  We study age of information (AoI) in a single-source dual-server status
update system for the generate at will (GAW) scenario, consisting of an
information source, dual servers, and a monitor. For this system, the method of
stochastic hybrid systems (SHS) was used to obtain the mean AoI for the
work-conserving ZW (zero wait) policy with out-of-order packet discarding at
the monitor. In this paper, we propose a non-work-conserving F/P
(freeze/preempt) policy for which the sampling and transmission process is
frozen for an Erlang distributed amount of time upon each transmission, and
out-of-order packets are preempted immediately at the source, rather than being
discarded at the monitor upon reception. We use the absorbing Markov chain
(AMC) method to obtain the exact distributions of AoI and also the peak AoI
(PAoI) processes, for both ZW and F/P policies. Numerical results are presented
for the validation of the proposed analytical model and a comparative
evaluation of ZW and F/P policies.
