Page 40 - DCAP608_REAL TIME SYSTEMS

Unit 3: Hard Versus Soft Real-time System

instruments with the critical time constraints, rocket and satellite control, aircraft control and Notes
navigation, industrial automation and control, and robotics.
Soft real-time systems also have time constraints; however, missing some deadline may not
lead to catastrophic failure of the system. Thus, soft real-time systems are similar to hard real-
time systems in their infrastructure requirements, but it is not necessary that every time constraint
be met. In other words, some time constraints are not strict, but they are nonetheless important.

!

Caution A soft real-time system is not equivalent to non-real-time system, because the
goal of the system is still to meet as many deadlines as possible.
Some applications with soft real-time requirements include web services such as real-time
query, call admittance in voice over internet protocol and cell phone, digital TV transmissions,
cable and digital TV set-top-boxes, video conferencing, TV broadcasting, games, and gaming
equipment. Multimedia systems in general are examples of soft real-time systems (e.g., dropping
frames while displaying video).

Even in some typical hard real-time applications, some functions have soft real-time constraints.
For instance, in Apollo 11, the lunar module guidance computer could not keep up with the data
stream from the landing radar. However, it was discovered that the missed deadlines were
nonfatal, and the scheduler automatically adjusted, to meet soft real-time behaviour for the
landing tasks.
Self Assessment

State whether the following statements are True or False:
7. Deadline is defined as an instant of time a job’s execution is required to be completed.
8. If deadline is infinity, then job has finite deadline.
9. Absolute deadline is equal to release time minus relative deadline.

10. Timing constraint is a constraint imposed on timing behaviour of a job: hard or soft.
11. Hard real-time systems have very strict time constraints, in which missing the specified
deadline is acceptable.

12. Soft real-time systems also have time constraints; however, missing some deadline may
not lead to catastrophic failure of the system.

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Case Study Process and Machine Description

“Punching” is a general term describing the process of cutting a hole in a metal sheet. The
punching process results from the motion of two sharp, closely adjoined edges on a
material placed between them and comprise three stages:
 The Deformation Phase, as the cutting edges begin to close;
 The Penetration Phase, as the cutting edges penetrate the material causing initial
fracture lines on both sides of the material;
 The Fracture Point, the point where the upper and lower fracture lines meet.
Contd...

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