What’s the Difference in Power over Ethernet—Now and Then?


Since the ratification of the first Power over Ethernet (PoE) standard in 2003, PoE use has increased dramatically and made headway into new applications. PoE provides huge benefits in relation to ease of installation, saving CAPEX and OPEX costs, and providing a unified and safe power standard for worldwide use.

Today there’s a demand for even higher power. In particular, new IoT and Big Data applications enabled by next-generation 5G technology are driving unprecedented demand for connecting additional types of powered devices (PDs) to Ethernet networks, including IP surveillance cameras, 802.11ac and 802.11ax access points, LED luminaires, 5G small cells, and other IoT appliances.

The biggest differences between PoE now and in years past involve power limits and data rates. The latest technology and standards push the power limit of power sourcing equipment (PSE) and PDs to 90 W and 71 W, respectively, and the supported data rate can be up to 10 Gb/s.

In the past, PoE was only required to deliver 15.4 W at the power source, which was ample for most IP phones and 802.11a/b/g access points. Accordingly, when IEEE 802.3at was released in 2009, the Institute of Electrical and Electronic Engineers (IEEE) specified 30 W at the PoE source.

Following these early solutions and concurrent with the IEEE’s work on today’s latest technology and standards, there were parallel efforts to increase the amount of power that could be delivered to PDs. These efforts were initiated with the IEEE 802.3af-2003 PoE standard, which provided up to 15.4 W of output power to each device over two pairs of Category 5e (Cat5e) cables.

The next step was the IEEE 802.3at-2009 standard, also known as PoE+, which introduced the “Type 2” PSE/PD for supporting 30-W output power and 25.5-W load power. Then, the HDBaseT Alliance standardized the HDBaseT protocol, which extends HDMI links up to 100 m over Cat5e or better cables. Finally, the HDBaseT Alliance in 2011 created the Power over HDBaseT (PoH) standard, extending the maximum power that can be delivered over four pairs to 95 W.

IEEE 802.3bt

Today, a growing variety of PDs needs even more power than can be supported with these earlier standards.  To meet these needs, the new IEEE 802.3bt standard ratified in September 2018 utilizes all four pairs of the structured wiring. It also extends the power classification information exchanged during initial negotiation to allow for meaningful power-management capability. This makes it possible to support multiple PoE classes while also ensuring backward compatibility.

In addition to enabling PD input power to reach up to 95 W if channel length is known, today’s IEEE 802.3bt standard adds several other important new capabilities. Two new categories of Type 3 and Type 4 PSEs/PDs have been introduced, along with the ability to operate over four pairs. The standard also supports both the single- and dual-signature PD constructions and adds class 5 to 8 to help improve the mutual identification process. Automatic class functionality is included, and IEEE 802.3bt supports low standby power as well as 10GBase-T with PoE, too. The following table summarizes the new PoE capabilities provided by the ratified IEEE 802.3bt standard.

PD input power can reach up to 60 W for Type 3 and up to 90 W for Type 4 PSE/PDs, as long as the channel length is known.

Power can’t exceed 100 W per port with the IEEE 802.3bt standard, ensuring compliance with ISO/IEC 60950 limited power source and Safety Extra Low Voltage (SELV) requirements. This power level is still adequate for applications that previously could not be supported with prior IEEE standards, opening the door to a larger number of PoE port deployments.

Finally, today’s IEEE 802.3bt specifications differ from yesterday’s PoE standards by ensuring that legacy Type 1 and Type 2 devices will work automatically with the IEEE 802.3bt system. This is true as long as the PSE and PD are both standard-compliant, and the PSE has the necessary power capability. If the PD needs the higher power offered by IEEE 802.3bt. and the PSE (i.e., an IEEE 802.3af/at PSE) can’t support it, the PD will either remain off or it will turn on but only draw the power that is available from the PSE.

PoE Chipsets

As an example of how this works, Microchip offers a PSE chipset that enables pre-standard switches to interoperate with new IEEE 802.3bt-2018-compliant products. The chipset is based on the company’s earlier PSE chipset for implementing the widely adopted PoH four-pair power standard for 95-W PDs. It also forms the foundation for its IEEE 802.3bt-2018-compliant PoE injectors and midspans that are installed between PDs and an existing switch.

Such systems solve the interoperability challenge for users, enabling them to power any mixture of pre-standard and IEEE 802.3bt-2018-compliant PDs. With their choice of single- and multi-port options, the IEEE 802.3bt-compliant switches can be used to power pre-standard PDs.

Today’s PoE chipsets give system developers the scalability to create a single-board design containing both the two- and four-pair systems required to support pre-standard PoE as well as the latest IEEE 802.3bt-2018-compliant capabilities. The chipsets balance thermal dissipation evenly across the system and include the necessary manager and controller functionality for building PSE equipment that can source 90 to 99.9 W of power per port.

At the same time, this equipment can support up to 48 ports for IEEE 802.3bt Type 3 (Classes 1-6) and Type 4 (Classes 7-8) applications. All that’s required is a software update to upgrade them from earlier standards to IEEE 802.3bt. 

Another difference between yesterday’s and today’s PoE solutions is their ability to protect PDs against reverse-polarity connections while reducing power space and the costs associated with delivering IEEE 802.3bt Type 4 Class 8 power. This is accomplished by using full-bridge rectifier devices on the powered side of the PoE connection.

Conclusion

Much has been achieved in the nearly two decades since the first PoE standards were ratified. Today’s PoE level of 90 W delivered over four pairs of Cat5e cables and above is expected to be the maximum defined by the IEEE. It’s generally agreed that higher levels may not be safe for today’s existing infrastructure of cabling and connectors. The IEEE 802.3bt standard will replace all of today’s existing pre-standard solutions that deliver 60 W/75 W/95 W, including those supporting UPOE or 4Pairs PoE.  

As these suppliers move forward with their respective product roadmaps, they will have the confidence that, with the proper implementation, their pre-standard and new IEEE 802.3bt-2018-compliant PDs can coexist in the same Ethernet infrastructure without the need to make any changes to existing switches or cabling.

Galit Mendelson is Senior Manager, Marketing and Business Development, at Microchip Technology.



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