All the signaling standards and communication methods discussed in the previous section typically focus on the connection to one massive voice network, known as the PSTN. If you have ever made a call from a home telephone, you have experienced the results of the traditional telephony network. This network is not unlike many of the data networks of today. Its primary purpose is to establish worldwide pathways to allow people to easily connect, converse, and disconnect.
Components of the PSTN
When the phone system was originally created, individual phones were wired together to allow people to communicate. If you wanted to connect with more than one person, you needed multiple phones. As you can imagine, this solution was short lived as a more scalable system was found. The modern PSTN is now a worldwide network built from the following components, as shown in Figure 1:
- Analog telephone: Able to connect directly to the PSTN and is the most common device on the PSTN. Converts audio into electrical signals (and vice versa).
- Local loop: The link between the customer premises (such as a home or business) and the telecommunications service provider.
- CO switch: Provides services to the devices on the local loop. These services include signaling. digit collection, call routing, setup, and teardown.
- Trunk: Provides a connection between switches. These switches could be CO or private.
- Private switch: Allows a business to operate a “miniature PSTN” inside its company. This provides efficiency and cost savings because each phone in the company does not require a direct connection to the CO switch.
- Digital telephone: Typically connects to a PBX system. Converts audio into binary Is and Os, which allows more efficient communication than analog.