Reversing course: Issues in modeling legacy systems

Abstract

The Enhanced Traffic Management System (ETMS) is a legacy system used by the Federal Aviation Administration (FAA) to support air traffic flow management. Air traffic flow management is the strategic control of air traffic to minimize delays and congestion and maximize the throughput of aircraft throughout the National Airspace System (NAS). This paper discusses the reasons for modeling a legacy system, problems and advantages encountered in modeling an operational system, and describes the construction of a simulation model of ETMS. Originally written in Pascal to run on Apollo workstations under the Aegis Domain operating system, ETMS has been converted to C/C++ and ported to HP servers and workstations running HP-UX, a POSIX-compliant version of UNIX. The objectives of the modeling task were to assess performance of the ported system and to provide a basis for evaluating a possible redesign/re-architecture 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 relatively high level, and then to develop a more detailed model to look at specific workstation/server issues. As is shown in this paper, issues of existing system design and documentation and the availability (or the lack) of data continually arose. Nevertheless, a reasonable set of working assumptions were derived which allowed modeling and evaluation to proceed. Thus, the quantitative and qualitative results obtained provided information and lessons learned that can be built upon. Moreover, the second of the stated goals (to provide a basis for possible redesign) was also achieved because there is now a baseline for future design/architecture studies. The focus of this paper is to provide insights into the issues involved in modeling an existing system rather than the results of the model itself.