  Enhancing information freshness in wireless networks has gained significant
attention in recent years. To optimize or analyze information freshness, which
is often characterized by the age of information (AoI) metric, extensive
theoretical studies have been conducted on various wireless networks. Early
research has demonstrated the significance of last-come-first-served (LCFS)
packet scheduling and controlled status sampling (i.e., packet generation) in
improving information freshness. These mechanisms have been widely adopted in
subsequent studies. However, the effective implementation of these mechanisms
in commercial off-the-shelf (COTS) wireless devices has not been thoroughly
investigated, which could limit the practical application of information
freshness-oriented protocols in real-world systems. Our work aims to address
the gap by exploring the effective implementation of the information
freshness-oriented mechanisms mentioned above in COTS WiFi devices that use the
Linux operating system. Our attempts reveal that implementing these mechanisms
in COTS systems is not a straightforward task. Specifically, we found that the
physical layer queue of WiFi devices operates on a first-come-first-served
(FCFS) basis, and the packet generation process cannot be precisely controlled
by default. To overcome these challenges, we develop Fresh-Fi, an information
freshness-oriented protocol stack that involves careful customization to the
lower layers of the Linux networking protocol stack. Fresh-Fi mainly
incorporates a mac80211 subsystem-based LCFS queue and a real-time kernel-based
cross-layer tunnel between the mac80211 subsystem and the application layer for
triggered packet generation.
