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.