State Machine Diagram Example: Phone


A state diagram is a graph whose node are state and whose directed arcs are transition between states which describes sequence caused by event sequences. In this example, the phone line is idle at the start of a call. When the phone is removed from the hook, it emits a dial tone and can accept the dialing of digits. Upon entry a valid number, the phone system tries to connect the call and route to the proper destination. The connection can fail if the number or trunk are busy. If the connection is successful the called phone begins ringing. When put on hook again, the phone line will go back to idle.

What is State Machine Diagram?

State machine designs are widely used for sequential control logic, which forms the core of many digital systems in a variety of applications covering a broad range of performance and complexity from software system like: use case or classes, or hardware digital devices like: low-level controls of micro-processor to VLSI peripheral interfaces. State machine diagram typically is used to model the possible states of a system or entities, show how state transitions occur as a consequence of events, and depict what behavior the system or object exhibits in each state.

How to draw State Machine Diagram?

  1. Identify objects or entities that need state diagrams
  2. Identify the states of the entity
  3. Define hierarchy of states, sub-states, orthogonal regions
  4. Link states and sub-states with transitions
  5. Add pseudo-states (i.e. entry point, exit point, choice join, fork junction, end etc.)
  6. Specify details of states and transitions using complete and appropriate notation

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