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8. The Application Layer
The application layer is comprised of a variety of standards, each of which provides a set of useful services for the benefit of end-user applications. Only those services
which can be provided in a system-independent fashion are subject to standardization. These include: virtual terminal handling, message handling, file
transfer, job transfer, and many others.
Describing all existing application layer standards is beyond the scope of this book, as they are numerous and detailed. This chapter will look at a few of such
standards. The aim is to illustrate the flavor of the services provided rather than provide an exhaustive coverage of the standards.
We will first discuss application services, and explain some of the relevant terminology. Then we will describe two groups of application service elements. The
first group is commonly used by other application service elements. The second group addresses very specific requirements. Finally, we will list a number of
application layer standards.
8.1. Application Services
Depending on their scope of use, application services may be divided into two broad categories:
•
General, low-level services which are used by most applications. This group includes three sets of services: association control, reliable transfer, and remote
operations. They are further described in section 8.2.
•
Specific, high-level services which are designed to support task-oriented application requirements. Examples include: virtual terminal handling, message
handling, and file transfer. These directly utilize the general application services, and are further described in Sections 8.3-5.
User applications should make as much use of the above services as possible so that they can benefit from the standardizations. A user application can be imagined as
consisting of two parts: i a standardized part, called Application Entity AE, which is immersed in the application layer and is implemented in terms of the
application layer services, and ii a non-standard part, called Application Process AP, which remains outside the scope of the application layer. Typically, the
application entity deals with the inter-application communication, and the application
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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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