O

The basic symbols are defined as follows :

State boxes. The state boxes are labelled with state number and state description. If necessary, additional information can also be included. The combination of the present state and a new event defines a task and performing this results in next state. Sometimes more than one state occurs, the choise depending on external information.

Event boxes. The intended arrow of the symbol indicate whether the event corresponds to the recipt of forward or backward signal. The forward signal and backward signal refers to the flow of signal from calling to called and called to calling subscriber through exchange respectively.

Action boxes. The rectangular box represents the action taken on the event. The portruding arrow indicates whether the signal is sent forward or backward.

Decision boxes. The diamond shaped box is used for the cases where two divisions are possible. For multiple decisions, another symbol shown in Fig. 6.2 (e) is used.

Connectors. This sysmbols are used to connect one flow chart to another diagram.

Fig. 6.3 shows the s.t.d. diagram for a typical local call. Let the calling subscriber is A and the called subscriber is B.

f State O

f State O

Subscriber A associated feature identification

JUnobtaln-

Providing dial tone to A

Waiting for answer J *

Remove ringing when customer answer

Subscriber A associated feature identification

Number unavailable tone_i

JUnobtaln-

Providing dial tone to A

iwajtii

Waiting for answer J *

Remove ringing when customer answer

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