  This paper constructs a finite state abstraction of a possibly
continuous-time and infinite state model in two steps. First, a finite external
signal space is added, generating a so called $\Phi$-dynamical system.
Secondly, the strongest asynchronous $l$-complete approximation of the external
dynamics is constructed. As our main results, we show that (i) the abstraction
simulates the original system, and (ii) bisimilarity between the original
system and its abstraction holds, if and only if the original system is
$l$-complete and its state space satisfies an additional property.
