Learn Power over Ethernet (PoE) Power Interfaces

Introduction

Power over Ethernet (PoE) is a technology that enables the delivery of electrical power along with data over standard Ethernet cables, simplifying the installation of networked devices by eliminating the need for separate power supplies. This technology is particularly useful for devices such as IP cameras, wireless access points, and VoIP phones. PoE power interfaces are defined by several key concepts, including Alt A/Alt B power delivery, single/dual signature powered devices (SSPD/ DSPD), 2-pair/4-pair powering and and endspan or midspan architectures. 

System Overview

A PoE network is comprised of Power Sourcing Equipment (PSE) and Powered Devices (PDs) using a link section. The IEEE^® 802.3 Power over Ethernet standard requires the PSE to valid the PD before power is applied.

There are two types of PSEs. Endpoint PSE are Ethernet switches with built-in PSE functionality. A midspan PSE is connected between the switch and the PD to provide power into the link segment. They can be used to provide power into non-PoE link sections. 

Interfaces

Let’s take a look at the power interface and its terminology. On the left side we have the PSE that is delivering power to the PD on the right side through a network cable. The power can be delivered over data pair set alternative A or Alt A over pins 1, 2, 3 and 6 of the network cable. Power can also be delivered over spare pair set alternative B or Alt B over pins 4, 5, 7 and 8 of the network cable.

2-Pair Power

In order to have 2-pair powering supporting the af or at standard, the PSE can inject power over the data pair (referred to as the endspan architecture) or over the spare pair (referred to as the midspan architecture).

In the endspan architecture, the PSE is connected to the central taps of the transformer of the data pair set. This architecture is also called phantom feeding and supports up to Class 4 power levels.

In the midspan architecture, the PSE is connected to the spare set, while data is on the data pair. This architecture also supports up to Class 4 power levels.

4-Pair Power

With 4-pair powering the PSE will inject the power over the data set and spare set at the same time, enabling higher power delivery suitable for more demanding devices. This is illustrated below for both single signature and dual signature PDs.

Single-Signature and Dual-Signature Powered Device (SSPD and DSPD)

IEEE 802.3bt™ standardized both Single-Signature (SS) and Dual-Signature (DS) PD. Any IEEE 802.3bt PD, SS or DS is capable of receiving 4-pair power. Any IEEE 802.3bt PSE must support both SSPD and DSPD.

IEEE 802.3af/at did not standardize DSPD, only SSPDs. But at the same time, prior to the release of IEEE 802.b bt, DS were not forbidden, they did exist. Note that a IEEE 802.3af/at SSPD is capable of receiving 4-pair power. Any Dual Signature PD that is not IEEE802.3bt is not capable of receiving 4-pair power. For this reason the power up sequence in IEEE 802.3bt includes a 4PID or 4-pair capability check which is a logical function of connection check, detection and classification.

Let’s cover SSPD and DSPD architectures in more depth. SSPDs are designed to support a single load or application, that is, provide power to a single device such as an IP phone or Wi-Fi^® access point for example.

SSPDs are designed to receive power as a single entity, simplifying the design and reducing costs for devices that do not require separate power management for different components. SSPDs have a single power interface and draw power uniformly across both pairs.

Conversely, DSPDs have two separate power interfaces, allowing them to draw power independently from different pairs. Therefore, they can manage power separately for different parts of the device, offering enhanced flexibility and efficiency for complex systems that need distinct power allocations, such as cameras with an embedded heater.

There was an evolution of DSPDs before they were formalized in the IEEE 802.3bt standard. Before bt, there were no rules against using 4-pair power, so these types of applications did exist even though their implementations were likely not bt-compliant. 

Endspan and Midspan Architecture

The PSE may be either an endspan PSE or a midspan PSE. An endspan PSE has the PoE integrated inside the Ethernet switch. A midspan PSE, or PoE injector has the PoE in a dedicated system that is placed between a non-PoE Ethernet switch and the powered device:

Summary

PoE power interfaces offer a versatile and efficient means of delivering power and data over Ethernet cables. Understanding the nuances of power delivery, single and dual signature powered devices, 2- and 4-pair powering and endspan and midspan architectures is essential for designing and implementing robust PoE systems that meet the diverse needs of modern networked devices.

Learn More

• Power over Ethernet (PoE), Power Delivery (PD) and DC-DC Design Considerations
• Designing a Type 1/2 802.3 or HDBaseT Type 3 Powered Device Front End Using PD702x0 and PD701x0 ICs
• Designing a Type 1/2 802.3 or HDBaseT Type 3 Powered Device Using PD702x1 and PD701x1 ICs
• Implementing Aux Power in PoE
• Power over Ethernet (PoE) Solutions
• Power over Ethernet (PoE) Terminology

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