Page 29 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes base station signals in the control channel. A cell phone responds to the communications received
from the base station within certain specified time bounds. For example, a base station might
command a cell phone to switch the on-going communication to a specific frequency. The cell
phone must comply with such commands from the base station within a few milliseconds.

2.4.8 Defence Applications

Typical defines applications of real-time systems include: missile guidance systems, anti- missile
systems, satellite-based surveillance systems.

Example : Missile Guidance System
A guided missile is one that is capable of sensing the target and homes onto it. Homing becomes
easy when the target emits either electrical or thermal radiation. In a missile guidance system,
missile guidance is achieved by a computer mounted on the missile. The mounted computer
computes the deviation from the required trajectory and effects track changes of the missile to
guide it onto the target. The time constraint on the computer-based guidance system is that the
sensing and the track correction tasks must be activated frequently enough to keep the missile
from diverging from the target. The target sensing and track correction tasks are typically
required to be completed within a few hundreds of microseconds or even lesser time depending
on the speed of the missile and the type of the target.

2.4.9 Miscellaneous Applications

Besides the areas of applications already discussed, real-time systems have found numerous
other applications in our everyday life. An example of such an application is a railway reservation
system.

Example: Railway Reservation System
In a railway reservation system, a central repository maintains the up-to-date data on booking
status of various trains. Ticket booking counters are distributed across different geographic
locations. Customers queue up at different booking counters and submit their reservation
requests. After a reservation request is made at a counter, it normally takes only a few seconds
for the system to confirm the reservation and print the ticket. A real-time constraint in this
application is that once a request is made to the computer, it must print the ticket or display the
seat unavailability message before the average human response time (about 20 seconds) expires,
so that the customers do not notice any delay and get a feeling of having obtained instant results.
However, as we discuss a little later, this application is an example of a category of applications
that is in some aspects different from the other discussed applications. For example, even if the
results are produced just after 20 seconds, nothing untoward is going to happen - this may not be
the case with the other discussed applications.
Self Assessment

State whether the following statements are True or False:
7. The term real-time refers to systems in which the correctness of the system depends not
only on the logical result of computation, but also on the time at which the results are
produced.
8. Examples of real-time systems exclude command and control systems, process control
systems, flight control systems, and so on.

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