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 following illustration (see Figure 1) to help us. Party A will be the SIP user and Party B will be the PSTN user.

 

Figure 1: SIP-to-PSTN Call Flow

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

1. Party A picks up the phone and dials the number.
2. An off-hook state is noticed by the SIP client.
3. The SIP client sends a request to the SIP proxy (at ISP).
4. The SIP client sends the SIP tel URL with the request.
5. ISUP message is prepared by the ISP PSTN Gateway.
6. The ISP Proxy finds the local terminating PSTN to send the call through (Network PSTN Gateway NGW).

The digit receipt sequence of steps begins:

1. Since Party A already sent the entire dialed number through the SIP phone prior to the call being sent through the Network PSTN Gateway, all the dial information is already there, so when the call is sent to the PSTN the switches already have all the information they need to process and route the call (i.e., no overlap sending is required).
2. This is sent through ISUP Messaging by the ISP PSTN Gateway.

Now, the ring down sequence begins:

1. Party A’s switch establishes a one-way voice path.
2. Party A’s switch sends a ringing tone.
3. At the same time, Party B’s switch is establishing its voice path.
4. Party B’s switch completes the set up.
5. Party B’s phone rings.

Parties A and B can begin their conversation after the following sequence of steps is completed:

1. Party B picks up the phone.
2. Switches receive an answered call indication.
3. Party A’s switch sets communication to bidirectional.
4. Parties A and B are able to speak on the established voice path.

When the two parties end their conversation, the on-hook sequence of steps begins:

1. The conversation ends with Party A hanging up the phone.
2. The SIP client sends a BYE message to Proxy at ISP.
3. The ISP Proxy sends a BYE signal to NGW.
4. Switches release established paths (termination).
5. The call is ended.