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A PoE installation can also save energy. Since it supplies DC power, there is no need for DC equipment to incorporate AC current conversion devices, which consume energy, take up space and add weight, which can make powered devices smaller and cheaper. Another benefit is that it enables these devices to be shut down remotely when not in use. It is also safer, since the maximum voltage of the DC is 57V, which is too low to cause an electric shock. Operational reliability can also be improved. By connecting an Uninterruptable Power Supply to a PoE source in the communications room, the entire network is capable of continuous operation during a power outage which avoids the need for local back-up to each AC outlet.
PoE Standards The standards provide the guidelines for a PoE installation. In 2003 the original PoE standard, IEEE 802.3af, specified a power supply of 15.4W from the power sourcing equipment. Potential power losses over the cable length were calculated at up to 2.45W, so the actual power delivered under this standard to the powered device is 12.95W. Maximum current flow is 350mA per wire pair, with a nominal voltage of 48V. To supply this quantity of power Ethernet cables of Cat3 or better are required. Since only two of the four pairs of wires are needed for 10Base-T and 100Base-T, two-pair can be used for data transfer while power is transmitted on the unused pair of conductors. Alternatively, the same two-pair used for data transmission can be used to supply power. In answer to the demands of end device manufacturers for increased power delivery, in 2009 the IEEE defined a new standard 802.3at. This came to be known as PoE+. Under this standard the minimum supplied power increased to 30W. This delivered 25.5W at the PD, once power losses over the cable are included at a maximum current of 600mA. To comply with this standard Cat5e cables, or better, are required. Like Cat 3, the Cat5e cable has 4 pairs of copper wire; the standard, however, only permits two of the pairs to deliver power, even if four pairs are used for data. The IEEE is currently looking at the next PoE level termed 1000Base-T, or as it is more commonly known 1GBase-T. This requires all 4 twisted pairs for data transmission, which means that it is not possible to separate 2-pair for data and 2-pair for power transfer. Transmitted power is increased to 60W. However, because the same total current for power transfer is sent through all four pairs, power loss through the cable is reduced by half and energy efficiency is increased. With the power sourcing equipment providing 60W the received power on the powered device port is 51W. Note: while research is still underway these figures have not yet been defined by the IEEE.
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The number of prospective applications served by the 1GBase-T is significant. Building management systems offer the highest growth potential, along with industrial automation and IP security cameras; similarly thin client devices, such as stateless desktop computers that rely on a server, are also in the 50W range. Figure 1: 4-pair PoE+ market opportunities (4-Pair Power Over Ethernet, Call for Interest Consensus Presentation, IEEE 802.3 Working Group, March 2013).
Sources: VDC Research IMS Research - Jenalea Howell http://seekingalpha.com/article/101408-the-global-lighting-marketby-the-numbers-courtesy-of-philips and other research reports Gartner Forecasts, BT Turret, Cisco Partners
In the future there is the potential to power devices beyond 60W. At a power range of 100W, for example, many more devices such as laptops, 40-inch LED TVs etc will be able to take advantage of PoE. For this power range, the Next Generation PoE will aim for 1A current per pair.
PoE Restrictions Power transmission rates over the Ethernet are restricted by heating of the cables that carry power. Cables have a maximum operating temperature of 60°C. To give cables sufficient headroom to heat up under PoE applications, the maximum ambient temperature for a PoE cable installation is 50°C. As a result, there is only a 10°C margin available to accommodate heating of the Ethernet cable as a result of power transfer. In the standards, the maximum current that can be passed through the cable without increasing its temperature by more than 10°C is determined. For example, when 600mA is passed through a Cat5 cable, its temperature will increase by 10°C, which defines the PoE+ standard. The measurements used in the standard are based on the installation of a 100-cable bundle, with power through all pairs and cables in series to each other. Shielded cables have better heat dissipation than unshielded ones, even if both are in the same category.
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Heating is caused by power losses in the cable. In PoE+, for example, 30W is supplied but only 25.5W of power is received at the powered device, which means a power loss of 4.5W in the Ethernet cable. Given that one of the aims of PoE is to improve energy efficiency, such power losses need to be minimised. In addition, data attenuation increases in line with cable temperature. This imposes a maximum limit on the length of PoE cable possible for a defined amount of data transmission. A further limitation comes from the number of wiring pairs used to power devices. While the standards for PoE and PoE+ recommend 2-pair for power and the other 2-pair used for data. There is the opportunity to use 4-pair for both power and data, which will increase the supplied power to 60W with higher efficiency.
Solutions for Ethernet cabling infrastructure Not all cables have the same characteristics; they perform differently under different conditions. Prysmian Group carried out a series of laboratory tests, in accordance with IEC 46C/912/PAS, on a variety of its Draka branded Ethernet cables in order to assess their performance in providing PoE. The results are outlined below.
Heat increase Figure 2: Increase in installed cable temperature compared to electrical current for 100-cable bundle
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The graph, Figure 2, shows that with increased power comes increased heat. The inefficiency of Cat5e cable is indicated by it having the highest temperature levels. At 600mA, the maximum current flow for PoE+ Cat 6A cable heats up by almost 73% more than the Cat7A cable. In fact, the Cat7A cable has 42% better heat dissipation than the Cat6A and 50% better than Cat 5e. The maximum current carrying capacity based on a 10°C temperature increase for Cat6A cable is 900mA, and 1250mA for the Cat7A. When considering Next Generation PoE with a current of 1A per pair, Cat7A is still capable of carrying this level of current while remaining below the 10°C temperature increase, while Cat6A is not compatible.
Power dissipation The increased focus on energy efficiency means the amount of power lost in cables is becoming increasingly significant as energy prices continue to rise. Figure 3: Power dissipation for PoE and PoE+ for different installed cable categories
The graph shows that for PoE+ power levels the Cat7A cable will save 38% more energy than the Cat6A cable and 42% more than the Cat5e. The more power dissipated in the cabling the shorter the maximum possible distance to a powered device. Based on the graph, a PoE+, Cat5e cable will reduce a 30W power supply to 25.5W, whereas a Cat7A will only reduce the power to 27.7W. In addition to increased energy efficiency the Cat 7 A cable also has more power dissipation headroom, which makes it capable of transmitting higher levels of current.
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Figure 4: The maximum possible current levels keeping to PoE and PoE+ specifications for different categories of installed cable.
While IT network components are changed or updated once every 3 to 4 years, the cabling infrastructure to support those components will have to last considerably longer, perhaps as long as a decade-and-a-half. It makes sense, therefore, to keep power efficiency as high as possible while ensuring the installation is compatible with future changes. This can be achieved by investing in a Cat7A infrastructure to ensure future resilience and compatibility.
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Tayfun Eren, Technical Sales Support, Draka Tayfun Eren supports Technical Sales in the UK, Turkey and ASEAN regions of Prysmian Group’s Multimedia Solutions business unit. Tayfun has experience in R&D, operations and sales in telecommunications and electronics industries. His responsibility within Draka is centred on product management and sales support in connectivity solutions as well as copper and fibre optic cables. Tayfun holds a Masters Degree in communications and electronics engineering.
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