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Abstract Power over Ethernet (PoE) allows system designers greater flexibility for structuring Local Area Networks. However, the ability to provide power via the data cable plant is not without specific challenges and requirements. This paper explains the history and mechanics of PoE, examines some of the design and implementation issues and presents findings that demonstrate the superior capabilities of Berk-Tek’s LANmarkTM2000 for supporting current and future PoE applications.
Introduction to PoE The convergence of evolving Ethernet-based applications on today’s network is demanding increased performance from the installed cabling infrastructure and its components. Applications that were once installed over separate, proprietary systems are now being designed to run over the same network as voice and data, utilizing common backbone and horizontal cabling. Some of these applications include security (CCTV and access control), telephony over the Internet (Voice over IP) and even other electronic applications, such as building controls (lighting and HVAC). An additional trend is the practice of supplying electrical power to these IP-enabled/LAN connected devices using the structured cabling network. Powering equipment using the physical cabling plant is known as Power over Ethernet (PoE). One of the biggest benefits for PoE is centralized control and management of the devices over the installed voice and data network.
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The ability to supply power directly to each network element over the data cables increases design flexibility. It is no longer necessary to install separate power cables, conduit and AC outlets for each device, which adds up to significant time and cost savings. Examples of equipment that may use PoE include phones, security cameras and wireless access points. However, once power is supplied over the same low-voltage cabling as voice and data, a whole new set of rules and standards needs to be put in place to assure safety and proper performance of the Ethernet-based cabling. Trying to make sense of the continual stream of new IP-enabled products and the associated cable performance differences creates an interesting challenge to system integrators and network installers. Proper planning becomes increasingly important because without proper planning of the physical infrastructure to prepare for these added IP-based devices, the cable may be pushed beyond its limit and outdate the network before its time. A proactive approach starts with understanding the evolving applications and their affects on the cabling infrastructure from the backbone to the device or desktop. Standardizing in an evolving environment To integrate running power over the structured cabling system, industry standards have been created to govern both the electrical and physical characteristics for PoE applications. IEEE developed the 802.3af standard in 2003 to define the methodology for the provision of power via balanced cabling to connect Data Terminal Equipment (DTE). The DTE is the device that controls data flowing to or from the computer through Ethernet interfaces. The amount of power specified in this standard is limited by cabling physics and regulatory considerations. Because the 802.af standard specified compatibility with existing equipment, the transmission guidelines honed in on delivering power over Category 5e. This was due to the fact that most networks were running 10BASE-T or 100BASE-TX over this existing media.
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Ethernet-delivered power was originally standardized at 12.95 W. However, many evolving applications will require higher power to run over the existing network. This list includes: -
Dual band Access Points (approx. 20W)
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Pan Tilt Zoom Network Cameras (15W – 20W)
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IP Telephony video phones (15W – 25W)
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RFID Readers and Access Control Systems (up to 25 W)
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Point of Sales and Information Kiosks (13W – 60W)
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Laptops (up to 70W)
In September of 2005, IEEE authorized work to begin on enhancements to existing PoE specifications. The working group, called IEEE 802.3at, or PoE Plus, is developing the next-generation standard. PoE Plus is being designed to operate over existing ISO/IEC 11801-1995 Class D or higher cabling (TIA/EIA Category 5e or Category 6). The proposed standard will increase the powered device (PD) load to a minimum of 30 W, while conforming to the safety standards such as ISO/IEC 60950 (Safety Information for Information Technology Equipment). Cables must also meet the safety guidelines set forth in the National Electrical Code (NEC) 2005 Codes. This increased power will mean an increased amount of current. Increased current in a cable yields a temperature increase of the conductor. Usually the heat generated inside a cable containing insulated conductors is dissipated via conduction, convection and radiation through the insulation and jacket. High cable density can lead to added heat within the cable. This is usually found in heavily populated cable trays, equipment rooms and data centers. The industry trend to install higher-grade cables that utilize larger conductors, such as Category 6, should minimize the affect of the internal heat. In addition to complying with the safety requirements, cable performance, particularly attenuation, must be considered. If the current flow is very large, the cable may heat to a point that exceeds the maximum cable rating, usually 60°C for communications cabling.
