96 Communication Networks
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process provides the user front-end and interfaces to the local environment see Figure 8.85.
Figure 8.81 User applications in relation to the application layer.
Presentation Layer Presentation Layer
Application Association Application Layer
Presentation Connection Application Layer
Application Entity Application Process
Application Entity Application Process
Figure 8.85 illustrates a further point. Unlike earlier layers, the application layer does not provide connections. This should not come as a surprise, because there are
no further layers above the application layer that would otherwise use such a connection. Instead, two applications use an association between their application
entities for exchange of information over the presentation connection. An association serves as an agreement so that the two parties are aware of each other’s
expectations.
Application processes will be no further discussed, as they are outside the scope of the OSI model and standards. Application entities are the focus of what the
application layer standards are about, and are discussed further below.
8.1.1. Application Entity
As mentioned earlier, an application entity accounts for that part of a user application which falls within the scope of the application layer. An application entity consists of
one or more application service elements and a control function see Figure 8.82. An Application Service Element ASE represents an application service and its
associated standard protocol. A Control Function CF regulates the activities of an application entity. This includes managing the ASEs and the association with the peer
application entity.
Peer application entities must share a common application context by ensuring
that either entity uses exactly the same set of ASEs and uses them in the same fashion.
There are two classes of ASEs:
•
A Common Application Service Element CASE denotes a general application service.
•
A Specific Application Service Element SASE denotes a specific application service.
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We will look at these in turn.
Figure 8.82 Makeup of an application entity.
Presentation Layer Application Association
Application Entity
ASE ASE
ASE ASE
ASE ASE
Control Function
8.2. Common Application Service Elements
This section describes three CASEs used by many other ASEs.
8.2.1. Association Control
The Association Control Service Element ACSE is used by every application entity. It provides a set of functions for managing the association between peer
application entities. Since managing an association is independent of the activities the association is used for, ACSE serves as an appropriate and highly useful standard.
An association is established using the A-ASSOCIATE service primitive. This maps to the P-CONNECT primitive and hence also establishes a corresponding
presentation connection. The application context and the abstract syntaxes are also conveyed during this process. The A-RELEASE primitive maps to P-RELEASE and
handles the orderly release of an association as well as the release of the corresponding presentation connection after ensuring that all data in transit has been
delivered. Error-initiated releases are managed by A-ABORT and A-P-ABORT which, respectively, map to and are similar to P-U-ABORT and P-P-ABORT .
The ACSE service primitives are implemented as Application Protocol Data Units
APDUs. Each of these represents a unique application-wide type and is tagged accordingly see Figure 7.8.
ISO 8649 and CCITT X.217 standards describe the ACSE service. The ACSE protocol is described in ISO 8650 and CCITT X.227 standards.
8.2.2. Reliable Transfer
The Reliable Transfer Service Element RTSE hides much of the underlying complexity of the session dialogue management services by providing high-level error
handling capabilities to other ASEs for data transfer. RTSE segments a data transfer
