  We investigate classes of systems based on different interaction patterns
with the aim of achieving distributability. As our system model we use Petri
nets. In Petri nets, an inherent concept of simultaneity is built in, since
when a transition has more than one preplace, it can be crucial that tokens are
removed instantaneously. When modelling a system which is intended to be
implemented in a distributed way by a Petri net, this built-in concept of
synchronous interaction may be problematic. To investigate this we consider
asynchronous implementations of nets, in which removing tokens from places can
no longer be considered as instantaneous. We model this by inserting silent
(unobservable) transitions between transitions and some of their preplaces. We
investigate three such implementations, differing in the selection of preplaces
of a transition from which the removal of a token is considered time consuming,
and the possibility of collecting the tokens in a given order.
  We investigate the effect of these different transformations of instantaneous
interaction into asynchronous interaction patterns by comparing the behaviours
of nets before and after insertion of the silent transitions. We exhibit for
which classes of Petri nets we obtain equivalent behaviour with respect to
failures equivalence.
  It turns out that the resulting hierarchy of Petri net classes can be
described by semi-structural properties. For two of the classes we obtain
precise characterisations; for the remaining class we obtain lower and upper
bounds.
  We briefly comment on possible applications of our results to Message
Sequence Charts.
