The fundamental difference is that ISDN (Integrated Services Digital Network) is a set of communication standards for digital transmission over ordinary telephone lines, offering services like voice, data, and video, while ATM (Asynchronous Transfer Mode) is a specific high-speed transport technology used as an underlying layer in various networks, including Broadband ISDN (B-ISDN).
Understanding ISDN
ISDN represents an earlier evolution of the telephone network, moving from analog to digital transmission end-to-end. Its main goal was to integrate different types of communication services (voice, data) over a single digital line.
Key aspects of ISDN:
- Service Integration: Designed to carry multiple service types simultaneously.
- Digital: Provides a completely digital connection from the telephone exchange to the user's premises.
- Channels: Operates using 'B' (bearer) channels for carrying data or voice and 'D' (delta) channels for signaling and control.
- Variants:
- BRI (Basic Rate Interface): Typically offers two 64 kbps B channels and one 16 kbps D channel (2B+D).
- PRI (Primary Rate Interface): Offers multiple B channels (e.g., 23 in North America/Japan, 30 in Europe) at 64 kbps and one 64 kbps D channel.
- B-ISDN (Broadband ISDN): A later concept aimed at much higher speeds, often utilizing ATM technology.
ISDN was a significant step up from dial-up modem speeds and was popular for internet access, video conferencing, and business lines in the late 20th century.
Understanding ATM
Asynchronous Transfer Mode (ATM) is a high-performance, connection-oriented switching technique designed for transporting a wide range of traffic types, especially those sensitive to delay (like voice and video).
Based on the reference: "ATM is a packet-switching technology that divides data into fixed-length cells and routes them through a network of ATM switches."
Key aspects of ATM:
- Fixed-Length Cells: Unlike variable-length packets used in other technologies (like IP), ATM breaks all data into small, fixed-size cells (53 bytes, consisting of 5 bytes for the header and 48 bytes for the payload). This predictability helps manage delay and quality of service.
- Connection-Oriented: Requires setting up a virtual circuit path before data transmission begins, ensuring cells arrive in order.
- High Speed: Designed from the ground up for high-speed digital networks.
- Underlying Transport: Often used as a transport layer for other technologies and services, including T1/E1 lines, DSL, and notably, Broadband ISDN.
The reference explicitly states that "Broadband ISDN (B-ISDN) is a network architecture that uses asynchronous transfer mode (ATM) to deliver high-speed data, voice, and video services." This highlights that ATM is a foundational technology employed by the B-ISDN architecture.
Key Differences Summarized
| Feature | ISDN (BRI/PRI) | ATM |
|---|---|---|
| Nature | Digital Network Service/Architecture | High-Speed Transport Technology/Protocol |
| Primary Goal | Integrated digital services (voice/data) over existing phone lines (at moderate speeds) | Efficient high-speed transport for various traffic types, especially delay-sensitive ones |
| Data Unit | Variable-length packets/frames | Fixed-length cells (53 bytes) |
| Speed | Up to 128 kbps (BRI), 1.5/2 Mbps (PRI) | Designed for much higher speeds (Mbps to Gbps) |
| Relationship | Can use ATM (B-ISDN) | Can be the underlying technology for ISDN (B-ISDN) |
| Primary Use | Voice, slower internet access, video conferencing (historically) | Backbone networks, DSL transport, B-ISDN implementation |
In essence, ISDN (particularly the original BRI/PRI forms) is a service delivered over digital phone lines, while ATM is a high-speed data transport mechanism that can serve as the engine for more advanced digital network architectures like Broadband ISDN. ISDN is the service concept or architecture you subscribe to; ATM is often a technology running beneath the surface to make high-speed versions of such services possible.
