The inventory example in Chapter 
presents an appropriate
environment to illustrate the situation. Suppose data on
specific items in Store A are sent to Store B, which uses a
different system and, possibly, a different programming language. Each
system might be flexible and large enough to handle the
communication by translating each representation of the data
into the representation in the other system. However, this approach
would become untenable as the number of stores with different
systems increased or as systems changed at individual stores over time.
A better solution is to make communication independent of change. This
can be achieved by using a defined set of
rules, an abstract syntax, for representing the data
types and structures of the items stored in the inventory
database, and another set of rules for transmitting the data, a
transfer syntax. These sets of rules would be
common for all systems. Each
system would then require the capability to translate the
abstract representation into a realization for the system.
More specifically, assume the item can be represented by the four-tuple [PN, Q, WP, SP], for part number (a string), quantity (an integer), wholesale price (a real), and sale price (a real), respectively, and that an abstract syntax has been defined for these variables and their data types. A Pascal program in Store A's system and a C language program in Store B's system include references to the item. Before the program is run, a compiler in Store A's system translates the abstract representation into a concrete syntax consisting of a Pascal structure (as if it were part or all of the declaration section) and a set of encoding/decoding procedures. A corresponding Application process is established in Store B's system using C instead of Pascal. When the data are sent from Store A, the encoding procedure is called and produces an encoded version to send to Store B's system (using appropriate aspects of each of the lower layers in the OSI model). There, the decoding procedure is called and the C structure is stored. Although programmers in Stores A and B could develop their own syntax and encoding rules, they would be better off using the ASN.1 abstract syntax and BER transfer syntax established as international standards.