  Achieving connectivity reliability is one of the significant challenges for
5G and beyond 5G cellular networks. The present understanding of reliability in
the context of mobile communication does not adequately cover the stochastic
temporal aspects of the network, such as the duration and spread of packet
errors that an outage session may cause. Rather, it simply confines the
definition to the percentage of successful packet delivery. In this letter, we
offer an elaborate modeling of the outage for a cellular mobile network by
showcasing the different types of outages and their contiguity characteristic.
Thereafter, using the outage metrics, we define two new key performance
indicators (KPIs), namely mean outage time and mean time between outages as
counterparts to akin KPIs that already exist in classical reliability theory,
i.e., mean down time and mean time between failures. Using a system-level
simulation where user mobility is a crucial component, it is shown that these
newly defined KPIs can be used to quantify the reliability requirements of
different user applications in cellular services.
