Information Technology Reference
In-Depth Information
6. calculation of existence of elements or records from other DVs in a logical
expression.
7. comparison expressions, i.e. string comparisons etc.
8. existence values.
9. choice statements of elements or records.
7.3.4 Formal Structure Description Languages
In this section we look at a number of formal languages which support automation.
These formal languages are rather powerful but not really applicable
to digital objects such as Word files.
Each method has its own strengths.
7.3.4.1 East
The EAST (
E
nhanced
A
da
S
ubse
T
) language [
47
] is a CCSDS and ISO stan-
dard language used to create descriptions of data, called Data Description Records
(DDRs). Such DDRs aim to ensure a complete and exact understanding of the struc-
ture of the data and allow the data values to be extracted and used in an automated
fashion. This means that a software tool should be able to analyze a DDR and inter-
pret the format of the associated data. This allows the software to extract values
from the data on any host machine (i.e., on a different machine from the one that
produced the data).
EAST is fully capable of describing the physical structure of integer, real float-
ing point and enumerations. It does not support boolean data types. The exception
bit patterns of real floating point values are not supported. The byte-order for the
data can be specified globally for the digital object, but not for individual DVs.
Characters are restricted to 8 bit and the code points are specified in the EAST spec-
ification. String made up of 8 bit characters are allowed with a fixed length. The
appropriate restrictions and facets for strings are supported. The lack of ability to
define dynamic offsets for the logical structure is the main restriction; file formats
such as TIFF cannot be described with EAST. No path language is specified in the
EAST standard.
EAST has a comprehensive set of tools (see [
47
] and [
48
]).
The EAST standard gives the following examples.
A communications packet format is illustrated in Fig.
7.9
