ISDN

What is ISDN?

ISDN stands for “Integrated Services Digital Network” is a set of communication standards that enables the digital transmission of voice, video, data, and other network services over the telecommunication networks and it operates over copper-based systems to provide higher data speed and better quality than analog transmission. It is a circuit-switched telephone network system for voice or data and it also allows access to packet-switched networks.

The main future of ISDN is that on the same telephone line it can integrate both voice and data at the same time and ISDN was design to run on the digital telephone system. In some areas, ISDN found as a major market application for internet access, in which ISDN typically provides a maximum of 128 kbit/s bandwidth in both upstream and downstream directions. Through channel bonding, it can achieve a greater data rate that is the ISDN B-channels of three or four BRIs (Six to eight 64 kbit/s channels) are bonded. In the OSI model, ISDN is also used with specific protocols, such as Q.931 where it acts as a network layer, data link layer, and physical layer.

History of ISDN

The first telecommunication technology Integrated Services Digital Network was defined in 1988 by the CCITT organization and therefore, which is now the ITU-T (International Telegraph and Telephone Consultative Committee). Prior to ISDN, the classic telephone system only used to transmit voice signals, with some special service available for data. ISDN was commonly used as high-end internet service in the 1990s to 2000s and offered by many ISPs as a faster alternative to dial-up internet access, nowadays ISDN is still used in some network connections, but it is rarely used for internet access.

However, in a videoconference, ISDN provides continuous voice, video, and text transmission between the individual (desktop) and group (room) videoconferencing systems. The technology used is the H.320 standard for audio coding and video coding, including audio codecs such as G.711 Pulse Code Modulation (PCM), G.728 (CELP), and Discrete Cosine Transform (DCT), and video codecs such as H.261 and H.263.

ISDN Network Architecture

ISDN Network Architecture

Variants of ISDN Interfaces

Basic Rate Interface (BRI)

It supports two 64 kbps bearer channels (or B channels) for a data transfer rate of 128 kbps.

Primary Rate Interface (PRI)

It supports 30 B channels and two additional channels in a single E1 connection, providing a data transfer rate of 2,048 kbps.

Always on Dynamic ISDN (AODI)

It is a consistent ISDN connection that uses the X.25 protocol and supports speeds up to 2 Mbps.

Broadband ISDN (B-ISDN)

A B-ISDN relies mainly on the evolution of fiber optics and according to CCITT B-ISDN is best described as ‘a service requiring transmission channels capable of supporting rates greater than the primary rate.

Narrow ISDN (N-ISDN)

It was an attempt to replace the analog telephone system with a digital one. It supports 64 kbit/s channels as a basic unit of switching. Its major contribution was frame relay and therefore, it describes telecommunication that carries voice information in a narrow band of frequencies.

ISDN Services

ISDN provides a fully integrated digital service to users and there are three categories of services such as

• Bearer Services
• Teleservices
• Supplementary Services

Bearer Services

Bearer network provides transfer of information likes voice, video and data between users without the network manipulating the content of the information. Therefore, it provides using circuit switched, packet switched, frame switched, and cell switched networks. In this, it defines as ISDN standards. It belongs to the first three layers of OSI model.

Teleservices

Tele network provides transfer of information between users with manipulating the content of the information in network. It includes telephony, telefax, videotex, telex and teleconferencing. In this, the ISDN yet not become standards. It belongs to the layers four to seven of OSI model.

Supplementary Services

Supplementary services provide an additional functionality to the bearer services and teleservices. The supplementary services are such as reverse charging, call waiting, and message handling.

Principles of ISDN

The various principles of ISDN as per ITU-T recommendation are

• Intelligence in the network
• Layered protocol architecture
• To support voice and non-voice applications
• To support switched and non-switched applications
• Variety of configurations
• Reliance on 64-kbps connections

Benefits of ISDN

It provides a number of significant advantages over analogue systems they are.

• Faster call connection
• Enables two simultaneous telephone calls to be made over the same line simultaneously.
• Digital stream can carry any form of data from voice to faxes and internet web pages to data files - this gives the name 'integrated services'.
• Data can be sent more reliably and faster than with the analogue systems.
• Noise, distortion, echoes and crosstalk are virtually eliminated.

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