  Age of information (AoI) and reliability are two critical metrics to support
real-time applications in Industrial Internet of Things (IIoT). These metrics
reflect different concepts of timely delivery of sensor information. Monitoring
traffic serves to maintain fresh status updates, expressed in a low AoI, which
is important for proper control and actuation actions. On the other hand,
safety-critical information, e.g., emergency alarms, is generated sporadically
and must be delivered with high reliability within a predefined deadline. In
this work, we investigate the AoI-reliability trade-off in a real-time
monitoring scenario that supports two traffic flows, namely AoI-oriented
traffic and deadline-oriented traffic. Both traffic flows are transmitted to a
central controller over an unreliable shared channel. We derive expressions of
the average AoI for the AoI-oriented traffic and reliability, represented by
Packet Loss Probability (PLP), for the deadline-oriented traffic using
Discrete-Time Markov Chain (DTMC). We also conduct discrete-event simulations
in MATLAB to validate the analytical results and evaluate the interaction
between the two types of traffic flows. The results clearly demonstrate the
tradeoff between the AoI and PLP in such heterogeneous IIoT networks and give
insights on how to configure the network to achieve a target pair of AoI and
PLP.
