f the most popular encryp- tion schema associated with the presentation layer is the secure sockets layer (SSL) protocol. The presentation layer is not always used in network communication. This is because most of its functionalities are optional. The encryption and compression can be formed at other layers in the OSI model. For data translation to come up with a common format is only required when handling heterogeneous systems. The remaining functions of the presentation layer can be performed by the application layer. Most of the protocol stack implementations may not have a presentation layer. Fundamentals of EMS, NMS and OSS/BSS There are three syntactic versions that are important when two applications interact: Syntax of the application entity that requested the connection. Syntax of the application entity that receives the connection. Syntax used by the presentation entities in giving a common format. The syntax of the application entity at the transmitting or requesting end is con- verted to syntax used by the presentation entity at the presentation layer of a request- ing system. In the same way, the syntax used by the presentation entity is converted to the syntax used by an application entity at the receiving end at the presentation layer of the receiving system. Another example of a presentation layer protocol is XDR (external data rep- resentation). XDR is used as the common data format mostly when interaction is involved with a system running UNIX OS. For a UNIX system to interact with a heterogeneous system running another OS, using remote procedure calls (RPC), the usual presentation layer format is XDR. XDR has its own set of data types and the application layer types will get converted to XDR format before transfer of data and the response data also needs to be converted to XDR. In a distributed system having heterogeneous systems, the presentation layer plays a key role to facilitate communication. 10.8 Application Layer This is the top most layer of the OSI model. Being the seventh lowest lay