While many users took advantage of the cost-effective benefits of Power over Ethernet (PoE), 12.95W maximum of af is not enough power to serve all the applications and implementations. The IEEE 802.3at task force was formed to create an improved Power over Ethernet standard, building on the original IEEE 802.3af specifications. Designated PoEPlus, it will more than double the wattage available to powered devices, delivering up to 30W of power and possibly even 45W. If accepted as a new IEEE standard, PoEPlus will allow increased power to many more devices, enhancing their capabilities, such as those used for wireless multi-radio access points, Pan Tilt Zoom security cameras, streaming video, VoIP (define - news -alerts), and IP security
The most recent focus of the task forces debate has centered on whether a two-pair solution is feasible, or if a four-pair solution will be necessary in providing enough power for high power applications such as multi-band access points and controllable security cameras. The greatest concern is rising temperatures in the cabling which can cause permanent deformity and eventual data degradation.
At the task forces January 12th meeting, a proposal was approved stating the groups objectives can most likely be met without having to resort to four-pair powering, and that the members would focus on developing two-pair solutions, which are lower in cost and easier to implement when compared to the alternative. The current proposal is to use a source voltage from the power sourcing equipment (PSE) of 5157V with 650mA on Category 5 or better wire. The drop per pair total in the worst case scenario of 100m cable and connectors is 12.5ohms. This will provide at least 30W at the power device with a voltage range of 4057. While these values are not final and still subject to additional thermal dissipation work being conducted by IEEEs cabling working group, what makes the two-pair solution both viable and preferable?
Why is PoE being implemented?
PoE technology offers an innovative solution to systems designers in eliminating the need for local power outlets or in getting power to a device without calling an electrician. PoE allows power and data to be carried over a single Ethernet cable, allowing for greater flexibility in office design, higher efficiency in systems design, and faster turnaround time in set-up and implementation.
To date, the most pervasive applications have been wireless access points and IP phones, for which PoE provides a single cable connection as well as a single point of backup for a UPS in case of power failure.
Some applications, however, require significantly more power, and the original IEEE802.3af specification limits the user to 350mA due to cable and patch panel limitations of Category 5 wiring systems. This was done as a safety precaution, even though several demonstrations showed drawing up to 750mA was possible with no safety hazard. Since that original specification was written, Category 5e and 6 cabling systems have become more common.
While manufacturers have been able to develop proprietary PoE products that meet the growing demand for higher power, users would prefer that all products meet a single interoperable standard that provides for those high power applications specifically.
Why High Power?
The current IEEE802.3 standard only allows for power up to 12.95W to be used by a powered device (PD). Because of the significant advantages of implementing PoE, many users would like to make the switch but require even higher power. For example, several manufacturers of VoIP phones have been adding features such as large color screens capable of providing streaming video (which greatly benefit from the use of PoE) typically require 24W and the new broadband wireless access points designed to meet IEEE802.3N and support A/B/G standards require up to 25W.
Today several manufacturers are delivering proprietary systems that are delivering PoE from 30W up to 75W. The IEEE802.3at working group is in the process of re-defining a standard to increase the power and provide full interoperability. Defining specifications for higher power will allow a whole new group of applications to take advantage of PoE.
Four-Pair Versus Two-Pair
The current standard delivers the power over two pairs of the Ethernet cable. To increase the power capability you can either increase the power delivered by two pairs, use all four pairs of the cable, or both. The constraining issues restricting how much power you can deliver on the cable are safety concerns and the cable system itself.
The safety issues are pretty straightforward. The IEC 60950 safety standard for information technology limits voltage and power for any electrical circuit that a user can touch to 60Vdc maximum and 100VA. This would mean the maximum limits would be 100Watts delivered from the source. With practical system and cable drops this would give the PD approximately 75 watts. This is enough power to run most laptops and LCD displays up to 17.
Cable System Limits
The task forces cabling working group is currently testing the viability of the two-pair solution, testing 196 cables bundled in a conduit and measuring the temperature rise in all the conductors when current runs through them. This testing does not reflect a safety concern or a fire hazard, but the cables can permanently deform above certain temperatures, causing data attenuation. It is anticipated that the Cat 5e and 6 wire is going to be rated for 760mA per conductor. The real problem is not the current but the temperature rise in a building conduit with a worst case of all the conductors delivering high power.
The cable is rated for 60 C. Actually, the problem is not a fire hazard but that the cables become soft above 60C and data properties degrade permanently. The ongoing testing will verify temperature rise and practical limits. The conclusion is still pending test reports, but for the lowest drops for the same current it seems it would favor a four-pair system.
The real issue is going to be the hotspot temperatures at the center of bundled cables. In addition, some significant analysis of the connectors has been done and it appears from initial results that the connectors are capable of handling over 420mA per conductor without degradation.
Complexity for Four-Pair Systems
It is common, in some countries, for installations to use one cable to distribute two 10/100 ports. This is obviously not suitable for four-pair powering and adds a level of idiot-proof detection before allowing pair power. To ensure power is not applied to wires that are not designed to receive it, the new systems need to detect if the four-pairs are connected to the same source.
Another issue with the four-pairs that adds an additional level of complexity is the requirement to share the current between multiple conductors. Even if you add current sharing, it is essential to current share in all conductors, especially as they are generally all derived from the same power supply. Due to differences in impedances, especially with short cables, all the current may go down one path. Connected to this you need to detect if one of the connections is broken, otherwise all the current is draw down the one path. All of these issues can be solved but add the complexity of cost.
Practical Power Possibilities
Although there are several examples of systems that are delivering up to 75W safely and reliably over the Ethernet for individual applications, the standard that will cover a general application will need to set lower limits. From the work done so far, it is conservative to say that two-pair systems can deliver up to 30 watts over existing cable infrastructure and 60 watts for four-pair. Voltage delivered from the source will be 50-57V with current of 600mA.
What will the standard be?
An IEEE802.3at working group has decided to support a two-pair medium power standard and four-pair high power solution. The specification will still take some time to complete but the direction and objectives are clear. The task force has formed three adhoc groups, each comprised of leaders in the power implementation field, to define voltage limits, advanced classification and a management interface base. The groups will work to develop a pragmatic standard and ensure that their products, as well as those manufactured throughout the industry, comply to the standard as soon as it is released. IT
Keith Hopwood is vice president of sales and marketing at Phihong USA (news -alerts). For more information, please visit www.phihong.com.