  We consider a network of status updating sensors whose updates are collected
and sent to a monitor by a gateway. The monitor desires as fresh as possible
updates from the network of sensors. The gateway may either poll a sensor for
its status update or it may transmit collected sensor updates to the monitor.
We derive the average age at the monitor for such a setting. We observe that
increasing the frequency of transmissions to the monitor has the upside of
resetting sensor age at the monitor to smaller values. However, it increases
the length of time that elapses before a sensor is polled again. This motivates
our investigation of policies that fix the number of sensors s the gateway
polls before transmitting to the monitor.
  For any s, we show that when sensor transmission times to the gateway are
independent and identically distributed (iid), for independent but possibly
non-identical transmission times to the monitor, it is optimal to poll a sensor
with the maximum age at the gateway first. Also, under simplifying assumptions,
the optimal value of s increases as the square root of the number of sensors.
For non-identical sensor transmission times, we consider a policy that polls a
sensor such that the resulting average change in age is minimized. We compare
our policy proposals with other policies, over a wide selection of transmission
time distributions.
