There are several tools that are frequently used by systems engineers:
The first significant systems engineering was performed for telephone systems. All the different parts of the phone system have to interoperate reliably. An excellent overview of the interfaces and logic, with some history, is "Digital Telephony" by John C. Bellamy[?].
When a system manipulates some physical process[?], simulation and modeling are important. Aircraft are usually modeled and simulated before flight. In this way the initial aeroelastic engineering and control equations[?] can be drafted initially and improved before the physical system is constructed. Since aircraft are often very expensive, this reduces the expense and difficulty of debugging the controls and reduces the risk of crashing real aircraft. Careful initial testing and flight envelope expansion are typically still required to reach acceptable levels of safety and performance in advanced aircraft.
System engineers perform testing and validation when a system has to have predictable behavior. For example medical machinery such as heart and lung machines usually consist of several parts, engineered by different companies. Testing and validation assures that normal operation and possible failures of each part will not harm the patient. Other applications are communications systems, or banking software, where failures can cause loss of property or liability. Test plans can often be adjusted to save significant amounts of money, by testing partial systems, or including special features in a system to aid testing.
The techniques of safety engineering can be applied by everyday people to planning complex events to assure that the systems cannot cause harm. . Most of safety engineering is just a way of making plans that cope with failures.
Usually a failure in safety-certified systems is acceptable if less than one life per 30 years of operation (10^9 hours) is lost to mechanical failure. Most Western nuclear reactors, medical equipment and commercial aircraft are certified to this level.
Interface design[?] and specification are concerned with making the pieces of a system interoperate. For example, the plugs between two computer systems can be a fertile source of failures. Sometimes something as simple as gold-plating the plugs can lower the probability of a failure enough to save millions of dollars.
Another issue is assuring that the signals that pass from system to the next are in tolerance, and that the receivers have a wider tolerance than transmitters.
Another issue is that the interface should be able to accept new features. Most often this is a problem in a plug and jack, of the transmission speed, although it sometimes affects computer data formats. The rule of thumb is that roughly 20% of the space in an interface should be reserved for future additions.
Interface design principles also have been used to place reserved wires, plug-space, command codes and bits in communication protocols.
Systems engineering principles are applied in the design of network protocols.
Security engineering can be viewed as a field of systems engineering.