Voltage V1 is the voltage drop across Port 1 of the two port network, and I1 is the current into Port 1. The quantities V2 and I2 are the corresponding values at Port 2. Impedance Z0 is the characteristic impedance of the network, and is usually 50 Ω. The factor of ½ applies where the voltage and current are peak values. The factor is dropped for rms voltages and currents. The voltages and currents are vector (real and imaginary) quantities. The s-parameters can then be defined as: