  Data center networks (DCNs) are essential infrastructures to embrace the era
of highly diversified massive amount of data generated by emerging
technological applications. In order to store and process such a data deluge,
today's DCNs have to deploy enormous length of wires to interconnect a plethora
of servers and switches. Unfortunately, wired DCNs with uniform and inflexible
link capacities expose several drawbacks such as high cabling cost and
complexity, low space utilization, and lack of bandwidth efficiency. Wireless
DCNs (WDCNs) have emerged as a promising solution to reduce the time, effort,
and cost spent on deploying and maintaining the wires. Thanks to its
reconfigurability and flexibility, WDCNs can deliver higher throughputs by
efficiently utilizing the bandwidth and mitigate the chronic DCN problems of
oversubscription and hotspots. Moreover, wireless links enhance the
fault-tolerance and energy efficiency by eliminating the need for error-prone
power-hungry switches. Accordingly, this paper first compares virtues and
drawbacks of millimeter wave (mmWave), terahertz (THz), and optical wireless
communications as potential candidates. Thereafter, an in-depth discussion on
advances and challenges in WDCNs is provided including physical and virtual
topology design, quality of service (QoS) provisioning, flow classification,
data grooming, and load balancing. Finally, exciting research opportunities are
presented to promote the prospects of WDCNs.
