PSTN Basics: Understanding the Public Switched Telephone Network

This page covers the basics of the PSTN (Public Switched Telephone Network). We’ll explore the PSTN hierarchy in both the US and the UK. We’ll also touch on the differences between PSTN, ISDN, and PSDN.

What is PSTN?

PSTN stands for Public Switched Telephone Network. It’s essentially the traditional circuit-switched telephone network that’s been around for ages. Telephone lines are connected to local exchanges, which in turn connect to toll offices. These toll offices are interconnected using various transmission lines, such as fiber optic cables, microwave links, or satellite connections.

Figure 1 illustrates the typical components of a PSTN. The PSTN can be divided into two main parts:

• Access Network: This is the connection between individual telephone lines and the local exchange.
• Core Network: This is the connectivity between different exchanges.

Figure 2 shows PSTN network components in more detail. Individual telephones are connected to telephone poles via twisted pair cables. These poles terminate at a Distribution Panel (DP) in the area. DPs are then connected at a cabinet, which interfaces with the Main Distribution Frame (MDF).

Equipment on the telephone handset side is called “customer premises equipment,” while the equipment on the exchange side is known as “operator” or “service provider” equipment.

Voice communication over a telephone line typically uses a bandwidth of 300 Hz to 3300 Hz. Before any conversation can take place, a dedicated circuit-switched connection is established between the calling and called parties (refer to Telephone Signal Call Flow for more detail).

Data transmission uses a slightly different bandwidth, from 600 Hz to 3000 Hz. Because PSTN lines are analog, a modem is needed for data communication. The modem on the transmitting side converts digital data to analog, and the modem on the receiving side converts the analog data back to digital. Refer to the Telephone System tutorial for more information.

US PSTN Hierarchy

The US PSTN has a hierarchical structure that efficiently routes calls across the country.

Key terms:

• EO: End Office
• IXC: Inter-exchange Carrier

UK PSTN Hierarchy

The UK PSTN also has its own hierarchical structure.

Key terms:

• DLE: Digital Local Exchange
• DMSU: Digital Main Switching Unit
• DISC: Digital International Switching Centers

PSTN vs. ISDN vs. PSDN

Understanding the differences between these networks is important:

• PSTN (Public Switched Telephone Network): Traditional circuit-switched telephone network, primarily analog.
• ISDN (Integrated Services Digital Network): A digital circuit-switched network that provides higher bandwidth and can carry voice, data, and video.
• PSDN (Packet Switched Data Network): A network that transmits data in packets, allowing for more efficient use of bandwidth, commonly used for internet traffic.