Models for Open System Protocol Development : A Technical Report (1994)

6. Application Layer Structure

6.1 Use of Multiple TC46 Application Protocols in One Application Association

6.2 TC46 Application Protocols With JTC1 Application Protocols

Both SR and ILL may be used in connection with Directories. The Origin, the Requester or an Intermediary may use the Directory Services to get the network address of a specific library system. Then switch to the SR or ILL respectively.

6.3 Construction of AEs from Various ASEs

As the basic building block of any AE an ASE is defined as a self contained group of interrelated communication functions. The varied needs of user applications have given rise to definition of ASEs that differ widely in functional scope, operational characteristics , styles of communication, requirements of support services, etc. Although application designers can generally pick and choose from a variety of independently defined ASEs, they are naturally constrained by the functional scope of the communication services required and the mapping requirements and operational behavior of component ASEs.

Whether a specific user application should have one or more AEs to satisfy its communication requirements would depend on the functional range and operational characteristics of the communication services required as well as the technical design of the user application. When the user application is rather complex and requires communication services of varied nature then several AEs of different types may be needed to support the application. The coordination of the different AEs will be the responsibility of the application itself.

When several ASEs are combined to operate within an AE the individual ASEs will have no notion of each others existence and some higher level control functions will be needed to ensure the coordinated operation of ASEs. It is this control function (CF), which is an integral part of the definition of any AE, that unifies the individual services of the component ASEs into what looks like a single communication service to the user application. The CF within the AE relieves the user application of any concern with the coordination and supervision of the services provided by individual ASEs.

In describing the internal structure of the OSI Application Layer, a clear distinction is made between the type of an object which is its specification and its invocation which is its actual use in an instance of communication. An AE definition addresses both these aspects. The AE definition specifies,for example, the number of ASE invocations of any given type that may exist in an AE invocation of this particular AE-type. An AE definition must also specify whether the AE is to support communication over a single or multiple application associations. The extended Application Layer Structure standard (ISO 9545:1993) allows a highly flexible and recursive structuring of AE using the broadened concepts of application layer objects and relationships. This enables definition of AEs and the corresponding AEIs that can support a very wide variety of communication needs of user applications.

6.4 Definition Of Application Context

There is no standard for defining application contexts although check lists of some kind may be used with profit. An application-context definition deals with such issues as invocation dependencies among the component ASEs, ordering of events generated by the component ASEs, use of ACSE and Presentation services, APDU concatenation, action to be taken in the event of rule or constraint violation, etc. An agreed application-context is a precondition of any communication to be possible between open systems applications. Application-context definitions are therefore given names and the names are registered. When applications wish to communicate with each other it is this name that they must exchange first.