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Heating the cable affects the cable’s electrical characteristics. For example, insertion loss increases as the temperature increases. The IEEE has requested technical data and guidance on these topics from ISO, TIA/EIA and ICEA. They are validating existing equations used to calculate heat production on the cable and connectors due to the current, developing new equations that explore the interaction between current and insertion loss, and working to measure heat transfer. PoE System Architecture In the PoE schematic, there are two types of Power Sourcing Equipment (PSE) to provide the DC power to each powered device. The PSE is either endspan or a midspan device. Power can be added to the Ethernet cable if the switch has a built-in power source known as “Endspan.” Endspan devices are often implemented when a new network is created and work in 10/100BASE-T (Ethernet and Fast Ethernet) channel configurations that utilize the two transmission pairs (Pins 1, 2 and 3, 6). Figure 1 shows examples of endspan PSE. Switch (device power source)
VoIP phone Outlet
Patch panel
Patch panel Webcam
Wireless access point
Figure 1: Endspan Power Sourcing Equipment
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Midspan equipment enables end users to utilize the existing infrastructure to deliver power. It is placed within the structured cabling system, between the switch and the PD, and sends power down the unused pairs (Pins 4, 5 and 7,8) to each end device for 10/100BASE-T. Switch VoIP phone Outlet Powered patch panel (device power source) Patch panel Webcam
Wireless access point
Figure 2: Mid-span equipment
PoE midspans offer a cost-effective solution for upgrading systems to IEEE 802.3at. Midspan, when designed correctly, does not require rationing of power among devices, as is found with an endspan switch that requires a larger power supply device. In a midspan environment, data is passively sent through the device. A single midspan may be used to support multiple PDs. There are basically two types of midspan devices: a power hub and a powered patch panel. A powered hub is placed between the switch and a passive patch panel. Patch cords from the switch to the hub are then matched with a corresponding port and patch cord out to the patch panel that subsequently provides power to the PD. A powered patch panel eliminates the hub and is connected directly from the switch to power the end device. Power is provided on the unused data pairs on the cable that is punched down into the powered patch panel (Figure 3).
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All IEEE 802.3af devices accept power from either midspan or endspan equipment even though different pairs are powered. This includes newer products that are designed to support 1000BASE-T and higher. Surveillance server
Switch
Powered patch panel (device power source) Patch panel Control Center
RFID Scanner Storage (H/D)
IP security cameras
Figure 3: An example of LAN Architecture for Mid-span PoE
Cabling Challenges The IEEE 802.3af standard specifies delivering power over the existing cable plant, namely Category 5, 5e, patch cables, patch-panels and connecting hardware. This standard allows either the spare pairs (10/100BASE-T) or the data pairs (1000BASE-T) to be used to carry the power. Only two out of the four pairs in Category 5e were originally used for data and the spares were used for power. While IEEE 802.3af was ratified after gigabit Ethernet, it was decided to not support 1000BASE-T, and to instead save that work for a future standard. The wide spread adoption of Gigabit Ethernet today employs all four pairs over Category 5e or Category 6 for both data and power and creates new challenges for cable performance. PoE Whitepaper
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A crucial issue of deploying PoE is the uncertainty of running data over all four pairs simultaneously with higher power levels. The PoE Plus working group is studying fourpair transmission and the capability to transmit the necessary power from the switching equipment to each device or each port. For example, a 24-port Ethernet switch requires a total of approximately 370 W and power must be available to supply at least 30W to each port. With the increase in power critical electrical operational parameters, particularly attenuation, are affected when the cable’s conductors are heated, especially from the output of the PSE. IEEE 802.3af addressed these operational issues by requiring ports to comply with the requirements for a limited power source from the PSE of less than 100 W. Protection of the telecommunications wiring from overheating is currently achieved by limiting the maximum current, which is dependent on gauge size.
