The Enhanced Traffic Management System (ETMS) is a legacy system used by th
e Federal Aviation Administration (FAA) to support air traffic flow managem
ent. Air traffic how management is the strategic control of air traffic to
minimize delays and congestion and maximize the throughput of aircraft thro
ughout the National Airspace System (NAS). This paper discusses the reasons
for modeling a legacy system, problems and advantages encountered in model
ing an operational system, and describes the construction of a simulation m
odel of ETMS. Originally written in Pascal to run on Apollo workstations un
der the Aegis Domain operating system, ETMS has been converted to C/C++ and
ported to HP servers acid workstations running HP-UX, a POSIX-compliant ve
rsion of UNIX. The objectives of the modeling task were to assess performan
ce of the ported system and to provide a basis for evaluating a possible re
design/rearchitecture of the system. The initial plan was to develop one or
two models aimed at the network aspects (both LAN and WAN) of ETMS at a re
latively high level, and then to develop a more detailed model to look at s
pecific workstation/server issues.
As is shown in this paper. issues of existing system design and documentati
on and the availability (or the lack) of data continually arose. Neverthele
ss, a reasonable set of working assumptions were derived which allowed mode
ling and evaluation to proceed. Thus, the quantitative and qualitative resu
lts obtained provided information and lessons learned that can be built upo
n. Moreover, the second of the stated goals (to provide a basis for possibl
e redesign) was also achieved because there is now a baseline for future de
sign/architecture studies. The focus of this paper is to provide insights i
nto the issues involved in modeling an existing system rather than the resu
lts of the model itself.