4.14 Data Communication Services 113 numbering) and the recovery in case of failures. The session layer closes the session at the end of transmission. The presentation layer asks the session layer to set up a call. It specifies the destination's name and type of transmission (e.g., datagram, high priority). The presentation layer translates between the local syntax used by the application process and transfer syntax. The presentation layer also performs the required encryption and data compression. The application layer provides information transfer services for user application programs. The user interacts with the application layer through a user interface. The application layer is composed of Specific Application Service Elements (SASEs) that use the services of Common Application Service Elements (CASEs). A CASE establishes are association between SASEs and may include an Association Control Service Element (ACSE), a Remote Operation Service Element (ROSE), and the Commitment Concurrency and Recovery (CCR) element. 4.14 Data Communication Services End-to-end communication services are classified as either synchronous (sync) or asynchronous (async) [2]. A sync communication service delivers a bit stream with a fixed delay and a given bit error rate. The voice communication is an example of