Published in Electronic Design
Power over Ethernet (PoE) devices can transmit and receive Ethernet data and electric power over the same network cable. Although, PoE has been around for almost two decades, for many military and aerospace applications, rugged deployable PoE solutions have only recently become available.
With Ethernet now the de facto backbone for mission situational awareness and network-centric operations in the military and aerospace applications, reliable MIL-STD qualified PoE-compatible devices can help system integrators reduce their platform SWaP and cost, while simplifying cabling, power, and device management for modern networked electronic payloads.
A Brief History of PoE
In the beginning, PoE was called “power injection.” It referred to power sourcing equipment (PSE), such as a network switch that could provide dc current to a powered device (PD), utilizing the unused twisted pairs in traditional Cat 5 copper Ethernet cabling that weren’t being used for 10Base-T or 100Base-TX operation.
In its early days, there were few standards or safety considerations, which meant that the use of power injection could unintentionally damage a device, especially one that wasn’t designed to accept power. To address the standardization of PoE, the IEEE 802.3 Working Group, the body that defines standards for Ethernet technology, got involved in 1999.
By 2003, the IEEE 802.3 Working Group had ratified the first standard, called IEEE 802.3af for “Type 1” PoE devices. Under this standard, power is transported on the same wire pairs, or spare wire pairs, as is the data for 10- and 100-Mb/s Ethernet variants. For Gigabit Ethernet (GbE) and faster, all four twisted pairs of the Ethernet cable are used for data, but just two pairs for power transmission. Power is transmitted on the data conductors by applying a common voltage to each pair. Because twisted-pair Ethernet uses differential signaling, this doesn’t interfere with data transmission.
The original PoE standard allowed for PSE to source up to 15.4 W and deliver up to 12.95 W per port to PDs. PoE switches or routers that provided power over the Ethernet cable were dubbed “endspan” or “endpoint” devices. External power injectors, called “mid-span injectors,” were to be used in combination with a non-PoE switch.
While 12.95 W may not seem like a lot of power, it was sufficient for many popular PDs, including voice over IP (VoIP) phones, stationary cameras, and door access control units, among others. However, it wasn’t long before some industries demanded even more power per port via PoE.
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