6. Database Tailoring and Retrofitting Techniques › 6.2 Tailoring the Database › 6.2.1 Tailoring Data Elements › 6.2.1.2 Common Data Elements

6.2.1.2 Common Data Elements

The example file definition contains the component generator
definition statements:

...
TYPE N 3 . 3 . 3 .
NAME NCPMNFBL  00  0 0 0 0 0 Free Buffer Low Water Mark
NAME NPANRRLS  99  0 0 0 0 0 .
NAME NPANRSLS  99  0 0 0 0 0 .
NAME NPATMAOT  99  0 0 0 0 0 .
...

Notice the data elements whose option code is 99.  These
data elements are NPANRRLS, NPANRSLS and NPATMAOT.  They are
also contained in the generation library member $DEGENIN:

...
TYPE XR  3  .  3  .  3  .
NAME NPANRRLS Receiving Line Speed (line/link)
TYPE XR  3  .  3  .  3  .
NAME NPANRSLS Sending Line Speed (line/link)
TYPE XA  5  TIME11.2  6  TIME15.2  6  TIME15.2
NAME NPATMAOT Actual Observation Time
...

The reason for having data elements defined in this
common manner is to assure consistency of definition of such
common data elements across all files in which they appear.
Data elements that are not of this "common" nature are called
"standard" data elements.

Most common data elements are named generically,
according to their function or contents.  Examples of such
data elements are ENDTS (end time stamp) and INTERVLS (the
number of recording intervals in a measurement
summarization).

Some common data elements take their names from the
component.  The first three characters of the data element
name are the component identifier, and the remaining 5
characters are a cryptic abbreviation of the data element's
function.  Examples are NPANRRLS (NPA component receiving
line speed) and NPATMAOT (NPA component actual interval
observation time).

It is possible to redefine common data elements into
standard data elements.  For example, the above extract could
have been coded:

...
TYPE N 3 . 3 . 3 .
NAME NCPMNFBL  00  0 0 0 0 0 Free Buffer Low Water Mark
TYPE XR 3 . 3 . 3 .
NAME NPANRRLS  00  0 0 0 0 0 Receiving Line Speed (line/link)
NAME NPANRSLS  00  0 0 0 0 0 Sending Line Speed (line/link)
TYPE XA  5  TIME11.2  6  TIME15.2  6  TIME15.2
NAME NPATMAOT  00  0 0 0 0 0 Actual Observation Time
...

Redefining these elements in this manner defeats the main
purpose of common data elements - single source format
definition.  Common data elements are often key data elements
in the file, used in sequencing the file's observations.  If
you attempt to bring two files together, and both files have
data elements of the same name in the key (eg. both have
ENDTS in the file sequence structure), then those data
elements must have the same format on both files.