Information Technology Reference
In-Depth Information
enable higher layers to specify the location and size of the bitstream of interest
and receive the bits as a string of 1 and 0 bits. In modern computing systems
device drivers and chips built into the physical storage interface provide much of
this functionality.
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The Stream Layer hides the unique characteristics of the transport medium by
stripping any artefacts of the storage or transmission process (such as packet for-
mats, block sizes, inter-record gaps, and error-correction codes) and provides the
higher levels with a consistent view of data that is independent of its medium.
The interface between the Stream Layer and higher layers allows the higher lay-
ers to request Data Blocks by name and receive a bit/byte string representing
those Data Blocks. The term “name” here means any unique key for locating the
data stream of interest. Examples include path names for files or message identi-
fiers for telecommunication messages. In modern computing systems, operating
system file systems often provide this layer of functionality.
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The Structure Layer converts the bit/byte streams from the Stream Layer inter-
face into addressable structures of primitive data types that can be recognized and
operated on by computer processors and operating systems. For any implemen-
tation, the structure layer defines the primitive data types and aggregations that
are recognized. This usually means at least characters and integer and real num-
bers. The aggregation types typically supported include a record (i.e., a structure
that can hold more than one data type) and an array (where each element con-
sists of the same data type). Issues relating to the representation of primitive data
types are resolved in this layer. The interface from the Structure Layer to higher
levels allows the higher levels to request labelled aggregations of primitive data
types and receive them in a structured form that may be internally addressable.
In modern computing systems programming language compilers and interpreters
generally provides this layer of functionality.
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The Object Layer, which converts the labelled aggregates of primitive data types
into information, represented as objects that are recognizable and meaningful in
the application domain. In the scientific domain, this includes objects such as
images, spectra, and histograms. The object layer adds semantic meaning to the
data treated by the lower layers of the model. Some specific functions of this
layer include the following:
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define data types based on information content rather than on the representa-
tion of those data at the structure layer. For example, many different kinds of
objects - images, maps, and tables - can be implemented at the structure level
using arrays or records. Within the object layer, images, maps, and tables are
recognized and treated as distinct types of information.
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present applications with a consistent interface to similar kinds of information
objects, regardless of their underlying representations. The interface defines
the operations that can be performed on the object, the inputs required for
each operation and the output data types from each.
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provide a mechanism to identify the characteristics of objects that are visible
to users, operations that may be applied to an object, and the relationships
between objects. The Interface between the Object Layer and the Application
