PSTN vs. PSDN vs. ISDN: Key Differences Explained

This article breaks down the distinctions between PSTN (Public Switched Telephone Network), PSDN (Public Switched Data Network), and ISDN (Integrated Services Digital Network). Let’s dive in!

PSTN: The Public Switched Telephone Network

PSTN, short for Public Switched Telephone Network, is the traditional telephone network.

PSTN: Public Switched Telephone Network

It’s essentially comprised of two main parts:

• Access Network: The connection that runs between your telephone and the local exchange.
• Core Network: The connections that link different exchanges together.

To use PSTN for data communication with a computer, you’ll need a modem. The modem handles the conversion between digital data (from your computer) and analog signals (used by the PSTN) for transmission. At the receiving end, another modem converts the analog signal back to digital data.

Key Features of PSTN:

• Circuit Switching: PSTN uses circuit switching, where a dedicated path is established for the duration of the call.
• Reserved Bandwidth: Bandwidth is reserved specifically for your call in advance.
• Dedicated Path: A dedicated communication path is established between the caller and the receiver.
• Cost Based on Distance and Time: Call costs are typically calculated based on the distance between the parties and the duration of the call.

PSDN: The Public Switched Data Network

PSDN, which stands for Public Switched Data Network, is designed specifically for data transmission.

PSDN: Public Switched Data Network

Key Features of PSDN:

• Data-Oriented: Developed and optimized specifically for data transmission, not voice.
• Shared Communication: Many users (e.g., different companies) share the same switching network infrastructure.
• Packet Switching: PSDN uses packet switching, where data is divided into packets and transmitted independently.
• Bandwidth on Demand: Bandwidth is allocated and released as needed.
• No Dedicated Path: No dedicated, fixed path exists between the sender and receiver. Packets can take different routes.
• Cost Based on Usage: Costs are typically based on the amount of data transmitted, not distance or time. PSDN services are often more affordable than dedicated leased lines.

Examples of PSDN Technologies:

• X.25
• Frame Relay
• ATM (Asynchronous Transfer Mode)

ISDN: The Integrated Services Digital Network

ISDN (Integrated Services Digital Network) is designed to transmit voice, data, and video over digital lines.

ISDN: Integrated Services Digital Network

There are two common types of ISDN:

• BRI (Basic Rate Interface): Provides two 64 Kbps “B” channels (for voice/data) and one 16 Kbps “D” channel (for control/signaling). The total data rate for BRI is 144 Kbps.

  $2 \times 64 \text{ Kbps} + 1 \times 16 \text{ Kbps} = 144 \text{ Kbps}$

• PRI (Primary Rate Interface): Provides 23 64 Kbps “B” channels and one 64 Kbps “D” channel (in North America; in Europe it typically offers 30 B channels + 1 D channel). The total data rate for PRI in North America is 1.544 Mbps.

  $23 \times 64 \text{ Kbps} + 1 \times 64 \text{ Kbps} = 1.544 \text{ Mbps}$