  Age of Information (AoI) has become an important concept in communications,
as it allows system designers to measure the freshness of the information
available to remote monitoring or control processes. However, its definition
tacitly assumes that new information is used at any time, which is not always
the case: the instants at which information is collected and used are dependent
on a certain query process. We propose a model that accounts for the discrete
time nature of many monitoring processes, considering a pull-based
communication model in which the freshness of information is only important
when the receiver generates a query: if the monitoring process is not using the
value, the age of the last update is irrelevant. We then define the Age of
Information at Query (QAoI), a more general metric that fits the pull-based
scenario, and show how its optimization can lead to very different choices from
traditional push-based AoI optimization when using a Packet Erasure Channel
(PEC) and with limited link availability. Our results show that QAoI-aware
optimization can significantly reduce the average and worst-case perceived age
for both periodic and stochastic queries.
