Emerging PoE standard aiming for 49W per-port minimum

July 16, 2014
The high-power promise of IEEE 802.3bt carries with it certain demands of a network's twisted-pair cabling infrastructure.

From the July, 2014 Issue of Cabling Installation & Maintenance Magazine

The high-power promise of IEEE 802.3bt carries with it certain demands of a network's twisted-pair cabling infrastructure.

The Institute of Electrical and Electronics Engineers' 802.3bt Task Force (www.ieee802.org/3/bt/) currently is working to specify a set of next-generation Power over Ethernet (PoE) specifications, and the levels of power likely to be delivered ultimately via the 802.3bt standard will require certain physical and electrical-performance characteristics of the twisted-pair cabling carrying that power. This article reviews some of the current available information about the development of IEEE 802.3bt and discusses technological advancements that have given rise to this standard-development effort.

When the IEEE formed a study group in early 2013 to examine the possibility of a four-pair PoE specification, it said the group would "consider a four-pair solution's capability to enhance energy efficiency and provide greater than 25.5 watts of power in improving PoE." The group's chair, Chad Jones, added, "PoE has become the dominant powering method in many Ethernet-based products-access points and IP phones, for example-demonstrating that customers will migrate toward convenience. Enabling applications beyond 25.5 watts will extend this convenience to other products. An additional benefit is the reduction of power lost in the delivery channel by at least half. This benefit can be extended to existing powered products by moving from a two-pair powering system to a new four-pair powering system."

Since that time, the effort has received a project authorization request (PAR) approval and is now officially in development as 802.3bt. Also since then, research into the effects of distributing more power over twisted-pair cabling has been undertaken, incorporating cabling-system vendors into the development process of 802.3bt.

A brief history

In a couple recent web-delivered seminars-one hosted by Cabling Installation & Maintenance and one hosted by Berk-Tek, a Nexans company (www.berktek.com)-the technical manager of Nexans' advanced design and applications lab, Paul Vanderlaan, detailed both the background and the current status of PoE specifications and technology. When the original PoE standard, IEEE 802.3af, was ratified in 2003 it specified the delivery of approximately 13 watts of power, with 350 milliamps, over Category 3 or Category 5 cabling, Vanderlaan explained. The specification for Category 3 or 5 was in keeping with the IEEE's consistent efforts to leverage the installed base of cabling and, to the extent possible, develop applications that can run over that installed base.

Vanderlaan pointed out the two different means of supplying power per 802.3af, called Alternative A and Alternative B. Each of these alternatives used two of the cable's four pairs for power transmission.

IEEE 802.3at (commonly referred to as "PoE Plus" but not officially called that in the standard) was ratified in 2009. Along with increasing power delivery to 25.5 watts with 600 mA, the PoE Plus standard also raised the minimum cabling-performance requirement to Category 5. By 2009 1000Base-T had taken firm hold in the market, and that form of Gigabit Ethernet implements bidirectional transmission over all four pairs of a cable. Yet, Vanderlaan explained, the two alternatives for delivering power specified in 802.3at changed little from what they were in 802.3af. Both PoE Plus power-delivery options use two of the cable's four pairs.

Efforts at efficiency

As Chad Jones pointed out, research has shown that a PoE variant using all four pairs to deliver power is more efficient than the currently standardized two-pair variants in the "af" and "at" standards. In the aforementioned web seminars, Vanderlaan provided some quantifications and real-life consequences of the use of two- or four-pair powering. "Take the example of a typical device," he said, "that consumes 8 watts of power with an average link length of 40 meters, operating continuously throughout the course of a year [8,760 hours]. If there are 100 million of these devices deployed throughout the world, the potential energy savings would be 60.8 million kilowatt hours."

When one considers that most electricity is generated by coal-fired plants, the 60.8 million kilowatts saved annually equates to more than 66 million pounds of coal saved annually. This research and calculations like this one have motivated the IEEE to establish four-pair powering as one of its objectives for the 802.3bt standard.

