What is the architecture of ISDN

ISDN (Integrated Services Digital Network) is a circuit-switched network technology that provides end-to-end digital connectivity over traditional telephone infrastructure. It bridges the Central Office's local loop and customer premises by using digital switching and transmission techniques to deliver voice, data, video, and fax services over a single line.

The term "Integrated Services" refers to ISDN's capability to simultaneously deliver multiple types of communication services?data, voice, video, and fax?over a single digital connection. Multiple devices can be connected to the line and used as needed, making it a versatile solution for both residential and business applications.

ISDN Services

ISDN provides two primary service levels:

• Basic Rate Interface (BRI) ? Consists of 2B + D channels, providing two Bearer channels of 64 kbps each for data transmission and one D channel of 16 kbps for signaling and control. The separate control channel enables "out-of-band" signaling.

• Primary Rate Interface (PRI) ? Consists of 23B + D channels (North America) or 30B + D channels (Europe), providing 23 or 30 Bearer channels of 64 kbps each for data and one D channel of 64 kbps for signaling and control. Bearer channels can be combined as required for higher bandwidth applications.

ISDN Architecture and Layer Structure

ISDN architecture aligns with the OSI model layers, with different channel types handling specific functions:

OSI Layer                           ISDN Channel
---------                           ------------
Network                             D Channel
Data Link                           B Channel
Physical                            B Channel

The D channel operates at the Network layer, handling call setup, teardown, and signaling. The B channels operate at the Data Link and Physical layers, carrying the actual user data, voice, or video traffic.

Line Encoding

ISDN uses 2B1Q (2 Binary 1 Quaternary) line encoding, where two binary bits are encoded into one quaternary symbol. Each dibit (two-bit combination) corresponds to a specific voltage level:

ISDN Reference Architecture

Key Components

• Network Termination 1 (NT-1) ? Converts the 2-wire U interface from the Central Office to a 4-wire S/T interface at the customer premises

• S/T Interface ? A 4-wire bus topology that allows multiple ISDN devices to connect, terminated with a 100-ohm resistor

• Terminal Equipment (TE) ? ISDN-compatible devices that connect directly to the S/T bus

Conclusion

ISDN architecture provides a structured approach to digital communications, using separate channels for data and signaling while maintaining compatibility with existing telephone infrastructure. The NT-1 device serves as the critical interface point, converting between the service provider's 2-wire system and the customer's 4-wire multi-device bus topology.