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Software archiecture lecture07
- 1. Air Traffic Control: A Case Study in Designing for High Availability
- 2. Introduction • Air traffic control (ATC) is among the most demanding of all software applications. ▫ It is hard real time, meaning that timing deadlines must be met absolutely ▫ It is safety critical, meaning that human lives may be lost if the system does not perform correctly; ▫ It is highly distributed, requiring dozens of controllers to work cooperatively to guide aircraft through the airways system. • This chapter is a case study of one part of a once- planned, next-generation ATC system for the United States • We will see how its architecture
- 3. Case • In the United States, air traffic is controlled by the Federal Aviation Administration (FAA) • The FAA is the customer for the system we will describe • As a flight progresses from its departure airport to its arrival airport, ▫ it deals with several ATC entities that guide it safely through each portion of the airways (and ground facilities) it is using. ▫ Ground control coordinates the movement of aircraft on the ground at an airport. ▫ Towers control aircraft flying within an airport's terminal control area, a cylindrical section of airspace centered at an airport. ▫ Finally, en route centers divide the skies over the country into 22 large sections of responsibility
- 4. From Take-off to Landing
- 5. En route centers in United States
- 6. ISSS • The system we will study is called the Initial Sector Suite System (ISSS), which was intended to be an upgraded hardware and software system for the 22 en route centers in the United States. • The fact that ISSS was to be procured as only one of a set of strongly related systems had a profound effect on its architecture. • In particular, there was great incentive to adopt common designs and elements where possible because the ISSS developer also intended to bid on the other systems. • After all, these different systems (en route center, tower, ground control) share many elements: interfaces to radio systems, interfaces to flight plan databases, interfaces to each other, interpreting radar data, requirements for reliability and performance, and so on. • Thus, the ISSS design was influenced broadly by the requirements for all of the upgraded systems, not just the ISSS-specific ones. • The complete set of upgraded systems was to be called the Advanced Automation System (AAS).
- 7. The ABC applied to the ATC System
- 8. Requirements and Qualities • Ultrahigh availability, ▫ meaning that the system is absolutely prohibited from being inoperative for longer than very short periods. ▫ The actual availability requirement for ISSS is targeted at 0.99999, meaning that the system should be unavailable for less than 5 minutes a year. • High performance, ▫ meaning that the system has to be able to process large numbers of aircraft—as many as 2,440—without "losing" any of them. ▫ Networks have to be able to carry the communication loads, and the software has to be able to perform its computations quickly and predictably.
- 9. Requirements and Qualities • Openness, ▫ meaning that the system has to be able to incorporate commercially developed software components, including ATC functions and basic computing services such as graphics display packages • The ability to field subsets of the system, ▫ to handle the case in which the billion-dollar project falls victim to reductions in budget (and hence functionality)—as indeed happened • The ability to make modifications to the functionality and handle upgrades in hardware and software (new processors, new I/O devices and drivers, new versions of the Ada compiler) • The ability to operate with and interface to a bewildering set of external systems, both hardware and software, some decades old, others not yet implemented
- 10. Architectural Solution • Physical view • Module decomposition view • Process view • Client-Server view • Code view • Layered view • Fault tolerance
- 11. ISSS Physical view •Host Computer System (HCS) •Local Communications Network (LCN) •LCN interface units (each called LIU- H) •Enhanced Direct Access Radar Channel (EDARC) •Backup Communications Network (BCN) •Monitor-and-Control (M&C) consoles
- 12. Process view •Functional Group (FG) •Primary Address Space (PAS) •Secondary Address Space (SAS)
- 14. Layered View
- 15. How the ATC System achieves its Quality goals Goal How Achieved Tactic(s) Used High Availability Hardware redundancy (both processor and network); software redundancy (layered fault detection and recovery) High Performance Distributed multiprocessors; front-end schedulability analysis, and network modeling Openness Interface wrapping and layering
- 16. How the ATC System achieves its Quality goals Goal How Achieved Tactic(s) Used Modifiability Templates and table- driven adaptation data; careful assignment of module responsbilities; strict use of specified interfaces Ability to Field Subsets Appropriate separation of concerns Interoperability Client-server division of Adherence to defined functionality and protocols; maintain message-based interface stability communications