Information Technology Reference
In-Depth Information
Layer allows the higher levels to specify the operation that is to be applied
to an object, the parameters needed for that operation and the form in which
results of the operations will be returned. One special interface allows the user
to discover the semantics of the objects, such as operations available and rela-
tionships to other objects. In modern computing systems subroutine libraries
or object repositories and interfaces supply this functionality.
•
The Application Layer contains customized programs to analyze the Data Objects
and present the analysis or the data object in a form that a Data Consumer
can understand. In modern computing systems application programs supply this
functionality.
7.3 Structure Representation Information
OAIS has the following to say about Structure Representation Information (SI):
Structure Information: The information that imparts meaning about how other
information is organized. For example, it maps bit streams to common computer
types such as characters, numbers, and pixels and aggregations of those types
such as character strings and arrays
.
The Digital Object, as shown in
Fig.
7.3
, is itself composed of one or more bit
sequences. The purpose of the Representation Information object is to convert
the bit sequences into more meaningful information. It does this by describing
the format, or data structure concepts, which are to be applied to the bit sequences
and that in turn result in more meaningful values such as characters, numbers,
pixels, arrays, tables, etc. These common computer data types, aggregations of
these data types, and mapping rules which map from the underlying data types
to the higher level concepts needed to understand the Digital Object are referred
to as the Structure Information of the Representation Information object. These
structures are commonly identified by name or by relative position within the asso-
ciated bit sequences. The Structure Information is often referred to as the 'format'
of the digital object
.
We have seen the following figure several times before, but this time we will
move from the very abstract view to the concrete.
An obvious example of Structure RepInfo is a document or standard that
describes how to “read and write” a file format.
Structure RepInfo can be broken down into levels, the first level being the struc-
ture of the bits and how they map to data values. This involves the exact specification
of how the bits contain the information of a data value and involves the definition of
several generic properties. This bit structure will be referred to as the Physical Data
Structure, and often is dictated by the computing hardware on which the data was
created and the programming languages used to write the data. Data values are then
