ISDN Telephony: Architecture, Components and Functions

Item 7: ISDN Telephony Fundamentals

1. Main Differences: ISDN vs. Analogue Telephony

ISDN utilizes digital signals, providing several transmission advantages:

• Faster speeds.
• Increased reliability.
• Unified data transmission: Ability to send all information types once digitized.

2. Advantages of ISDN Services

ISDN offers three primary service categories:

• Bearer Services: Methods to send digital information effectively.
• End Services: Services offered directly to the user (e.g., telephony).
• Additional Services: Supplementary features for the user (e.g., Call Waiting).

3. ISDN System Architecture and Benchmarks

Key components of the ISDN architecture include:

• TE2: Analogue telephone.
• TA: Terminal adapter allowing analogue phones to function with ISDN.
• TE1: ISDN telephone.
• NT2: Network Termination 2. An ISDN user PBX that manages multiple extensions when they exceed the capacity of a passive bus.
• NT1: Network Termination 1. Converts the ISDN line code for indoor installation, adjusts line speed, and converts cabling from 2-wire to 4-wire (6P2C to 8P4C connector).

Reference Points (Benchmarks)

• R: Point between an analogue telephone and the adapter.
• S: Point between a digital phone and the user PBX (NT2).
• T: Point between the ISDN PBX (NT2) and NT1.
• U: Entry point of the ISDN line to NT1.

4. What is a Rosette C?

It is a service entrance point used to physically connect the external cable to the internal user network.

5. ISDN ADSL Filter Functionality

The filter receives input from the LT terminal and separates the ADSL signal from the ISDN signal, preventing interference between equipment.

6. The Role of the NT1

The Network Termination 1 (NT1) adapts ISDN data formats from the PBX to the format required by ISDN telephones. It establishes the boundary of responsibility between the service provider and the property owner. It can also be programmed for subaddressing and to maintain emergency call functionality during power outages.

7. Passive Bus Definition

A passive bus is a set of rosettes with RJ45 connectors that distributes data from the NT1 to ISDN telephones efficiently.

9. Terminal Rosette Specifications

A terminal rosette is the final point of a passive bus. It differs from standard rosettes by including a 100-ohm resistor connected across pins 3 and 6 (reception) and another across pins 4 and 5 (transmission).

10. Verifying Passive Bus Installation

When the ISDN line is active, you can verify the connection by placing a tester near the NT1 to detect a specific buzzing sound.