What is the standard impedance value for copper twisted-pair Ethernet cables?

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The concept being tested is the characteristic impedance of copper twisted-pair Ethernet cables. These cables are designed as a balanced transmission line, and their geometry and dielectric determine a specific characteristic impedance. For Ethernet over twisted pair, that impedance is standardized at 100 ohms. This value is chosen to match the input and output impedance of Ethernet transceivers and to minimize signal reflections and distortion across the frequency range used by the network. Keeping the impedance at 100 ohms helps ensure reliable data transmission for all common Ethernet speeds (including faster varieties like gigabit Ethernet) and maintains compatibility across Cat5e, Cat6, and Cat6a cables. The other impedance values correspond to different media (for example, coax or other telecom systems) and would not provide the proper matching for Ethernet signaling.