In order to add these terms to the centered difference approximation in such a way that we may still use an interleaved scheme, we can use the semi-implicit  approximations to , and given by
Losses and sources can be added to the waveguide network scheme rather easily, by introducing new ports at each series or parallel junction. In fact, as per wave digital filters, each pair of terms , and can be interpreted as a resistive source , and only requires the addition of a single new port at each junction. (The resistive voltage source was discussed in §2.3.4.) For any parallel port we will call the new port admittance , and the voltage wave variable entering the port . For a series port, we call the new impedance , and the incoming voltage wave variable . The generalized network is shown in Figure 4.15, with the new loss/source port immittances marked.
Note that in the case where the loss parameter is zero, or close to zero, will become infinite, or very large. For this reason, it will be necessary in this case to use the dual type of wave; i.e., if is small, set , and use current waves at the series junctions. The other impedances in the network remain unchanged under the addition of losses and sources; thus all the stability criteria mentioned in §4.3.6 remain the same. It is rather interesting to note, however, that in the case of the current-centered network, for example (type II), scattering at the series junctions is no longer trivial if we have non-zero sources or loss . That is, the series junctions cannot be treated as simple throughs. A similar statement holds for the dual case of the voltage-centered network (type I) in (1+1)D, but will not be true when we generalize to the (2+1)D mesh (see §4.4).