Now that we have discussed what makes up the PSTN, let s put it all together and walk through a messaging sequence. Here we will start from a caller picking up the phone attempting to make a call. The flow will be broken down into off-hook, digit receipt, ring down, conversation, and on-hook sections. We will start by imagining someone (Party B) picking up the phone to make the call (to Party A, on the same CO switch). The following list outlines, in order, the actions performed by the network:

Party B picks up the phone, and the off-hook sequence begins:

1. The off-hook state is detected by the switch (loop or ground start).

2. The switch establishes the time slot and sends a dial tone on the voice path.

3. The switch awaits digits pressed by Party B.

The digit receipt sequence is as follows:

1. Party B dials digits on the touch pad.

2. Each digit is received by the switch and sends a silence tone and starts Inter Digit Timer (IDT).

3. IDT starts when the switch is awaiting a dialed digit and stops when the digit is pressed.

After Party B dials the last number, the ring down sequence begins:

1. When the digit receipt stops (or when the maximum dialed digits are pressed), the switch sends the request to the called number to allocate a time slot.

2. When the called switch allocates a time slot the path is switched to the call handler.

3. Party...