Data centers (DCs) became an important part of computing today. A lot of services in Internet are run on DCs. Meanwhile a lot of research is done to tackle the challenges of high-performance and energy-efficient data center networking (DCN). Hot node congestion, cabling complexity/cost, and cooling cost are some of the important issues about data centers that need further investigation. Static and rigid topology in wired DCNs is an other issue that hinders flexibility. Use of wireless links for DCNs to eliminate these disadvantages is proposed and is an important research topic. In this paper, we review research studies in literature about the design of radio frequency (RF) based wireless data center networks. RF wireless DCNs can be grouped into two as hybrid (wireless and wired) and completely wireless data centers. We investigate both. We also compare wireless DCN solutions in the literature with respect to various aspects. Open areas and research ideas are also discussed.