  Age of Information (AoI), which measures the time elapsed since the
generation of the last received packet at the destination, is a new metric for
real-time status update tracking applications. In this paper, we consider a
status-update system in which a source node samples updates and sends them to
an edge server over a delay channel. The received updates are processed by the
server with an infinite buffer and then delivered to a destination. The channel
can send only one update at a time, and the server can process one at a time as
well. The source node applies generate-at-will model according to the state of
the channel, the edge server, and the buffer. We aim to minimize the average
AoI with \emph{independent and identically distributed} transmission time and
processing time. We consider three online scheduling policies. The first one is
the optimal long wait policy, under which the source node only transmits a new
packet after the old one is delivered. Secondly, we propose a peak age
threshold policy, under which the source node determines the sending time based
on the estimated peak age of information (PAoI). Finally, we improve the peak
age threshold policy by considering a postponed plan to reduce the waiting time
in the buffer. The AoI performance under these policies is illustrated by
numerical results with different parameters.
