[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"article-pd-and-pse-explained-the-poe-powering-architecture":3},{"id":4,"slug":5,"title":6,"category":7,"summary":8,"tags":9,"publishTime":14,"views":4,"seoTitle":15,"seoDescription":16,"seoKeywords":17,"content":18},8,"pd-and-pse-explained-the-poe-powering-architecture","PD and PSE Explained: The PoE Powering Architecture","standards","Understand PoE's two roles: PSE (endspan or midspan) sources power; PD receives it. Learn detection, classification and the 25k-ohm signature.",[10,11,12,13],"PSE","PD","PoE architecture","classification","2025-11-23","PD and PSE Explained: PoE Powering Architecture","How PoE works: PSE (endspan\u002Fmidspan) sources power, PD receives it. Learn detection, classification and the 25k-ohm signature resistance.","PSE, PD, PoE architecture, endspan, midspan, PoE detection classification, 25k ohm signature, Alternative A B","\u003Cp>Every PoE link has two roles: the Power Sourcing Equipment (PSE) that injects power, and the Powered Device (PD) that consumes it. Understanding how these two negotiate is the key to predictable, safe PoE - and to designing splitters and modules that behave correctly as PDs.\u003C\u002Fp>\u003Cimg src=\"\u002Fbrand\u002Fnet\u002Fswitch-cabling.jpg\" alt=\"switch cabling\" loading=\"lazy\" \u002F>\n\u003Ch2>The PSE: Where Power Originates\u003C\u002Fh2>\n\u003Cp>The PSE is responsible for detecting valid PDs, classifying them, and supplying regulated DC power within the standard voltage range (44-57 V depending on Type). There are two physical forms:\u003C\u002Fp>\n\u003Cul>\n\u003Cli>\u003Cstrong>Endspan:\u003C\u002Fstrong> A network switch with PoE circuitry built into each port. This is the most common form in modern installations - power and data come from the same box.\u003C\u002Fli>\n\u003Cli>\u003Cstrong>Midspan:\u003C\u002Fstrong> A standalone power injector placed between a non-PoE switch and the device. Midspans are used to add PoE to existing non-PoE infrastructure without replacing the switch.\u003C\u002Fli>\n\u003C\u002Ful>\n\u003Ch2>The PD: Where Power Is Consumed\u003C\u002Fh2>\n\u003Cp>The PD is the endpoint - an IP camera, access point, phone, sensor, or a PoE splitter acting on behalf of a non-PoE device. A compliant PD presents the signatures the PSE looks for and accepts power only after the handshake completes. Because a splitter sits on the PD side, it must implement PD behavior correctly to be recognized and powered by any standard switch.\u003C\u002Fp>\n\u003Cimg src=\"\u002Fbrand\u002Fnet\u002Fethernet-switch.jpg\" alt=\"ethernet switch\" loading=\"lazy\" \u002F>\u003Ch2>Power Delivery Modes\u003C\u002Fh2>\n\u003Cp>PoE can place power on different pairs of the cable:\u003C\u002Fp>\n\u003Ctable>\n\u003Cthead>\n\u003Ctr>\u003Cth>Mode\u003C\u002Fth>\u003Cth>Pairs used\u003C\u002Fth>\u003Cth>Notes\u003C\u002Fth>\u003C\u002Ftr>\n\u003C\u002Fthead>\n\u003Ctbody>\n\u003Ctr>\u003Ctd>Alternative A\u003C\u002Ftd>\u003Ctd>Data pairs (2 & 3)\u003C\u002Ftd>\u003Ctd>Power superimposed on data; common on endspans\u003C\u002Ftd>\u003C\u002Ftr>\n\u003Ctr>\u003Ctd>Alternative B\u003C\u002Ftd>\u003Ctd>Spare pairs (1 & 4)\u003C\u002Ftd>\u003Ctd>Uses the unused pairs in 10\u002F100; common on midspans\u003C\u002Ftd>\u003C\u002Ftr>\n\u003Ctr>\u003Ctd>4-pair (802.3bt)\u003C\u002Ftd>\u003Ctd>All four pairs\u003C\u002Ftd>\u003Ctd>Required for Type 4; spreads current, cuts loss\u003C\u002Ftd>\u003C\u002Ftr>\n\u003C\u002Ftbody>\n\u003C\u002Ftable>\n\u003Cp>A well-designed PD accepts power on either polarity and either alternative, since it cannot assume which scheme the PSE uses.\u003C\u002Fp>\n\u003Ch2>The Detection and Classification Sequence\u003C\u002Fh2>\n\u003Cp>Standard PoE follows a defined power-up sequence that protects non-PoE equipment:\u003C\u002Fp>\n\u003Cul>\n\u003Cli>\u003Cstrong>Detection:\u003C\u002Fstrong> The PSE applies a low probe voltage (roughly 2.7-10 V) and measures the load. A valid PD presents a 25 kΩ signature resistance across the powered pairs. No valid signature, no power.\u003C\u002Fli>\n\u003Cli>\u003Cstrong>Classification:\u003C\u002Fstrong> The PSE raises the voltage into the classification window (about 14.5-20.5 V) and reads the PD's class so it can reserve the right amount of power. 802.3bt adds multi-event and Autoclass for finer granularity.\u003C\u002Fli>\n\u003Cli>\u003Cstrong>Startup:\u003C\u002Fstrong> The PSE applies full voltage (above ~42 V) and the PD powers on.\u003C\u002Fli>\n\u003Cli>\u003Cstrong>Operation:\u003C\u002Fstrong> The PSE maintains 37-57 V at the PD and monitors current; if the PD is removed or draws too little, the PSE disconnects power.\u003C\u002Fli>\n\u003C\u002Ful>\n\u003Cimg src=\"\u002Fbrand\u002Fnet\u002Fserver-rack.jpg\" alt=\"server rack\" loading=\"lazy\" \u002F>\u003Ch2>Signature Resistance and Why It Matters\u003C\u002Fh2>\n\u003Cp>The 25 kΩ detection signature is the single most important safety element of PoE. It is the fingerprint that tells the PSE \"a real PoE device is here.\" Ordinary non-PoE equipment does not present this resistance, so a compliant PSE withholds power and the device is never exposed to 48 V. When designing a PD or splitter, getting the detection and classification signatures right is what makes the product universally compatible with standard switches.\u003C\u002Fp>\n\u003Ch2>Endspan vs Midspan: Choosing\u003C\u002Fh2>\n\u003Cul>\n\u003Cli>Choose an \u003Cstrong>endspan switch\u003C\u002Fstrong> for new or refreshed networks - it is cleaner, centrally managed, and supports the latest 802.3bt power.\u003C\u002Fli>\n\u003Cli>Choose a \u003Cstrong>midspan injector\u003C\u002Fstrong> to add PoE to a working non-PoE switch, to reach a single high-power device, or to add power without re-cabling.\u003C\u002Fli>\n\u003C\u002Ful>\n\u003Ch2>Designing on the PD Side\u003C\u002Fh2>\n\u003Cp>Splitters and custom power modules live on the PD side of this architecture, which means correct detection, classification, and dual-mode\u002Fdual-polarity acceptance are non-negotiable. Our products implement standards-compliant PD behavior so they are recognized and powered by any compliant endspan or midspan, then deliver clean, regulated output to the equipment behind them.\u003C\u002Fp>"]