session layer
The communication control layer, formerly called the session layer, is layer 5 of the OSI reference model. A session of the Session Layer is the logical connection between two top- level working entities that communicate with each other. A session must be established when a user process on one computer wants to connect to a user process on another computer.
The communication control layer then determines the address of the corresponding session work unit of the partner computer and requests an appropriate connection from the transport layer. To control the communication, services are provided to the application processes that allow them to organize and synchronize their dialog.
The services are grouped into functional units and are agreed upon for the session setup. The International Standards Organization( ISO) proposes a grouping of the functional units into three classes: Basic Combined Subset ( BCS), Basic Synchronized Subset( BSS) and Basic Activity Subset(BAS).