When building sophisticated networked electronic systems, power and data distribution are two of the biggest challenges for military systems integrators. The labor, material and logistical costs of laying cable are growing as systems become more sophisticated. For next-generation high-performance military systems placing an increasing emphasis on data throughput and energy management and conservation, the struggle is significant. And, while wireless approaches limit cabling, they have significant restrictions in operational use, due to limitations in battery capacities and the ongoing need for clandestine operation.

Power over Ethernet (PoE) is a technology gaining significant momentum in commercial and industrial applications and which has significant potential for military systems in simplifying distributed system administration while increasing performance and managing costs. PoE utilizes standard Ethernet cables, such as Category 5/5e (Cat5), to transmit both power and data on the same lines to distributed devices. Because of the ubiquity and low-cost of Cat5 cabling, PoE installations could be used to provide both power and data in a variety of scenarios, while limiting cabling complexity and cost. Yet, while PoE has been utilized for several years in commercial/industrial circles, PoE is relatively new in military circles. While more COTS PoE solutions are becoming available, as with any technology, PoE’s utility depends on the application. So, proper evaluation of PoE’s advantages and disadvantages is necessary to determine whether it is appropriate for a given system.

PoE Is Standardized

In 2003, the venerable IEEE 802.3 Ethernet standard incorporated PoE as IEEE std. 802.3af. Today, IEEE std. 802.3at (known as PoE+) is close to ratification and incorporates new features that extend the original standard. The ideas behind these standards are not new, as there have been many attempts to standardize power and data distribution systems to reduce cabling costs.

A variety of power-line communications standards and approaches have been tried, but these approaches try to superimpose data on relatively low-quality power lines that are typically ill-conditioned for data. As a result, signal quality and signal distribution issues have limited the data bandwidth, distance and utility of power-line communications. More recently, USB has been a very successful approach for providing power and data to distributed devices along a single cable. The standardization and ubiquity of USB have led to a plethora of successful industrial and military implementations.

Advantages over Power Line and USB

PoE has significant advantages over both power-line and USB communications. Relative to power line, PoE is based on Ethernet so it starts with a high-fidelity data cable, injecting power on the cable. The result is that the limitations are on the power side, relative to the electrical current capacity of the cable, not on the data side. So PoE can be utilized in high-performance Gigabit networks. And, unlike USB, which has noteworthy current capacity limitations, PoE on standard cables can provide significantly more power. Furthermore, USB has major host/client topology limitations, while PoE benefits from the flexibility of the purely distributed nature of Ethernet. Table 1 summarizes the features of the current 802.3af standard for PoE.

While the power capabilities of the 802.3af standard are reasonable for typical PoE devices such as IP phones and wireless access points, many military applications that involve higher-powered peripherals such as IP cameras for security and sophisticated embedded computers with power-hungry displays utilize more power. The upcoming 802.3at standard extends the current standard by utilizing all wires for power and allows such PoE+ PSE devices to provide up to 24W of power (50W in certain implementations), while maintaining backward compatibility with the 802.3af standard. PoE+ implementations are not supported on Cat3 cables, which have greater limitations than Cat5e and Cat6 cables. It also must be noted that the higher power capabilities and other features of PoE+ are still under consideration and there are issues that remain to be sorted out with the pending standard update.

PSE devices that provide the power are typically of two types, endspans or midspans. Endspans (also known as endpoints), such as PoE-enabled Ethernet switches, provide power to their Ethernet interfaces, allowing any connected PD device to draw power. Midspans are essentially power supplies with two Ethernet connections and the conversion between PoE and non-PoE Ethernet. Midspans connect to standard non-PoE switches on one side and PoE-enabled Powered Devices (PDs) on the other side.

PoE Is Effective

For military systems integrators, PoE has many significant advantages over separately managed power and data distribution implementations. Primary among these is the concept of “no new wires,” since PoE utilizes standard cabling and works with common Ethernet connector interfaces. While standard Ethernet cabling and connectors may not be rugged enough for certain military applications, most Ethernet cabling in military installations actually will be over-specified from a current- and data-carrying capacity perspective, and thus still appropriate for PoE implementations.

