Models can be generated not only by simulations, but also from measurement results. This can be done by VNA (Vector Network Analyzer) or TDR/TDT measurements.
VNA measurements result directly in an S-Parameter set, that can be used as is in time or frequency domain simulations. Besides this, a lot of useful information can be read from the S-Parameter set regarding the frequency behavior of the DUT (Device Under Test) as well as Insertion Loss (S21), Return Loss (S11), FEXT (Far End X-Talk), NEXT (Near End X-Talk) and signal to noise ratio to name some additional outputs that can be read out of an S-Parameter set!
A TDR gives quite similar information – but directly in the time domain. Using fourier transformation (FFT), TDR measurement result can be transformed into the Frequency Domain and finally a S-parameter set can be used in the simulation. The dynamic range of the TDR/TDT is not as high as the VNA measurement, but it is more intuitive for many designers to stay within time domain. Here it is “directly” possible to measure capacitive or inductive parasitics of e. g. a connector. Additionally, transmission line length can be measured with high accuracy and, for instance, a short or open can be located. More often than not, the direct impedance readout by the TDR for a transmission line is used.
Below is an example of a TDR characterized PCB with a connector. A more detailed explanation on this picture can be found in the TDR/TDT section.