Consider a stream of status updates generated by a source, where each update
is of one of two types: high priority or ordinary (low priority). These updates
are to be transmitted through a network to a monitor. However, the transmission
policy of each packet depends on the type of stream it belongs to. For the low
priority stream, we analyze and compare the performances of two transmission
schemes: (i) Ordinary updates are served in a First-Come-First-Served (FCFS)
fashion, whereas, in (ii), the ordinary updates are transmitted according to an
M/G/1/1 with preemption policy. In both schemes, high priority updates are
transmitted according to an M/G/1/1 with preemption policy and receive
preferential treatment. An arriving priority update discards and replaces any
currently-in-service high priority update, and preempts (with eventual resume
for scheme (i)) any ordinary update. We model the arrival processes of the two
kinds of updates, in both schemes, as independent Poisson processes. For scheme
(i), we find the arrival and service rates under which the system is stable and
give closed-form expressions for average peak age and a lower bound on the
average age of the ordinary stream. For scheme (ii), we derive closed-form
expressions for the average age and average peak age of the high priority and
low priority streams. We finally show that, if the service time is
exponentially distributed, the M/M/1/1 with preemption policy leads to an
average age of the low priority stream higher than the one achieved using the
FCFS scheme. Therefore, the M/M//1/1 with preemption policy, when applied on
the low priority stream of updates and in the presence of a higher priority
scheme, is not anymore the optimal transmission policy from an age point of
view