The LANmark-2000 Solution
As new power-hungry devices surface to take advantage of expanding PoE opportunities, the demand to define “high-power PoE” cable limitations is becoming critical. Ultimately, the cable will be relied upon as the solution to efficiently carry sufficient power, while delivering error-free data throughout the network. As compared to Category 6 standards, Berk-Tek’s LANmark-2000 was designed with lower insertion loss, improved crosstalk immunity, and improved resilience to high temperatures. With its larger conductor size, LANmark-2000 has a current carrying capacity of 2.9 A compared to 2.0 A currently specified over Category 5e. In addition, the precisely engineered twist rate results in improved crosstalk immunity, translating to an increased signal-to-noise ratio and therefore, improved data transmission properties.
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The insertion loss (or attenuation) is the degradation of the digital signal or a loss of amplitude of an electric signal, which directly affects the performance of the cable. A cable that has a high attenuation will operate poorly at high data rates, which is critical in the operation of evolving applications which require 100 Mb/s or higher. The added robustness of LANmark-2000’s physical characteristics, such as larger gauge and uniquely constructed jacketing compounds, contribute to improved electrical characteristics. Chart 1 shows the signal strength of Berk-Tek’s LANmark-1000 Category 6 and LANmark-2000 Enhanced Category 6 as compared to the industry standards’ specifications (TIA/EIA-568-B).
LANmark 1000 LANmark 2000 CAT 6
Chart 1: Insertion loss comparison
As previously discussed, a major issue with providing power through data cable is the potential for elevating cable temperature above ambient temperature and the associated power dissipation. Higher temperatures will affect attenuation, which has an integral impact over the entire length of the cable, especially a cable installed in plenum spaces that may span as much as 295 feet. Once again, LANmark-2000’s superior design and larger conductor enable it to outperform the standards, and it has been tested to transmit Ethernet packets without errors in elevated temperatures as high as 70°C (158°F).
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Conclusion Structured cabling has become the core of all local area network (LAN) systems. When new services are added to the network, the reliability of the installed cabling can be degraded. Because the electrical and physical characteristics will affect the overall system performance, ongoing research and tests of the cable’s properties need constant exploration. When implementing a PoE system, voltage, bandwidth, current carrying capacity, temperature levels, and distance become significant parameters for cable selection. Whereas, Category 5e might have been suitable for low-frequency data and power operations two years ago, newer PDs and PSEs will require a higher performance cable, such as enhanced Category 6. With the objectives of the PoE Plus, which include pursuing the support of operations for 1000BASE-T and 10GBASE-T while evaluating the effects on cable heating, today’s solution is to install a cable that will provide the lowest insertion loss while maintaining consistent system performance. The larger gauge size of the LANmark-2000 combined with the unique separation of pairs improves the current-carrying capacity and resilience to higher temperatures, resulting in greater performance than standard Category 6 and superior performance to Category 5e.
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Data Communications Competence Center Nexans’ Data Communications Competence Center, located at the Berk-Tek Headquarters in New Holland, Pennsylvania, focuses on advanced product design, applications and materials development for networking and data communication cabling solutions. The Advanced Design and Applications team uses state-of-the-art, proprietary testing and modeling tools to translate emerging network requirements into new cabling solutions. The Advanced Materials Development and Advanced Manufacturing Processes teams utilize sophisticated analytical capabilities that facilitate the design of superior materials and processes. The Standardization and Technology group analyzes leading edge and emerging technologies and coordinates data communication standardization efforts to continuously refine Nexans’ Technology Roadmap. An international team of experts in the fields of cable, connectors, materials, networking, standards, communications and testing supports the competence center. The competence center laboratories are a part of an extensive global R&D network that includes nine competence centers and the Nexans Research Center headquartered in Lyon, France.
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132 White Oak Road, New Holland, PA 17557 - USA Tel: 717-354-6200 - Fax: 717-354-7944 - www.nexans.com PoE Whitepaper
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