How the PSTN Works

To talk about VoIP technology, it helps to understand a little about how the PSTN works
today. Here’s what has to happen when someone—the caller—makes a telephone call to
someone else—the callee—over the PSTN:
1) The caller picks up the telephone handset hears a dial tone.
2) A telephone number is entered, specifying the address of the callee.
3) Signals are sent through the PSTN to set up a circuit for the call. Capacity and bandwidth
are reserved for the call.
4) The destination phone rings, indicating to the callee that a call has arrived.
5) The callee picks up the telephone handset and begins a conversation. The audio, voice
conversation is translated to digital format in the center of the network, and then back
to analog at the edge.
6) The conversation ends, call billing occurs, the circuit is taken down, and resources are
released.
These steps must happen correctly and quickly for a telephone call to succeed with high
quality. When telephony professionals consider providing the same functionality and reliability
on relatively new and unreliable IP networks, you can see where some doubts and
skepticism can occur.