ISDN Introduction

Introduction to Integrated Services Digital Network (ISDN): history, concepts, and digital integration.

The Digital Revolution Over Copper Wires

Before widespread broadband internet, the world was connected by the . This network was analog, slow, and designed for one thing: voice calls. The advent of computers created a need to send digital data, but this had to be done by converting data into audible tones using modems, a slow and inefficient process.

ISDN, or the Integrated Services Digital Network, was a revolutionary technology designed to solve this problem. Its core idea was to replace the analog connection with a fully digital, end-to-end connection, all while using the same copper wire infrastructure of the existing telephone network. This allowed for the integration of multiple services (like voice, data, and video) over a single digital line.

ISDN Reference Configuration: The Building Blocks

To ensure that devices from different manufacturers could work together, the ITU-T defined a standard "reference configuration". This model consists of functional groups (the devices) and reference points (the interfaces between them).

Diagram of the ISDN reference configuration

Functional Groups (Devices)

TE1 (Terminal Equipment 1): An "ISDN-native" device, such as an ISDN phone or video conferencing unit.
TE2 (Terminal Equipment 2): A non-ISDN device, like an analog phone or a computer with a standard modem/serial port.
TA (Terminal Adapter): The "translator" that connects a TE2 device to the ISDN network. It's often called an "ISDN modem".
NT1 (Network Termination 1): Connects the user's premises to the two-wire public ISDN line (the local loop). It handles the physical layer functions.
NT2 (Network Termination 2): An intelligent device like a digital PBX or a LAN router that can switch and manage multiple internal ISDN devices.
LT/ET (Line/Exchange Termination): The equipment at the telecommunication provider's central office.

Reference Points (Interfaces)

R: The interface between a non-ISDN device (TE2) and its Terminal Adapter (TA).
S: The 4-wire bus interface connecting ISDN terminal equipment (TE1/TA) to the NT2. Up to 8 devices can be connected.
T: The interface between the user's internal network (NT2) and the network termination (NT1). When no NT2 is present, TE1s connect directly to the S/T interface on the NT1.
U: The 2-wire physical connection from the central office to the NT1. This is the actual subscriber line.

ISDN Channel Types

ISDN divides the total bandwidth of a line into several distinct digital channels, each with a specific purpose.

B Channel (Bearer Channel): This is the main data channel. It provides a $64 \text{ kbit/s}$ circuit-switched connection for user data, which can be digitized voice, computer data, or video.
D Channel (Delta Channel): This is the signaling and control channel. It operates at $16 \text{ kbit/s}$ or $64 \text{ kbit/s}$ and carries the information needed to set up, manage, and tear down calls on the B channels (e.g., dialing digits, caller ID). It can also be used for low-speed packet data.
H Channels (High-Speed Channels): These were defined for applications requiring more bandwidth than a single B channel. They are created by combining multiple B channels, offering speeds like $384 \text{ kbit/s}$ (H0), or around $1.5 \text{ Mbit/s}$ (H11) and $1.9 \text{ Mbit/s}$ (H12).

ISDN Access Interfaces: BRI and PRI

Based on these channel types, ISDN was offered in two main "packages" or interfaces, targeting different types of users.

Basic Rate Interface (BRI)

Designed for residential users and small businesses, BRI provided a significant speed boost over dial-up modems.

2B + D₁₆

Two 64 kbps B channels for voice or data.
One 16 kbps D channel for signaling.
Total user data throughput: 128 kbps.

Primary Rate Interface (PRI)

Designed for larger organizations, PRI offered much higher capacity by bundling many B channels, allowing it to connect to digital PBXs.

North America/Japan: 23B + D₆₄

Europe/Rest of World: 30B + D₆₄

23 or 30 64 kbps B channels.
One 64 kbps D channel for robust signaling.
Total throughput: 1.544 Mbps (T1) or 2.048 Mbps (E1).