Reference Model (OSI RM)-more Transport Layer : Layer 4. This layer responds to service requests from the Session Layer and issues service requests to the Network Layer. The purpose of the Transport Layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern
with providing reliable and cost-effective data transfer.
Session Layer : Layer 5. This layer responds to service requests from the Presentation Layer and issues service requests to the Transport Layer . The Session Layer provides the mechanism for managing the dialogue between end- user application processes. It provides for either duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures.
Presentation Layer : Layer 6. This layer responds to service requests from the Application Layer and issues service requests to the Session Layer . The Presentation Layer relieves the Application Layer of concern regarding syntactical differences in data representation within the end- user systems. Note: An example of a presentation service would be the conversion of an EBCDIC -coded text file to an ASCII -coded file.
Application Layer : Layer 7, the highest layer. This layer interfaces directly to and performs common application services for the application processes; it also issues requests to the Presentation Layer . The common application services provide semantic conversion between associated application processes. Note: Examples of common application services of general interest include the virtual file , virtual terminal , and job transfer and manipulation protocols.