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Mapping Services is a facility that gives NCL access to complex data structures.
Mapping Services is designed to separate the application’s data processing requirements from a need to understand the actual organization of the data.
It means that NCL procedures deal with the logical relationships and usage of data (the data protocol), while the system manages and maintains the physical representation of the data (the data format).
Conventional NCL processing deals with simple data items accessed as NCL variables (or tokens). If a number of variables are logically related, the programmer must understand, through naming conventions or other disciplines, how data items are related and how they must be managed.
Difficulties arise in a number of circumstances, such as where data to be processed by NCL is sourced externally, or where NCL must define an interface to some other processing system. In such cases data is exchanged across a program interface, according to a strict protocol, and can be conveniently represented as some sort of protocol data unit. Such a protocol data unit is usually composed of one or more logical components, but must be presented across the interface as a series of bytes, and hence is structured according to some encoding technique.
Nearly all management data conforms to this simple model; however, there are many different encoding techniques employed. These range from very simple rules involving fixed length fields, one following another, to more complicated rules involving variable length structures, and even more complex rules involving self-defining lengths, tags, or similar structures.
The use of variable length data items, and tagged data structures, is popular because it encourages programming precision, and provides a continuous upward migration path. By extending the length of existing structures, or inventing new ones within a data unit, it can retain its original character while evolving to keep pace with new requirements.
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