PSTN Call Flow

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 A’s phone rings (unless it is already off-hook).
Parties A and B can begin their conversation after the following sequence of
steps is completed:
1. Party A picks up the phone.
2.The switch receives an answered call indication (off-hook).
3.The ring-down signals stop.
4. Parties A and B are able to speak on the established voice path.
After the two parties finish their conversation, the on-hook sequence of steps
begins:
1.The conversation ends with either party hanging up the phone.
2.The on-hook indication is received by switches on access networks.
3.The switches release established paths (termination).
4.The call is ended.
During each of these sections there is traffic traveling in both directions to keep
the signal alive.There are numerous acknowledgement requests between the caller
and their access network, and the two access networks and the called party and their
network, to keep this communication path alive. Most of this traffic is happening
along the voice path.
This book is about securing voice over Internet networks, so later in the book
you will be introduced to a protocol called Session Initiation Protocol (SIP).Though
it is early on in the text we will now walk through a SIP to PSTN call. Remember
that PSTN is a voice network and the SIP is originating from a data-only network.
We will follow the sections of off-hook, digit receipt, ring down, conversation, and
on-hook.To better visualize this call sequence we will use the