The Public Switched Telephone Network (PSTN) is the common network of telephony systems across the world. Similar to the Internet, it can be considered a cloud through which phone systems (as opposed to computers) are connected. Protocol standards implemented across many different vendors is what allows for a common set of services such as making and receiving phone calls to work across the PSTN regardless of where the calls are placed. Connections to PSTN are analog phone lines, cellular connections, satellite based, or any other form, as shown in Figure 17.1. The PSTN serves as the backbone for voice services around the world.