Public Switched Telephone Network (PSTN)

The Public Switched Telephone Network is the common network of telephony systems across the world. For those with more of a systems administration background, consider it similar to the Internet, like a cloud through which phone systems (as opposed to computers) are connected. The PSTN is composed of circuit switched analog lines, digital trunks such as T1s or E1s, cellular connections, and satellite transmissions, all of which can call each other through connected switching centers around the world. There is an incredible number of varying protocols and standards between different vendors and regions of the world to the point where it’s remarkable how well the system works. Figure 17.1 shows a logical representation of the PSTN.

Figure 17.1. The Public Switched Telephone Network.