  Identifiability of linear dynamic networks requires the presence of a
sufficient number of external excitation signals. The problem of allocating a
minimal number of external signals for guaranteeing generic network
identifiability has been recently addressed in the literature. Here we will
extend that work by explicitly incorporating the situation that some network
modules are known, and thus are fixed in the parametrized model set. The
graphical approach introduced earlier is extended to this situation, showing
that the presence of fixed modules reduces the required number of external
signals. An algorithm is presented that allocates the external signals in a
systematic fashion.
