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POE (Power over Ethernet)

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POE (Power over Ethernet)

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Since the rapid development of network video surveillance, technical barriers are getting higher and higher, manufacturers offer technical support is also becoming comprehensive and systematic. Tom is a sales representative in video surveillance industry for several years, everyday exchanges ideas with security integrators. In the exchange of technology, integrators and installers often feel confused on Power over Ethernet (PoE) problems, Tom feel the importance of PoE technology, herein, Tom invited a PoE expert to provide comprehensive information about PoE.

#1 What's Power over Ethernet (PoE)?

PoE stands for Power over Ethernet, it's a technology that transmits both data and power supply simultaneously to network devices (such as VoIP phones, Wireless AP, network camera etc) without changing existing Cat.5 cable network structure.  PoE allows you to supply power to a networking device using the same cable that transmits the data, which in turn, greatly reduce the cost of installation and maintenance.

A complete PoE system includes Power Sourcing Equipment (PSE) and Powered Device (PD) two parts.

Power Sourcing Equipment (PSE): The equipment supports PoE function such as PoE switch, PoE injector (midspan) etc.

Powered Device (PD): IP/network cameras in video surveillance system. Note: Currently, PoE technology has two standards - IEEE802.3af and IEEE802.3at.

#2 Is PoE technology reliable?

On technical perspective, PoE technology has been developed for many years, now PoE technology is in mature stage. However, current video surveillance market faces cost pressure, the quality of selected PoE switch and Cat5 cable is too poor, or the project design is unreasonable, may result in the maintenance workload of PoE project is huge, so integrators hold the common view that PoE technology is not stable.

For network video surveillance projects, different from common network cabling, data transmission is huge, power consumption is high, additionally requires 24/7 days continuous working, choosing high quality PoE device and cable can ensure the reliability of system.

#3 What's the advantage of PoE?

1. Simple cabling, reduce installation cost

Centralized power supply, one network cable can transmit data and power supply, PoE enables network devices no longer require extra power supply, greatly reduce the installation cost and time.

2. Safe and convenient

Power Sourcing Equipment only provides power supply to PoE-enabled devices. For example, PoE switch only transmit power supply when it detects the connection of PoE-enabled devices. This can avoid the risk of leakage. Users can mix use existing non-PoE device and PoE-enabled devices, both of them can co-exist.

3. Convenient for maintenance

Similar to data transmission, PoE can be managed through Simple Network Management Protocol (SNMP). This function can offer shut down at night, remote reset etc functions.

Simple Network Management Protocol (SNMP) is a popular protocol for network management. It is used for collecting information from, and configuring, network devices, such as servers, printers, hubs, switches, and routers on an Internet Protocol (IP) network.

#4 Possible problems or disadvantages of PoE?

1. Power insufficient, powered device can't boot: 802.3af PoE standard maximum output power is below 15.4W, this is sufficient to provide power supply for most of IP cameras. However, this output power is not sufficient for high power consumption devices such as network PTZ camera.

2. Centralized risk: Generally, a PoE switch connects to multiple IP cameras, if PoE network switch encounters any problems, all IP cameras will stop working.

3. High equipment and maintenance cost: Compared to other power supplying methods, PoE technology increases maintenance workload, when considering security and stability, separate power supply is best.

#5 How to select PoE switch?

1. Power consumption of IP camera: PoE network switch adopts different standards, the output power is also different. For example, IEEE802.3af can support powered devices that don't exceed 15.4W. As cable transmission will consume power, IEEE802.3af can provide power supply to powered devices that don't exceed 12.95W. According to IEEE802.3at standard, IEEE802.3at PoE switch can work with powered devices that don't exceed 25W.

2. Max. support powered devices: PoE switch has an important parameter - total power. Under IEEE802.3af standard, If 24CH PoE switch's total power reaches 370W, then each ports can get sufficient power (370/15.4=24), but if based on IEE802.3at standard, each port supports 30W, the PoE switch only can provide power supply to 12 ports (370/30=12).

 PoE IEEE 802.3af Technical SpecificationPoE IEEE 802.3af Technical Specification

3. Required quantity of ports, whether having Optical Fiber port, or network management, bandwidth (10/100/1000M).

#6 What's the PoE transmission range? advice on selecting cable?

PoE transmission range is 100 meters, recommend to use Cat5 Ethernet cable.

You should be cautious when buying the Ethernet cable from China. PoE IEEE802.3af standard requires PSE output power is 15.4W, after 100 meters transmission, the PD can receive 12.95W power, according to 802.3af standard current value - 350ma to calculate, the resistance value of 100 meters Ethernet cable should be (15.4-12.95W)/350ma = 7 Ohm or (15.5-12.95)/350ma = 7.29 Ohm. 

Obviously, standard Cat5 cable can meet this requirement. but there are many non-standard Ethernet network cables selling in the market, they are produced not strictly in accordance with the requirements of the standard network cable. Non-standard network cable utilizes other materials such as copper clad steel, copper clad aluminum, copper coated iron, etc. The resistance of these cables is big, not suitable for POE power supply. POE power supply must use oxygen-free copper material, namely, the standard network cable.




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