ISO Presentation Service
Characteristics of the Presentation Service
The presentation service provides two general categories of service.
Its prime service is to handle the representation of application data so
that two application entities can successfully exchange data even if they use
different local representation of that data. To provide this service the
presentation layer has two functions that it carries out for presentation
service users.
- Negotiation of transfer syntaxes:
For any type of user data, a transfer
syntax is negotiated that can be used by each presentation entity for
converting to or from its user's data representation.
- Transformation:
The data that the user provides is converted into transfer
syntax representation for transmission. Data received for delivery to the
user is transformed from the transfer syntax representation to the user's
representation.
The presentation layer services are grouped into functional units for the
purpose of negotiation at connection establishment time. These are provided
so that a presentation user may request and negotiate the use of particular
session services.
Presentation Functional Units
- Presentation kernel
- Context management
- Context restoration
If two applications are using a common representation for all data to be
exchanged, then only the presentation kernel is needed. The presentation layer
just passes requests and responses down to the session layer and passes
session indications and confirmations up to the application layer. If two
applications do not share a common representation, then the presentation layer
performs the necessary transformation.
The presentation service deals with one or more presentation contexts. Each
context specifies the abstract syntax of the user's data and the transfer
syntax to be used in transmitting that user data. The defined context set
consists of presentation contexts that have been defined by agreement between
the two presentation users and the presentation service provider.
The default context is a presentation context that is always known to the
presentation provider and is used when the defined context set is empty.
With the context management functional unit, it becomes possible to change the
defined context set during the course of a presentation connection. Both
additions to and deletions from the context set are permitted.
The context restoration functional unit is available to deal with the
interaction between context management and session synchronisation.
The following is a list of other related links.
Aidan Mc Dermott