Reversing course: Issues in modeling legacy systems

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.