QuitExams

🔹 Introduction

The **Public Switched Telephone Network (PSTN)** is the world’s traditional circuit-switched telephone network. It was originally designed for voice communication but later adapted for data communication through **modems**. Modems convert digital computer data into analog signals that can travel over PSTN telephone lines.

This topic explains how PSTN works, modem functionality, modem types, and their role in early Internet access.

1️⃣ PSTN — Public Switched Telephone Network

PSTN is a global network of switches, telephone lines, fiber optics, and microwave links that establishes circuit-switched voice communication.

✔ Features of PSTN

• Circuit-switched network
• Analog last-mile connections
• Uses copper telephone lines (Twisted pair)
• Telephone exchanges connect users

✔ PSTN Architecture

User Phone → Local Loop → Local Exchange → Trunk Network → Remote Exchange → Receiver

✔ PSTN Components

• Local loops: Connect homes/offices to local exchange
• Switching centers: Route calls
• Trunks: High-speed digital links connecting exchanges
• Signal channels: Carry dialing information

✔ PSTN Limitations for Data

• Low bandwidth (0–4 kHz)
• Analog signal prone to noise
• Limited speed (56 kbps using dial-up)

2️⃣ Modems

The term Modem comes from Modulator–Demodulator. It enables digital devices to communicate over the analog PSTN.

✔ Modulation Function

The modem converts digital data → analog signals.

✔ Demodulation Function

The receiving modem converts analog signals → digital data.

✔ Diagram

Computer → [Modem] → Analog PSTN Line → [Modem] → Computer

3️⃣ Types of Modems

✔ External Modem

• Connected through serial port
• Has power adapter
• Visible lights (TX, RX, CD, OH)

✔ Internal Modem

• PCI/PCIe card inside computer
• No external power

✔ Soft Modem (WinModem)

• Uses system CPU for processing
• Cheaper but slower

✔ Cable Modem (Modern Alternative)

• Used in broadband cable networks
• 100 Mbps to 1 Gbps speeds

4️⃣ Modem Modulation Techniques

• ASK — Amplitude Shift Keying
• FSK — Frequency Shift Keying
• PSK — Phase Shift Keying
• QAM — Quadrature Amplitude Modulation

✔ Example: 56 kbps Dial-Up Modem

Uses a combination of PSK and QAM to maximize speed within PSTN’s 4 kHz audio bandwidth.

5️⃣ Applications of PSTN + Modems

• Early Internet access (Dial-up)
• Fax machines
• ATMs over telephone lines
• Point-of-sale (POS) terminals
• Remote monitoring systems

6️⃣ PSTN vs Modern Digital Networks

📝 Conclusion

PSTN provided the foundation for global voice communication and early Internet connectivity. Modems played a crucial role by bridging digital computers with analog telephone lines. While modern broadband technologies have replaced PSTN for data usage, the concepts of modulation, demodulation, and circuit switching remain essential in communication engineering.