Ethernet—one of the world’s most pervasive data communications technology standards—is celebrating its 40th anniversary this year. Ethernet was conceived in 1973, commercially introduced in 1980 and first published as a draft IEEE 802.3 standard in 1983. In the hyperactive world of data-transmission technology, 40 years can be an eternity, and most standards would be well past their prime. But thanks to Ethernet’s robust “technology DNA,” it is as young and restless as ever.
Ethernet’s technology DNA is in reality the IEEE 802.3 Ethernet Working Group, which for decades has spawned dozens of study groups and task forces whose members have solved the problems and met the challenges of each new market mutation.
From its roots in local-area networks (LANs), Ethernet spread to wide-area networks (WANs) and then to metropolitan-area networks (MANs). Many new markets are helping to fuel the next inflection points including automotive and industrial, expanding access and core and data center technologies.
Ethernet’s evolutionary success is based on its ability to morph into new standards usually determined by transport media—cable, twisted pair and fiber—and to keep getting faster and faster; now, 10 Gb/s Ethernet is widely deployed. Standards are in place for 40 Gb/s and 100 Gb/s Ethernet. But Ethernet’s reach goes far beyond the types of transmission media and data rates.
Although everyone knows Ethernet transports data, it can also transport the energy used to power end equipment such as network hubs, closed-circuit TV cameras, wireless access points (APs), IP phones and other network devices.
One of the most common uses for Power over Ethernet (PoE) is to power IEEE 802.11 “Wi-Fi” access points (APs). An Ethernet switch conveniently provides a central connection point for powering all the APs. Using PoE also eliminates the need to plug in to a wall outlet.
PoE has been around for about 10 years. Implementing the original IEEE 802.3af-2003 standard can provide up to 15.4 W of 48-V nominal DC power to each device being powered. (Owing to power dissipation in the cabling, only 12.95 W is assured at the powered device.) The updated IEEE 802.3at-2009 standard (also known as “PoE+”) can provide up to 25.5 W of 48-V nominal DC power. The link length can be up to 100 m. The work of IEEE 802.3af and IEEE 802.3at are now part of the recently published IEEE 802.3-2012.
PoE is presently limited to using two pairs of the four sets of twisted-pair wiring found in Cat5/6 RJ-45 Ethernet cable. The powered device must operate with either the spare pairs (wires 4–5 and 7–8) or data pairs (1–2 and 3–6).
PoE is specified for 10Base-T and 100Base-TX—both of which use two twisted pairs for data transmission—and also for 1000Base-T, which uses all four pairs for data transmission. Presently, it is not specified for 10GBase-T links.
The standard defines two types of power sourcing equipment (PSE) devices for sourcing the power: endspans and midspans. Endspans are essentially Ethernet switches with PoE circuits added. They are common in new installations or when the Ethernet switch must be replaced anyway while upgrading to a higher-speed version of Ethernet. Midspans are not integrated into the switch but are positioned between it and the powered device (PD). They are typically used when PoE is the only upgrade being made to the network such as when adding IP Phones or wireless APs to an existing non-PoE network.
PoE of the Future
The extension of PoE to some new applications would require improvement in three general areas: providing higher power by energizing all four pairs, moving to higher efficiency and studying the feasibility of power transport with 10GBase-T Ethernet.
As PoE evolved from a fairly low-power source to being capable of delivering up to 25.5 W/port, cable losses increased exponentially. This loss becomes an important operating expense in applications that have many devices to power. The most effective means of improving efficiency is to inject power into all four pairs instead of just two.
Simple circuit calculations illustrate the advantages of injecting current into all four twisted pairs. Fundamentally, the result is achieved by injecting half the current into one pair and the other half into the other pair. (The same amount of power is sourced as in the two-pair solution, but more power is delivered thanks to improved efficiency.)
For the present two-pair PoE solution, cable loss power (PCL) can be calculated as
where R is channel resistance and I is the current carried by the two pairs
For a four-pair solution, the equation becomes
Assuming the current remains the same, the four-pair solution has about one-half the cable loss.
For systems with an identical load—for instance, when upgrading a device from two-pair (2P) powering to four-pair (4P) powering—the total current in a 4P solution is actually lower than the total current in a 2P solution. This is because the cable current is directly related to cable impedance (I = V/R), but cable power is exponentially related (P = I2 * R). Therefore, for existing installations, upgrading to 4P power is slightly more than 50 percent more efficient than a 2P powering system.
The actual amount of power saved depends on cable characteristics, but one practical example would be a 24-port IEEE 802.3at-2009 Type 2 system delivering 25.5 W per port over a cable run of 100 meters, in which more than 50 W can be saved.
Table 1 shows comprehensive picture of power savings. PSE and PD are the two devices involved in any PoE scenario. Two use cases are presented: the “average” PoE installation today, which does not require the maximum power available and has a cable run of about 40 meters, and the “worst” case, in which the maximum 25.5 W are required by the load and the maximum recommended 100-meter cable run is used.
