  Age of information (AoI) is an effective performance metric measuring the
freshness of information and is popular for applications involving status
update. Most of the existing works have adopted average AoI as the metric,
which cannot provide strict performance guarantees. In this work, the outage
probability of the peak AoI exceeding a given threshold is analyzed in a
multi-source system under round robin scheduling. Two queueing disciplines are
considered, namely the first-come-first-serve (FCFS) queue and the single
packet queue. For FCFS, upper and lower bounds on the outage probability are
derived which coincides asymptotically, characterizing its true scaling. For
the single packet queue, an upper bound is derived whose effectiveness is
validated by the simulation results. The analysis concretizes the common belief
that single packet queueing has a better AoI performance than FCFS. Moreover,
it also reveals that the two disciplines would have similar asymptotic
performance when the inter-arrival time is much larger than the total
transmission time.