The other objectives for the "bt" standard are: to be backward-compatible with "af" and "at"; to support 10GBase-T operation (which, like 1000Base-T, uses four-pair full-duplex transmission); to specify parameters to limit pair-to-pair current imbalance; and to supply a minimum of 49 watts at the powered device. More on that 49-watt minimum shortly.

Technology marches on

Technology developers have not sat idly since the ratification of 802.3at in 2009, patiently waiting for the next PoE spec from the IEEE. Rather, several have brought to market device-powering systems that are not standardized by the IEEE but have been developed to meet users' current needs.

Probably most prominent among these systems is Cisco's Universal Power Over Ethernet (UPOE) technology, which delivers 60 watts of power to devices and was introduced in mid-2011. At that time Cisco explained that using the UPOE linecard and splitter, any device that requires up to 60 watts of power can be powered by the Catalyst 4500E; some of those devices include Cisco IP phones, personal telepresence systems, compact switches and wireless access points.

More recently the company produced a use-case for UPOE, telling the story of the system's deployment in the Mandarin Orient Hotel in Boston, MA. In 2012 the hotel was "in search for a new technology provider to help upgrade its legacy networking infrastructure and increase power across the property," Cisco explained. "The hotel called for two color-displayed phones and an access point to remain available during a power outage that could occur on all floors."

The UPOE system deployed at Mandarin Oriental Boston enables in-room switching while maintaining power over the switch port for up to 60 watts, Cisco further stated. "Cisco UPOE allows connectivity to Ethernet-powered devices … without having to depend on external electric power supplies. This technology also meets the 60-watt power requirements set forth by Mandarin Oriental Boston, allowing the property to keep one access point and multiple color-display phones per guestroom running at all times, even in a power outage."

The concern about the system's ability to remain up even during a power outage emanates in large part from the hotel's experience during a previous citywide outage when, as Cisco recalled, "the previous switching structure left guests without Internet or phone access … Guests had no means of communication, leaving them concerned since the cause of the power outage was unknown."

As fate would have it, three weeks after the UPOE installation was complete, the city of Boston experienced another power outage. "The hotel successfully kept phone and Internet access available for guests due to the completion of the UPOE project," Cisco concluded.

In addition to the specific systems mentioned by Cisco in the Mandarin Oriental Boston deployment, devices used in many other environments, systems and applications also require more than PoE Plus's 25.5 watts of power. From nurse-call systems in hospitals to point-of-sale devices in retail shops, multichannel wireless access points and a number of others, networked devices in many spheres also have served as an impetus for the development of 802.3bt.

The year 2011 also saw the introduction of Power Over HDBase-T by the HDBase-T Alliance (www.hdbaset.org), an organization that describes itself as a "crossindustry alliance formed to promote and standardize HDBase-T technology for whole-home distribution of uncompressed high-definition (HD) multimedia content." In September 2011 the alliance announced the publication of the Power Over HDBase-T (POH) specifications as an addendum to its HDBase-T 1.0 specification.

The alliance stated at that time that POH technology "delivers up to 100 watts of power to TVs and other devices over distances up to 100 meters/320 feet via a single Category 5e or 6 cable with standard RJ45 connectors. This means it is no longer necessary to connect a device's electrical jack to a power supply." It added that among the characteristics of POH is it significantly reduces overall network power consumption.

To recap, a lot happened since the ratification of IEEE 802.3at in 2009-Cisco's UPOE as well as the HDBase-T Alliance's POH technologies were brought to market, and consumer behavior put increasing numbers of devices requiring more (sometimes far more) than 25.5 watts of power into deployment. Against this backdrop, the IEEE has set a 49-watt minimum support level, but has not yet established a ceiling for wattage support with the in-development 802.3bt specification.

The cable matters

The 802.3bt objective of supporting 10GBase-T is evidence that a high-performance twisted-pair cabling system will be required. Nexans' Vanderlaan as well as other cabling-industry technical experts have explained that the considerations for cable type go beyond supporting 10G. And as the standard is a work-in-progress, current iterations are subject to change. Nonetheless, the stated desire to support 10GBase-T does not clinch the notion that Category 6A will be a minimum requirement. Issues related to cabling performance required to support networking applications are not so much the focus of the IEEE as they are standards-development groups like the TIA or ISO/IEC. That is the case here as well.