Source: IEEE 802.3 Ethernet Working Group
Table 1. Cable power loss of an average PoE installation versus one that pushes the limits of the standard.
PoE market analysts conservatively estimate the size of the market today at 100 million powered devices worldwide. Using this figure and the data from Table 1, average cable loss per year worldwide with a two-wire PoE solution is 121.7 million kWh. Using the four-wire PoE solution, the cable loss is 60.8 million kWh. In addition to reducing the environmental impact of PoE, a four-wire solution also offers operators of large Ethernet-based installations considerable utility cost savings.
The four-wire solution has been discussed for some time in the PoE community and has been implemented in a few proprietary solutions. As such, it is mature enough to be recognized as workable and as a potential enhancement to the family of PoE standards.
On April 1, 2013, IEEE announced formation of a new study group to investigate the options and means of extending the standard to include four-pair PoE. The IEEE 802.3 4-pair Power over Ethernet Study Group will consider a four-pair solution’s capability to enhance energy efficiency. Importantly, it will also consider a solid technology solution for increasing the power delivered beyond the present 25.5 W.
More power available to the powered device should extend the usefulness of PoE to new applications with higher power demands. Table 2 shows the first cut at markets that would find a four-wire solution useful, according to IEEE. Other applications, including some operations associated with data centers, would also be more likely if greater power at higher efficiency becomes available.
Table 2. A four-wire PoE solution that significantly increases today’s 25.5 W limit will open new markets.
Higher Data Rates
Increasing PoE efficiency and extending its maximum power capacity would save energy and add applications. The present PoE standard is not specified to operate over 10GBase-T, however, and as various markets convert to higher data rates, there may be an increased demand for PoE to operate on such links.
In particular, powering wireless APs is a very common PoE application today, and future generations of IEEE 802.11 will allow greater data bandwidth. Market forecasts for IEEE P802.11ac (Gigabit Wi-Fi) estimate that more than nine million APs worldwide will use the in-development standard by 2017. To keep up with this trend, the study group will also consider technology solutions for extending PoE over 10GBase-T links.
Forty years after the birth of Ethernet, the technology dominates large segments of the data communications market and continues to evolve—sometimes in unexpected ways. Ethernet’s success has, in fact, been possible because of the IEEE 802.3 working group’s commitment to constantly enhancing the standard.
As IT facilities have begun to play an increasingly important role of many market segments—medical, industrial and retail, to name just three—the advantages of PoE have become more and more apparent.
By delivering data and power over the same cabling system, installation costs can be reduced and ongoing maintenance simplified. Although it has already achieved significant market success, PoE adoption is beginning to be limited by its power efficiency and by the maximum power it can deliver. Presently, it is also restricted by the maximum-data-rate Ethernet version it supports. IEEE is working to meet those challenges by investigating a four-wire solution to increase efficiency, deliver more power and extend the utilization of PoE over 10GBase-T links.
About the Authors
Chad Jones lives in Doylestown, Ohio, with his wife and three daughters. He is a proud Zip, having graduated with his BSEE from the University of Akron in 1996. He has been at a large systems vendor since late 2000, first as a hardware engineer and later managing a group of hardware engineers, designing wireless access points, midspans and controllers. Chad participated in both the IEEE 802.3af and IEEE 802.3at standards and implements these standards in the product designs he manages. He currently holds six patents, most related to Power over Ethernet. In his spare time—oh yeah, he’s an engineer and a father. There is no such thing as spare time.
Wael William Diab is a Senior Member of the IEEE and Senior Technical Director in the Office of the CTO at Broadcom, where he is responsible for defining technical strategy for the Infrastructure & Networking Group (ING). In IEEE, Diab is the Vice-Chair of the IEEE 802.3 Ethernet Working Group, serving in that position since 2008. Before his role as Vice-Chair he served as secretary for the IEEE 802.3 Working Group and Editor-in-Chief for IEEE Std 802.3ah-2004. Wael is currently the chair of the IEEE 802.3 Working Group’s standing Maintenance Task Force and has served as editor and chair of multiple IEEE 802.3 Task Force projects.
 IEEE 802.11-2012 “Standard for Information technology--Telecommunications and information exchange between systems Local and metropolitan area networks--Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”
 IEEE 802.3af-2003 “Standard for Information Technology - Telecommunications and Information Exchange Between Systems - Local and Metropolitan Area Networks - Specific Requirements - Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications - Data Terminal Equipment (DTE) Power Via Media Dependent Interface (MDI)”
 IEEE 802.3at-2009 “Standard for Information technology-- Local and metropolitan area networks-- Specific requirements-- Part 3: CSMA/CD Access Method and Physical Layer Specifications Amendment 3: Data Terminal Equipment (DTE) Power via the Media Dependent Interface (MDI) Enhancements”
 IEEE P802.11ac “Draft Standard for IT - Telecommunications and Information Exchange Between Systems - LAN/MAN - Specific Requirements - Part 11: Wireless LAN Medium Access Control and Physical Layer Specifications - Amd 4: Enhancements for Very High Throughput for operation in bands below 6GHz”