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Although there is no “one-to-one” relationship between UTFS defined
message types
and
ISO/IEC 8583 defined message classes, this standard provides a flexible model, or
foundation
to further define transit industry related data elements in its body.
The
second element in the message structure defines whether it contains one or two
bitmaps.
A “1” in the bitmap indicates the presence of the corresponding data element
in the message. Each bitmap has 64 bits, thus two bitmaps can provide the total of 128
data
elements, as defined by the standard. The first (primary) bitmap is mandatory and
represents
the most frequently used data elements. The presence of the secondary
bitmap is signified by a “1” in bit 01 of the primary bitmap.
Bitmaps provide the application providers with the flexibility of choosing among a
greater pool of data elements for varying industry segments, however, they tend to
add
extra
overhead to the transaction speeds, especially in real-time data processing
environments. The effects of this overhead may not be as significant for backend,
processing environments such as clearing and settlement or reporting that are not as
time sensitive as card to reader transactions.
Messages are constructed using the message bit map as an index of present data
elements. There are three types of data elements:
• Primitive data element
• Constructed data element
• Composite data element
Primitive data elements represent a single piece of information, whereas the constructed
data element consists of a fixed number of sub-elements, all of which must be present.
Composite data element consists of a large number of sub-elements. Most of these sub-
elements fall into industry related categories such as auto rental data, or airline data. In
order to define these categories, the concept of a “dataset” has been introduced. All
the
sub elements in
a composite data element are divided into a certain number of datasets,
each
of which relates to an industry and contain a dataset identifier. The structure of a
dataset is based on the message structure explained in this standard. There is also a
provision
for identifying sub-elements using the TLV data encoding method as an
alternative to using a second level bitmap. Each composite data element can therefore
contain a variable number of different datasets and can include both TLV and bitmap
formats as illustrated in Exhibit 4.1-2.
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