  We consider networked sources that generate update messages with a defined
rate and we investigate the age of that information at the receiver. Typical
applications are in cyber-physical systems that depend on timely sensor
updates. We phrase the age of information in the min-plus algebra of the
network calculus. This facilitates a variety of models including wireless
channels and schedulers with random cross-traffic, as well as sources with
periodic and random updates, respectively. We show how the age of information
depends on the network service where, e.g., outages of a wireless channel cause
delays. Further, our analytical expressions show two regimes depending on the
update rate, where the age of information is either dominated by congestive
delays or by idle waiting. We find that the optimal update rate strikes a
balance between these two effects.
