Q. Explain Standard set of procedure used in Completing a call?
Completing a local telephone call between two subscribers linked to the same telephone switch is accomplished through a standard set of procedure which includes 10 steps listed below.
1. Calling station goes off hook.
2. After detecting a dc current flow on the loop, switching machine returns an audible dial tone to calling station, acknowledging that caller has access to switching machine.
3. Caller dials the destination telephone number using one of the two methods: Mechanical dial pulsing or more likely electronic dual-tone multi frequency (Touch-Tone) signals.
4. When switching machine detects first dialled number, it removes the dial tone from loop.
5. Switch interprets the telephone number and then locates loop for the destination telephone number.
6. Before ringing destination telephone , switching machine tests the destination loop for dc current to see if it is idle (on hook) or in use (off hook). At the same instance, switching machine locates a signal path through the switch amid the two local loops.
7. (a) If destination telephone is off hook, switching machine transmits a station busy signal back to the calling station.
(b) If destination telephone is on hook, switching machine transmits a ringing signal to destination telephone on the local loop and same time transmits a ring back signal to calling station to give the caller some assurance that something is happening.
8. When destination answers the telephone, it completes the loop, causing dc current to flow.
9. Switch recognizes the dc current as station answering the telephone. At this instance, switch removes the ringing and ring-back signals and completes the path through switch, allowing the calling and called parties to begin conversation.
10. When either end goes on hook, switching machine detects an open circuit on that loop and then drops connections through switch.