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POE Overview

PoE was created during the first wave of Voice-over-IP (VoIP) telephones and wireless access points (WAP) in the late 1990s. When VOIP and WAP were installed two cables were required, one telecommunications cable to connect to the data network, and another cable to a local power outlet. Older telephone systems that replaced VOIP telephones never required local power. Those older telephones were powered from a central telephone system, typically a private branch exchange (PBX). Many locations for VoIP telephones and wireless access points did not have local power available and power had to be installed at considerable costs. So the need for electric power for the first VoIP telephones and wireless access points undercut the attractiveness of those technologies.

That shortcoming of the VoIP technology spurred the effort to supply power to the telephones over the twisted pairs in the telecommunications network cable. The result was that in 2003 an IEEE standard was published allowing low voltage power, 48 VDC, to be transmitted over Category 3 and Category 5 Twisted Pair cable. With the current standard, IEEE 802.af, the maximum power that can be delivered to a powered Ethernet device is 15.4 watts. After counting losses, about 13 watts is the nominal power delivery available. Additional losses will occur with the use of switch mode power supplies.

Power is transmitted over the pairs of the Category 5 cable in one of two ways. For 1000BASE-T, which uses all four pair, cable pairs must carry both power and data. This method of powering is called Phantom Power (Fig.13.3). For 10BASE-T and 100BASE-TX Ethernet, which only use two out of the four cable pairs for data transmission, the power can be transmitted over the two idle pair. Transmitting the power over idle pair is called I (Fig. 13.4).

The device supplying power is called power sourcing equipment (PSE), and the device being powered, such as the VOIP telephone or wireless access point, is called the powered devices (PD). The PSE determines the method (galvanic injection or phantom power) used to power the PD. The PSE can either be a POE-enabled network switch (referred to as an endspan) or a device that injects power between a network switch not POE-enabled and the PD (referred to as a midspan). Midspans allow users without POE-enabled network switches to make their networks POE-enabled without procuring new network switches.

The initial standard is sufficient to power many low powered devices. However, the power rating is insufficient to power devices such as video surveillance cameras using pan, tilt and zoom (PTZ) capabilities, and video telephones. A second standard, IEEE 802, also known as POE Plus, is now being developed and will offer higher power levels, although still within the low voltage range. Expectations are that the standards for POE Plus will at least have a power rating of 30 watts, basically doubling the existing standard. One concern with the development of the standard is that higher power generates more heat on the cable pair(s) resulting in an increase in the attenuation of the cable. The result could be that cabling runs would need to be shortened to comply with the standards of TIA 568. Obviously, there is more to come on the development of the standard and its implications.

POE products started with VoIP telephones and wireless access points but have rapidly expanded into hundreds of certified products. These include surveillance cameras, clocks, intercom systems, paging systems, access control devices, time and attendance systems, and touch-screen flat panels. There's even a prototype POE electric guitar and a demonstration POE electric shaver for men. Here are a couple examples of POoE product types that improve the installation and functionality of systems:

b Access Card Reader: Network connected, POE powered access control card readers eliminate the need for a local AC/DC power supply and power the reader from the telecom equipment room, which typically is on an uninterruptible power supply.

Figure 13.2

b Paging Speakers: Each speaker is networkconnected and POE powered. The speakers become part of a VoIP telephone system, endpoints on the system much like the telephones. Thus a separate paging system, and interconnection of the paging system and telephone system is eliminated. In addition, the speakers do away with a limited number of paging zones and allow paging to specific speakers or predefined groups of speakers as needed. If your VoIP telephone system is networked between buildings or cities, so is the paging system and you can page from one building or city to the other.

b Clocks: POE clocks plug into the network and become end devices on the network much like VoIP telephones or card readers. The clocks can be automatically synchronized throughout a facility and managed by the network.

b Intercom Systems: Intercoms, which are used in classroom communications or as part of access control for doors and gates, are evolving to networked POE devices. As with many of the other POE devices, installation costs are lower and system flexibility is increased.

The benefits of PoE are numerous:

b At the top of the list is cost savings or cost avoidance. The cost of a power outlet includes conduit, wire, a back box for the outlet, and the labor of an electrician. If a power outlet does not have to be installed for the device, the cost of installation and construction is reduced. Purdue University installed over 1,100 PoE wireless access points across campus, and saved $350 to $1,000 per location by not having to install power. Others have estimated that an average cost to provide power to a device is about $864, while the cost of a POE network port is $47-$175.

b Increased Reliability: POE centralizes power distribution. Instead of a power outlet at each local device, power is now distributed from the telecom rooms, (a throwback to the older centralized telephone systems.) Centralized power makes it easier to provide uninterruptible and emergency power for critical hardware, thus increasing system reliability and uptime.

b System Management: POE allows the end device to be monitored and managed. Network switches provide management tools such as the simple network management protocol (SNMP), which allows staff to manage the end devices, including the power to the end device. You can remotely turn the device on or off or monitor the device's consumption of power.

b Move, Additions, and Changes: POE allows for slightly easier building renovations and rearranging of spaces since devices only need one cable. It's easier to install devices on walls or ceilings, and to setup temporary installations.

b International Applications: POE is being marketed and deployed worldwide, allowing manufacturers to avoid supplying different power cords for different countries and eliminating the need for installers to worry about power cords.

b Safer: When POE is used increased low voltage distribution is used to power devices, and less high voltage is used throughout a building. This results in a safer environment and lower power consumption.

In an environment where construction costs are steadily increasing, POE is a technology feature that can add long term value to a building. Over one hundred million network ports enabled by Power over Ethernet are currently shipping annually.

The initial POE deployments were providing 15 watts for VoIP telephones. The more recent advances of POE have included Cisco with 60w Universal POE (UPOE) technology driving the adoption of virtual desktop

Figure 13.3

infrastructure (VDI). The HDBaseT Alliance is promoting the POE HDBa- seT specification for consumer electronics, which can deliver up to 100w over twisted-pair cable, supporting full HD digital video, audio, 100BASET, and control signals in television and display applications. Finally, the IEEE is developing a standard called IEEE 802.3bt DTE Power via MDI over 4-Pair which will have remote powering applications and improved energy efficiency.

 
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