The ability to send electrical current over twisted-pair copper data cabling through technology known as Power over Ethernet (PoE) presents cabling-system users the opportunity to achieve network-design flexibility and cost savings. At the same time, the technology’s availability and the practical implications of its deployment raise questions for many who are considering deploying it.
This confluence of factors was the setting for a seminar on PoE delivered via the Web on September 28. The seminar, hosted by Cabling Installation & Maintenance, included presentations from Mike Posey of Belden CDT (www.beldenibdn.com), Michael Pula of Panduit (www.panduit.com), Sev Sadura of Transition Networks (www.transition.com), and John Schmidt of ADC (www.adc.com). Each presenter approached PoE from a different angle, yet they all referenced the same few “killer applications” that are expected to drive PoE use in the years ahead.
Chief among those killer apps is Voice over Internet Protocol (VoIP). VoIP appears to hold great promise for the PoE market because a VoIP system does not supply power to a telephone the way a POTS line does.
The two other applications most frequently mentioned in the same breath as PoE are network cameras and wireless local area network (LAN) access points (APs).
Posey went through some basics of the 802.3af standard, explaining that PoE injects balanced power on two pairs of Category 5, Category 5e, or Category 6 UTP cable. Power sourcing equipment (PSE) comes in two types: endspan and midspan. An endspan PSE, which for all intents and purposes is a PoE-enabled network switch, sends power down the same copper pairs that carry the data traffic. Midspan PSEs, which are placed within-and in fact become part of-a structured cabling system, send power down the normally unused pairs of a UTP cable. PSEs have the capability to send a minimum of 15.4 Watts of direct-current (DC) power, which because of loss ends up supplying the powered device (PD) with 12.95 Watts.
Those 12.95 Watts may not always be sufficient to fuel some power-intensive network devices, from laptop computers to the most-sophisticated network cameras and even the latest feature-rich VoIP phones. So the IEEE has convened the 802.3at Task Force to study what is being called PoE Plus, which will be based on the same technical concepts as the original PoE but will enable the distribution of more Watts of DC power over twisted-pair cable.
Panduit’s Michael Pula focused his presentation on the power patch panel.A cabling system that includes a midspan PSE patch panel differs from a traditional system in some obvious ways (sending power on pins 4-5 and 7-8 in addition to the data on pins 1-2 and 3-6), and some not-so-obvious ways (modified test procedures).
Pula spent time specifically addressing the manner in which PoE-enabled cabling systems should be tested. Before testing powered ports, he advised, set the wiremap function to recognize the Ethernet pins ,1-2 and 3-6,while ignoring the powered pairs 4-5 and 7-8. Test all ports using the two-pair PoE wiremap, and analyze characteristics including attenuation, crosstalk, and return loss during permanent-link testing.
Transition Networks’ Sadura introduced the concept of using media conversion as part of a PoE strategy. Because PoE systems are deployed within traditional structured cabling systems, he explained, they are subject to the 100-meter distance limitation defined in TIA/EIA-568B. The use of PoE-specific media converters makes any switch suitable for PoE applications, he said. Such products allow users to capitalize on the traditional benefits of media converters as distance-extension tools, and still allows them to send power to PDs at the system’s endpoints.
ADC’s Schmidt completed the Web-delivered seminar with a presentation comparing and contrasting midspan and endspan technologies, from usage and economic standpoints.
The most significant positive attributes of endspan systems, he said, are that they do not require the purchase or installation of any additional equipment other than the PoE-enabled switch, and that they work in channel configurations that have only two active pairs. Endspan has its downsides, including that by definition it requires a switch upgrade. Also, switches are inherently power-hungry and many PoE-enabled switches will require significantly larger power supplies than their non-PoE brethren. Implementing the forthcoming PoE Plus technology will require another switch upgrade, Schmidt advised; and PoE switches include the capability in each port, meaning there’s a good chance users will pay for PoE capability in ports that do not require it.
Turning his attention to midspan devices, he pointed out its position as a comparatively inexpensive method to upgrade existing installations and its ability to selectively deploy PoE, thereby not wasting investment in PoE-enabled ports. Additionally, midspan PoE technology represents straightforward installation, maintenance, and upgrade labor. And when designed correctly, it does not require rationing of power among devices. On the downside, midspan PoE requires all four pairs to be wired in the channel, and if not designed properly it can degrade channel performance.
This was the third Web-delivered seminar presented by Cabling Installation & Maintenance. All three seminars are archived and available for download at: www.cable-install.com.
-Patrick McLaughlin
