converters on each end-in the TR and at the device end. But, note that the media converters require local power.

Extended distance with composite cable

In long-distance scenarios where local power for the media converter is not feasible or accessible at the device end, there is a solution-a composite copper/fiber cable within the same sheath. Data and video can run over the optical fiber and the power is transmitted through copper conductors. Because the cable carries low-voltage power, this cable is actually defined as a Class 3 copper cable with fiber within the NEC codes. The stranded copper conductors are 12 AWG or 18 AWG and coupled with either tight-buffered or loose tube multimode optical fibers.

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