# S parameters measurement inside VNA

How does Vector Network Analyzer (VNA) measures S parameters in reality?

Namely we have equations:

$b_1=S_{11}a_1+S_{12}a_2$

$b_2=S_{12}a_1+S_{22}a_2$

In theory, if i wanted to calculate scattering parameters I would have to be able to make perfect matching on the source ($a_1=0$) and on the load side ($a_2=0$).

Unfortunately I do not understand how is this done inside the VNA. If I want to measure scattering parameters of a random network with input and output impedance different than 50ohm, how is in this case achieved $a_1=0$ and $a_2=0$?