 In the same manner the application can implement modules that perform the application services offered by the appli- cation layer. These modules are called application specific elements. Application layer protocols are classified into: Common application specific elements (CASE) Specific application specific elements (SASE) As the name signifies, CASE elements are the integrated set of functions that are commonly used by an application while SASE elements are written on a need basis for a specific functionality. Some of the CASE related functions include ini- tialization and termination of connection with other processes, transfer of message with other processes, or underlying protocol and recovering from a failure while a task is under execution. It should be understood that not all applications use the application layer for communication and applications are not the only users of the application layer. The operating system (OS) itself can use the services directly from the application layer. 10.9 TCP/IP Model and OSI Model The OSI model is the not the only network model available for implementing pro- tocol stacks for communication between network elements. The four layer TCP/ IP model was developed before the OSI model and is also one of the most popular models for implementing protocol stack (see Figure 10.3). The functions performed by the four layers in the TCP/IP model can be mapped to the functions performed by the seven layers of an OSI model. The layers in the TCP/IP Model are: 1. Application layer: This is the top most layer of the TCP/IP model and per- forms functions done by application, presentation, and session layer in the OSI model. In addition to all the processes that involve user interaction, the Fundamentals of EMS, NMS and OSS/BSS Application layer Presentation layer Application layer Session layer Transport layer Transport layer Network layer Internet layer Data link layer Network interface layer Physical layer OSI model TCP/IP model Figure 10.3 Mapping layers in the OSI model to layers in