Presentation Standards The Presentation Layer

96 Communication Networks Copyright © 2005 PragSoft 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.