Power over Ethernet Analysis

Posted on by FS.COM

Power over Ethernet (PoE) has found widespread applications in markets such as VoIP telephony, wireless LANs, IP video security and access control, since its standardisation in 2003. It is gaining popularity across the spectrum of IP surveillance and access control applications. But what is PoE? This article will analyze it step by step.

Basic Concepts of PoE

PoE is defined across a single network link which includes three basic components. The first component is an equipment delivering power to the cable (often referred to as a PSE, which stands for power sourcing equipment). The second component is a device receiving power from the cable (also known as a powered device, or PD). The third is the cable itself.

Typical PDs include IP cameras, wireless access points, and the PSE would normally be a PoE switch or a midspan power injector, patched in to add PoE capability to a non-PoE network switch channel or similar. These two configurations are shown in the following picture. Note: this part will be described more clearly later in this article.

PoE switch

Advantages of PoE

Its primary advantage is time and cost savings. By reducing the time and expense of having electrical power cabling installed, network cables do not require a qualified electrician to fit them, and can be located anywhere. Then it has great flexibility. Without being tethered to an electrical outlet, the PDs (IP cameras, wireless access points) could be located wherever they are needed most. Safety is the third advantage. PoE delivery is intelligent and it is designed to protect network equipment from overload, or incorrect installation. Also it has reliability and scalability. PoE power comes from a central and universally compatible source, rather than a collection of distributed wall adapters. It can be backed-up by an uninterruptible power supply, or controlled to easily disable or reset devices.

Applications of PoE

The original PoE application is VoIP phones, which have a single connection to a wall socket, and can be remotely powered down, just like with the older analog systems. PoE could also be used in IP cameras. It is ubiquitous on networked surveillance cameras where it enables fast deployment and easy repositioning. Wifi and bluetooth APs and RFID (radio frequency identification devices) readers are commonly PoE-compatible, to allow remote location away from AC outlets, and relocation following site surveys.

How PoE Works

PoE is designed to operate over standard network cable: Cat 3, Cat 5, Cat 5e or Cat 6 (often collectively referred to as Cat 5), using conventional RJ45 connectors. The principles of carrying electrical power over Cat5 are no difference to those of other power distribution systems, but as the power is being transferred over light-duty cable for long distances, the effects of the power loss and voltage drop become significant.

The arrangement and connection to the cabling used for PoE also differ slightly from conventional power wiring, in order to work around the existing standard for Ethernet data. Cat 5 network cables contain a bundle of eight wires, arranged as four twisted pairs shown in the following picture. In the most common type of Ethernet, 100BASE-T or Fast Ethernet, only two of the four pairs are used to carry data; each pair carrying a signal in one direction. These are known as the data pairs, and the remaining two are unused and are referred to as the spare pairs.

PoE switch

Although each data signal can be carried within a single pair, PoE treats each pair of wires as a single conductor (a reason for this is that using both wires halves the overall resistance). As electrical current must flow in a loop, two pairs are required to allow power to be carried by the cable, and either the data or spare pairs can be used for this. The PD must be able to accept power from whichever pairs the PSE delivers it to.

How to Connect PoE

Two ways are available with PoE connection. A PoE switch is a network switch that has power over Ethernet injection built-in. Simply connect other network devices to the switch as normal, and then the PoE switch will detect whether they are PoE compatible and enable power automatically. PoE switch is available to suit all applications, from low-cost unmanaged switches with a few ports, up to complex multi-port rack-mounted units with sophisticated management.

A midspan or PoE injector is utilized to add PoE capability to regular non-PoE network links. Midspans could be used to upgrade existing LAN installations to PoE, and provide a versatile solution where fewer PoE ports are required. Upgrading each network connection to PoE is as simple as patching it through the midspan, and as with PoE switches, power injection is controlled and automatic. Midspans are available as multi-port rack-mounted units or low-cost single-port injectors.

Conclusion

Like all technologies, PoE can be used most effectively if its working basis is known and understood. The above statements only briefly explain its concepts, advantages, applications, connection and working principles. More information about PoE would be desired and we should keep learning about it.

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