Controlling Age of Information in a Dual-Server System with Transmission Freezing and Preemption

We study age of information (AoI) in a status update system for the generate-at-will (GAW) scenario, consisting of a single information source, dual heterogeneous servers, and a monitor. For this system, stochastic hybrid systems (SHS) were used to obtain the mean AoI for the work-conserving zero wait (ZW) policy with out-of-order packet discarding at the monitor. In this paper, we propose a non-work-conserving freeze/preempt (F/P) policy for which the sampling and transmission process is frozen for an Erlang-distributed duration upon each new transmission, and out-of-order (obsolete) packets are preempted 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 the F/P policy. Numerical results are presented for the validation of the proposed analytical model and a comparative evaluation of ZW and F/P policies.