  We consider a two-way data exchanging system where a master node transfers
energy and data packets to a slave node alternatively. The slave node harvests
the transferred energy and performs information transmission as long as it has
sufficient energy for current block, i.e., according to the best-effort policy.
We examine the freshness of the received packets at the master node in terms of
age of information (AoI), which is defined as the time elapsed after the
generation of the latest received packet. We derive average uplink AoI and
uplink data rate as functions of downlink data rate in closed form. The
obtained results illustrate the performance limit of the unilaterally powered
two-way data exchanging system in terms of timeliness and efficiency. The
results also specify the achievable tradeoff between the data rates of the
two-way data exchanging system.
