The Application Layer is the highest layer of the OSI architecture, the purpose of which is to serve as a window between correspondent application processes so that they may exchange information on the open environment. The progams which use the Application Layer are known as Application Processes. A user program may interface directly with the Presentation Layer in which case it must include protocol modules to initialise communication with peer application processes, establish the appropriate Presentation context and transfer files or messages. Alternatively, the user process may include available modules which support commonly required application related services. Such modules are referred to as Application Specific Elements and are defined as an integrated set of functions which together provide one or more application related communication capabilities. These capabilities are defined by a protocol in a very similar manner to that used to specify a service provided by the layer below. The protocol may specify direct use of presentation layer services and/or those provided by other ASEs. The grouping of a user process (or application entity), relevant ASEs and interfaces between them is known as an application process.
- JTM - JOB TRANSFER AND MANIPULATION
- CASE functions
- X.200 - application Layer - see X217
- Services provided to application-processes
- Functions within the application layer
- Message Handling System
- X.400 - Use of OSI communications facilities
- Message Transfer Sublayer
- Summary of common application layer service elements
Application Layer protocols are classified into Common Application Specific
Elements (CASE) and Specific Application Specific Elements (SASE).
CASE elements are commonly required by user elements and SASEs, whereas SASEs
are only included in an application process when their particular service is
specifically required. For more information on SASEs Group Xs project can be
There are four CASEs currently defined. These are...
- Association Control ACSE, enables users to establish or terminate an association between application processes.
- Reliable Transfer RTSE, enables reliable transfer of information between peers.
- Remote Operations ROSE enables users to initiate operations at a remote site.
- Recovery CCR enables users to recover from failure during
execution of a task.