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Subsection 4.2 discusses that Representation Rendering Software is often used to end the Representation Network. A way to view this information is as a layered Information Model as shown figure E-1. In this model there are five layers. Each of these layers has well- defined interfaces to the higher layers of the model. These interfaces are known as Application Program Interfaces or Service Access Points in other layered models. The following is an overview of the functionality of each layer and the data that is exchanged at each interface. This overview illustrates the process of getting bits from the media and adding Representation Information needed to make the information usable by the Consumer.

Figure E-1 Layered Information Model 650x0m2.jpg Figure E-1: Layered Information Model

– The Media Layer simply models the fact that the bit strings are stored on physical or communications media as magnetic domains or as voltages. The function of this layer is to convert that bit representation to the bit representation that can be used in higher level (i.e., one and zero). This layer has as single interface, which enables higher layers to specify the location and size of the bit stream 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.

– The Stream Layer hides the unique characteristics of the transport medium by stripping any artifacts of the storage or transmission process (such as packet formats, 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 layers 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 identifiers for telecommunication messages. In modern computing systems, operating system file systems often provide this layer of functionality.

– The Structure Layer converts the bit/byte streams from the Stream Layer interface into addressable structures of primitive data types that can be recognized and operated by computer processors and operating systems. For any implementation, the structure layer defines the primitive data types and aggregations that are recognized. This usually means at least characters and integer and real numbers. 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 consists 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 labeled 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.

– The Object Layer, which converts the labeled 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:

• Defines data types based on information content rather than on the representation 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.

• Presents 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.

• Provides 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 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 relationships 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.

The problem with using Representation Rendering Software to end Representation Networks is that the programs that are saved do not include the information needed to enable the lower levels of the layered model to extract the information from the bits on the media. These services are usually provided by the vendor-supplied operating systems, device drivers, and file systems. When data is moved to other media or different software platforms, the interfaces to these levels may be changed. This migration process is further discussed in section 5 of this document.

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