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POWER OVER ETHERNET Evolving Power over Ethernet accommodates higher-power devices

Author / Editor: Nigel Charig / Johanna Erbacher

Power over Ethernet has long been popular for connecting network devices, as it eliminates the power cable connection. But how has PoE kept pace with the ever more power hungry devices essential to today’s networks? This article looks at the answer.

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Power over Ethernet (PoE) is a method of supplying network-compatible devices with power via the Ethernet cable.
Power over Ethernet (PoE) is a method of supplying network-compatible devices with power via the Ethernet cable.
(Source: gemeinfrei / Unsplash)

A conventional, cabled Ethernet device, such as a digital security camera, has two cable ports: one for the Ethernet data cable, the other for electrical power. However, an increasing number of applications are benefitting from a more simply-installed and attractive alternative – Power over Ethernet (PoE). As its name suggests, PoE technology allows power to be delivered to the device over the Ethernet cable, so eliminating any need for a separate power cable and power distribution infrastructure.

Advantages of Power over Ethernet

This approach has many advantages. Eliminating the power cable not only saves cost, space, wiring complexity, and installation time: with a 57 V operating voltage, PoE-enabled network cables do not need to be installed by a qualified electrician. This saves labour costs and allows IT departments to keep installations entirely under their control. Safety is further improved as PoE power delivery is intelligent, and designed to protect equipment from overload, underpowering, or incorrect installation.

IP devices can be located far more freely without being tethered to a mains outlet, and repositioned easily as required. Also, systems are more readily scaled when only the network infrastructure needs to be expanded. Meanwhile, reliability is improved, as PoE power comes from a central and universally compatible source, rather than a collection of distributed wall adapters. It can be backed-up by an uninterruptible power supply, or controlled to easily disable or reset devices.

However, these benefits have created challenges as well as opportunities for PoE. It was originally used with devices like static surveillance cameras, wireless access points and voice-only phones, with relatively modest power demands. However, as Ethernet data rates have increased, designers started applying it to higher-performance, more power-hungry equipment like video conferencing hardware, laptop computers, flatscreen TVs and IoT devices.

Today there is 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.

All of this has presented problems to PoE, as Ethernet cables were designed to carry data rather than power. Voltage drop and associated losses along the cables could become a real problem if not properly managed. In fact, the management has come from the IEEE 802.3 standard, which has evolved to specify increasing power levels.

Evolution of the IEEE802.3 PoE standard

Originally, 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. This was covered by IEEE 802.3af-2003 which specified that the 15.4 W dc power should be transmitted from the Power Sourcing Equipment (PSE) through two twisted pairs of Category 5e (Cat 5e) cable to the Powered Devices (PDs). Line losses were allowed to consume no more than 16 % of the power before reaching the powered device, delivering 12.95 W dc. PSEs and PDs operating at this level are known as Type 1 devices.

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, in 2018, the IEEE 802.3bt was ratified to support more powerful devices. It introduced 60 W Type 3 PSEs, powering 51 – 60 W Type 3 PDs, and 90 W Type 4 PSEs, powering 71 W – 90 W PDs, depending on losses.

All PDs are also specified by power class levels, currently from 1 to 8, based on how much power they require; this is to facilitate power management. When a PD is connected to a PSE, it provides its class to the PSE so that the PSE can supply the correct amount of power to it. Class 1, Class 2, and Class 3 devices require very low power, low power, and medium power, respectively. Class 4 and above devices require a high amount of power.

IEEE 802.3bt accommodates its enhanced power specification by utilizing all four pairs of the Cat 5e cable, and by extending the power classification information exchanged between PSEs and PDs during initial negotiation to allow for meaningful power management.

However, power cannot 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.

Another advantage of IEEE 802.3bt is backward compatibility. Legacy Type 1 and Type 2 devices will work automatically with the IEEE 802.3bt system, 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 while the PSE is an IEEE 802.3af/at type and cannot support it, the PD will either remain off or it will turn on but only draw the power that is available from the PSE.

Conclusion

It can be easy to think that, today, all networking is done wirelessly. Wireless is everywhere, from cellular to Wi-Fi, Bluetooth, and other local area networks. But this wireless trend has not replaced wired networks, with thousands of servers connected to PCs, Wi-Fi access points, VOIP phones, IP cameras, and the more powerful 5G and IoT applications as mentioned above.

Virtually every office building and workspace is pre-wired for Ethernet. Factories have continued to adopt the network for a wide range of industrial applications. A recent development is the inclusion of a special version of Ethernet for automotive equipment networking.

Overall, Ethernet still provides a reliable, secure, and flexible backbone to many facilities, and its future is assured with the enhanced performance capabilities of today’s PoE standards.

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