  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 assumed that new information is used at any time, which is not always
the case and 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. 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).
