Impedance is the ratio of the voltage divided by the current in an alternating current circuit. The impedances most encountered in wire telecommunications are 300, 600 and 900 ohms for voice-band analog, and 135 ohms for subrate; 75, 100 and 120 ohms for T1/ E1 digital lines.

These impedances must be closely maintained in order to reduce the possibilities of undesirable reflections and echoes, which in long distance circuits can be delayed long enough to confuse the telephone user or destroy the data being sent.

Repeat coils and other matching transformers are used to correct and maintain the impedances within the system. Impedance containment within plus or minus ten percent is usually required. In critical applications, impedance with respect to a standard network is generally specified. A simple circuit for measuring the magnitude of reflected impedance of a loaded transformer is shown in the drawing below.

Where:

Z_PRI = the impedance seen at the transformer's primary
e₁ = the voltage across the shunt resistor R1
e₂ = the voltage at the transformer's primary winding
R₁ = the shunt resistor whose value is typically much lower than Z_PRI

In cases where it is necessary to know both the real and imaginary components of the impedance, an LCR bridge would be more appropriate.