  This paper considers a communication system where a source sends
time-sensitive information to its destination via queues in tandem. We assume
that the arrival process as well as the service process (of each server) are
memoryless, and each of the servers has no buffer. For this setup, we develop a
recursive framework to characterize the mean peak age of information (PAoI)
under preemptive and non-preemptive policies with $N$ servers having different
service rates. For the preemptive case, the proposed framework also allows to
obtain mean age of information (AoI).
