support are shown in Fig. 11.27. A payroll tax return (PTR) can be submitted in two
main formats: on paper or electronically. The electronic tax returns have three
possible formats: Web-based messages, small messages sent via SMTP, and large
messages sent via FTP. The model shows the expected distribution of the total
number of messages over these different formats. The first part of the ‘Receive tax
return’ process transforms these formats into a common, medium-independent
format. We will refer to this phase of the process as ‘Medium-specific processing’.
The second phase of the ‘Receive tax return’ process, ‘Medium-independent
processing’, processes all the payroll tax returns in the same way, irrespective of
their original format