Ngspice implements both the original SPICE3f5 transmission lines models and the one introduced with KSPICE. The latter provide an improved transient analysis of lossy transmission lines. Unlike SPICE models that use the state-based approach to simulate lossy transmission lines, KSPICE simulates lossy transmission lines and coupled multiconductor line systems using the recursive convolution method. The impulse response of an arbitrary transfer function can be determined by deriving a recursive convolution from the Pade approximations of the function. We use this approach for simulating each transmission line's characteristics and each multiconductor line's modal functions. This method of lossy transmission line simulation has been proved to give a speedup of one to two orders of magnitude over SPICE3f5.