  Observabililty is an important topic of Boolean control networks (BCNs). In
this paper, we propose a new type of observability named online observability
to present the sufficient and necessary condition of determining the initial
states of BCNs, when their initial states cannot be reset. And we design an
algorithm to decide whether a BCN has the online observability. Moreover, we
prove that a BCN is identifiable iff it satisfies controllability and the
online observability, which reveals the essence of identification problem of
BCNs.
