{"id":3698,"date":"2012-07-12T18:53:25","date_gmt":"2012-07-12T22:53:25","guid":{"rendered":"http:\/\/rbachnet.wwwmi3-ss40.a2hosted.com\/?p=3698"},"modified":"2021-07-29T16:36:34","modified_gmt":"2021-07-29T20:36:34","slug":"power-over-eternet-802-3at","status":"publish","type":"post","link":"https:\/\/rbach.net\/index.php\/power-over-eternet-802-3at\/","title":{"rendered":"Power Over Ethernet 802.3at"},"content":{"rendered":"

\"Power<\/a>A client recently asked what happened to the network design rule of thumb which said do not install data cables anywhere near electrical cables? The fear of cross-talk, interference, and corruption of the data traffic seems to have disappeared with power over Ethernet. He rightly pointed out that now it seems OK to mix data and power in the same cable going to a networked device. Power over Ethernet (PoE<\/a>) is similar in principle to the way that the copper wire pair that carries your POTS (Plain Old Telephone System<\/a>) telephone signals into your house also carries enough electricity (48v DC) from the telco Central Office to power the phone’s core elements of the headset, dial, and ringer. The latest PoE standard is 802.3at.<\/em><\/p>\n

See part 1 here<\/a>.<\/em><\/p>\n

\"can<\/a>PoE end devices (PD) became popular with the bean-counters because these devices can cut installation costs by as much as 90% over traditional powering techniques. With cost savings comes demands for new products however, the power limitation of 802.3af\u00a0 prevented full support of\u00a0 \u201chigh power\u201d devices via the industry-standard PoE. In September 2005, the IEEE<\/a> launched a task force to begin reviewing new PoE<\/a> specifications to enhance the IEEE 802.3af guidelines into a next-generation standard.<\/p>\n

The IEEE ratified the new PoE standard in 2009 called IEEE 802.3at-2009 PoE standard<\/a> (aka PoE+ or PoE plus<\/a>). Under the new standard Power Sourcing Equipment<\/a> (PSE) must provide at least 30.0 Watts at the PSE port and 25.5 watts continuous power to up to 100 meters from the PSE over Cat5e or better cable to each PD. This will allow PoE to address applications such as multi-radio wireless APs, Pan-Tilt-Zoom (PTZ) gimbal-mounted cameras, 802.16 subscriber stations<\/a>, 3G\/LTE femtocells, perimeter security equipment, videoconferencing products, thin clients<\/a>, and even some notebooks, as well as emerging applications. The new standard also provides full backward compatibility and interoperability to existing 802.3af compliant PSE\u2019s and PD\u2019s.<\/p>\n

\"802.11n<\/a>The widespread use of 802.11n<\/a> Wi-Fi technology has been one of 802.3at\u2019s drivers. The multiple-input\/multiple-output (MIMO<\/a>) architecture used by 802.11n technology requires more power than earlier technologies, like 802.11a and 802.11b\/g.<\/p>\n

Installation practices and cable type impact the ability to use PoE+ to power a PD. Installation practices are outside the scope of the IEEE 802.3at standard. The Telecommunications Industry Association<\/a> (TIA<\/a>) has addressed these issues in a Telecommunications Systems Bulletin (TSB). TSB-184<\/a>, Guidelines for Supporting Power Delivery Over Balanced Twisted-Pair Cabling<\/em> describes how the impact of environmental conditions of the installed cabling and heat dissipation profiles of the cables’ effect the telecommunications cabling capacity to support the PoE+ application.<\/p>\n

But Wait There’s More<\/h3>\n

\"IEEE<\/a>One of the most important benefits of the IEEE 802.3at standard is it incorporates existing 802.3af PoE safety requirements. The new standard includes compliance detection features for safe powering as well as safe PD disconnection in overload, short-circuit or under-load conditions enabling fast and safe adoption of the new standard.<\/p>\n

The definition of a powered device changed under the IEEE 802.3at-2009 standard. The new standard considers the PD to be the powered interface, as opposed to the entire device being powered. This means that one device can have two power interfaces, each taking 25.5W, inside the same box. Nothing precludes these to be connected\u2014one over the two pairs using lines 1, 2, 3, and 6 and the other over the two pairs using lines 4, 5, 7, and 8. This is what makes it possible to double the standard 802.3at-2009 maximum of 25W and go up to 51W while fully complying with the standard. Applications that need 51W PoE are an exterior IP camera with a heater or an access control system that includes a controller, a reader, and a few door locks which can easily consume 45W of power or more.<\/p>\n

\"POE<\/a>Another way PoE system developers are addressing the growing device powering requirements is by employing the option in the IEEE standard to add PoE<\/a> functionality over all four pairs of the Ethernet cable. This option opens the door for safely delivering 60 watts of direct current (DC) power over a single Ethernet cable, using current levels of 600 milliamps (mA) than the 1.2 Amp level of two-pair 60W midspans.<\/p>\n

The manufacturers claim this configuration can cut power consumption by 15% compared to two-pair solutions. This could translate into savings of about $25 per year per powered device, assuming energy costs of $0.10 per kilowatt-hour.<\/p>\n

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