The advantages of “no new wires” are major, since existing cabling infrastructures can be utilized, greatly simplifying installation and reducing installation cost. And, when new network/power cabling is installed, expensive conduits and electrician labor are avoided, since PoE cables are classified as low voltage and not subject to Underwriters Laboratories (UL) or other regulations.

Another advantage of PoE is that individual devices on the network can eliminate AC power supplies, wall-wart transformers and other high-voltage AC power conditioning circuits. This capability reduces the cost and size of devices, while also reducing the potential for failure of individual devices. And, with all the focus on energy conservation and management within the military and civilian communities, PoE provides a level of consolidation and flexibility of power distribution that allows more centralized management of energy consumption, with the ability to consolidate battery backup and surge protection. PoE also provides remote power diagnostics and status reporting that is not available from conventional separate power and data wiring techniques. Finally, since distances of over 300 feet can be realized with Ethernet cabling, it is possible in certain applications to isolate PSEs and their power sources in protective surroundings, separate from harsh PD environments.

In general, PoE will make the most sense when local power sources are scarce and separately distributing power is relatively costly and cumbersome. Conversely, because PSE and PD devices by their nature incorporate PoE circuitry, unless that circuitry is utilized by a reasonable percentage of the devices on the network, PoE may represent a cost burden. This may occur on existing networks that sparsely supplement traditional power sourcing with PoE, so a careful cost-benefit analysis should be undertaken. Additionally, while PoE can minimize single points of failure by simplifying the power circuitry on PDs, PoE can be vulnerable to attack on PSEs, since a single PSE may provide power to multiple PDs. So, as with any distributed power sourcing approach, systems integrators may wish to supplement PoE with local backup power sources, such as batteries on critically important PDs, as appropriate.

Available Technology

The good news for military systems designers is that PoE is increasing in popularity and more and more COTS PoE-compliant devices and technologies are coming to market. While many of these devices are still rooted in commercial and industrial implementations, as can be seen from the listing of products below, a migration to more rugged military-grade solutions has started.

For rugged endspan PSEs, companies such as MOXA, Sixnet and Ethernet Direct offer industrial-grade PoE-enabled Ethernet switches with DIN-rail mounting. Parvus’ DuraMAR1000 (Figure 1) is a military-grade IP router with multiple PoE ports and supports MIL-STD-810F shock, vibration, humidity, crash safety and water immersion standards with MIL-STD-461E EMC compliance and a MIL-STD-704E power supply. The Parvus unit is designed for mobile applications such as in-vehicle installations. For midspan injection of power, Phihong and SL Power Electronics have several options, and Citel’s C2MJ8-505-RJ is designed for outdoor applications with a rugged housing and extended temperature operation.

Cameras and More

On the PD side, a variety of COTS PoE-based IP cameras are available for security and surveillance applications. Included in these are the Arecont Vision AV3105 PoE camera (with 3 Megapixel resolution and H-264 compression) and ACTi’s ACM-1431 PoE camera (with MPEG-4 and IP-66 weatherproofing for outdoor operation). For those looking to develop custom embedded computer systems, ADI Engineering’s extended temperature Cinnamon Bay SBC with PoE+ Option Module supports the new 802.3at standard, and because it is based on the advanced low-power Intel Atom processor, 20+W of PoE power is available for peripherals (Figure 2).

At the silicon level, several vendors provide single-chip solutions for PSE and PD devices, which manage PoE signaling and convert inputs to a regulated DC voltage suitable for powering electronic circuits. With the addition of appropriate transformers and other passives, complete PoE PD board-level solutions can be created with extended temperature components such as the On Semiconductor NCP1081 PoE-PD Interface & DC/DC Converter Controller and the Texas Instruments TPS23754 High Power/High Efficiency PoE Interface and DC/DC Controller. Additionally, Texas Instruments’ TPS23481 High-Power, Wide-Voltage Range, Quad-Port Ethernet Power Sourcing Equipment Manager component can be utilized in the creation of board- and system-level PSE solutions with extended temperature requirements.

ADI Engineering
Charlottesville, VA.
(434) 978-2888.
[www.adiengineering.com].