In January 2014 CommScope (www.commscope.com) published a white paper titled "Laying the groundwork for a new level of Power over Ethernet." In the 12-page paper, CommScope explains, "According to the current scope of the four-pair PoE discussion, all cabling must, at a minimum, meet the performance requirements for Category 5e cabling over a 100-meter channel, including a worst-case scenario of four connections. It should be noted that Category 5e cabling only provides the minimum level of performance required. Therefore, it is recommended to use Category 6 or Category 6A cabling-preferably solutions … that have been tested for compliance to the corresponding Category or Class per ANSI/TIA-568 or ISO/IEC 11801."

It looks like several of a cable's physical characteristics take on particular importance regarding 802.3bt. Vanderlaan simplified the complex electrical-engineering considerations when he explained, "As a general rule, increased copper content, or larger gauge size, will aid in power delivery. That's not always the case, but in general it can be said that when you migrate to a higher grade of cabling-from Category 5e to Category 6 to Category 6A-you should see larger gauge sizes and more copper content."

Users who expect to deploy 802.3bt and take advantage of its powering capabilities may have to pay attention to cabling-system performance characteristics that they previously did not. Specifically, characteristics like DC resistance unbalance and pair-to-pair resistance imbalance are under scrutiny as 802.3bt takes shape. The higher wattages covered by the forthcoming standard will be accompanied by higher amperages; some foresee the specification of 800 milliamps to 1 full amp (1,000 milliamps) for 802.3bt. These higher amperages will contribute to the challenge cabling systems will have in order to meet performance requirements for characteristics like DC resistance unbalance and pair-to-pair resistance imbalance.

Vanderlaan summed up: "For users, cable selection will be based not just on the speed that can be supported, but rather on speed as well as power delivery. What you simply plug in today, you may want to also power in the future."

The generation of heat within bundles of cables supporting IEEE 802.3bt also is under investigation. CommScope's paper noted, "In order to minimize cooling costs and maximize the usable lifespan of the cabling infrastructure, it is important to take into account the thermal load on the cabling. When remote power is applied to balanced cabling, the temperature of the cabling will rise due to heat generation in the copper conductors … Depending on the installation conditions, the heat generated will be dissipated into the surrounding environment until a steady state is reached with the temperature of the cable bundle higher than the ambient temperature of the surrounding environment. It is important that the temperature of any single cable in the bundle does not exceed the temperature rating for the cable.

"The IEEE 802.3bt four-pair PoE standard is expected to assume a maximum temperature rise of 10 degrees Celsius when all four pairs are energized. For cabling with an operational temperature range of -20 degrees Celsius to 60 degrees Celsius, the ambient temperature should not exceed 50 degrees Celsius. Using a higher category cable with lower DC resistance and improved heat dissipation can help reduce the rise in temperature. Consequently, CommScope recommends Category 6A cabling for four-pair PoE applications. Because increased thermal loading can also increase insertion loss, the maximum cable length should be de-rated for higher temperatures, per ANSI/TIA-568-C.2."

(In April of this year we published an article focused exclusively on the topic of cabling options for network-device powering. That article, authored by Siemon's Valerie Maguire, emphasized the benefits of using shielded cabling systems in these applications. You can read the full article, titled "Advantages of using higher-quality shielded cabling to power remote network devices," beginning on page 19 of our April 2014 issue.)

Ultimately the published IEEE 802.3bt standard will provide a means by which many more networked devices can be powered via IEEE-standardized Power over Ethernet technology than is currently the case. In the meantime, cabling-system owners and managers would be well-served to examine their current infrastructure with eyes toward the devices supported, and whether or not the next generation of those devices might be powered via more-capable PoE technology. Additionally, it is worth asking questions about the installed base of cable, and the likelihood that cable has the electrical-performance characteristics required to support a next generation of PoE.

Patrick McLaughlin is our chief editor.

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