This whitepaper explains how DOCSIS Proactive Network Maintenance (PNM) uses correlation groups and virtual Time Domain Reflectometry (vTDR) to accurately locate upstream network impairments. It describes how impedance mismatches, corrosion, and micro-reflections create measurable signal reflections that can be analyzed through pre-equalization data. By grouping affected cable modems with similar impairment signatures, operators can identify common fault locations and calculate the distance to defects with greater precision than traditional methods. The paper also demonstrates how mapping correlation groups alongside vTDR results enables technicians to pinpoint faulty sections of coaxial cable or passive devices before widespread service degradation occurs. Understanding the difference bet

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