  We derive key features of the Age-of-Information distribution in a system
whose activities are strictly limited to periodic instances on a global time
grid. In particular, one agent periodically generates updates while the other
agent periodically uses the most recently received of those updates. Likewise,
transmission of those updates over a network can only occur periodically. All
periods may differ. We derive results for two different models: a basic one in
which the mathematical problems can be handled directly and an extended model
which, among others, can also account for stochastic transmission failure,
making the results applicable to instances with wireless communication. For
both models, a suitable approximation for the expected Age-of-Information and
an upper bound for its largest occurring value are developed. For the extended
model (which is the more relevant one from a practical standpoint) we also
present numerical results for the distribution of the approximation error for
numerous parameter choices.
