A covert channel is a communication channel that allows parties to communicate and transfer data indirectly. Covert channel types are storage, timing, and behavior channels. Covert channels can be used for malicious and benign applications. A covert channel is a mechanism for violating the communication security policy that was not anticipated by the system creator. Recently, covert channels are used to transfer text, video, and audio information between entities. This article, discusses studies related to the development of covert channels as well as the research works that focus on improving the performance/throughput of covert channels. Also, it analyzes the previous studies in terms of publication type, year of publication, article title, article purpose, transferring file format used in covert channel, coding technique, throughput performance, time needed to transfer files, and article limitations.