  We present a new and simple method for the identification of a single
transfer function that is embedded in a dynamical network. In existing methods
the consistent identification of the desired transfer function relies on the
positive definiteness of the spectral density matrix of the vector of all node
signals, and it typically requires knowledge of the topology of the whole
network. The positivity condition is on the internal signals and therefore can
not be guaranteed a priori, in addition it is far from necessary. The new
method of this paper provides simple conditions on which nodes to excite and
which nodes to measure in order to produce a consistent estimate of the desired
transfer function. Just as importantly, it requires knowledge of the local
topology only.
