ISDN - Principles, Objectives, Services, Architecture, Channels

ISDN - Principles:

Integrated Services Digital Network (ISDN) is a high -speed, fully digital telephone service. It is an ITU-T (CCITT) standard for an end-to-end global digital communication system providing fully integrated digital services. The principles of ISDN from the point of ITU-T International Telecommunication Union - Telecommunications Standardisation Sector). CCITT (Consulative committee for International Telegraphy and Telephony are :

1. Support of voice and non-voice execution by means of a partial set of standardised services.
2. Support for switched and non-switched (committed lines) applications.
3. Reliance for 64 - kbps connections.
4. Aptitude in the network.
5. Layered protocol architecture.
6. Diversity of configurations are likely to execute ISDN.

ISDN - Objectives :

Activities currently under way are leading to the development of a worldwide ISDN. The key objectives of ISDN are :

1.Standardisation 

2. Transparency : It should provide a transparent transmission service, thereby permitting users to develop applications and protocols with the confidence that they will not be affected by the underlying ISDN.

3. Separation of Competitive Functions

4. Leased and Switched Services : The ISDN should provide dedicated point-to-point services and switched services, thereby allowing the user to optimise implementation of switching and routing techniques.

5. Cost-related Tariffs

6. Smooth Migration: ISDN interfaces should evolve from current interfaces, and provide a migration path for users.

7. Multiplexed Support : ISDN should provide low-capacity support and multiplexed support.

ISDN - Services :

The purpose of the ISDN is to provide fully integrated services to users. These services are divided into three categories :

1. Bearer Services : They can be provided using circuit -switched, packet - switched, frame - relay or cell - relay networks.

2. Teleservices : In teleservicing, the network may change or process the contents of the data. These services be in contact to layers 4 to 7 of the OSI model. Teleservices rely on the facilities of the bearer services and are designed to accommodate complex user needs without the user having to be aware of the process details. Teleservices include telephony, teletex, telefax, videotex, telex and teleconferencing.

3. Supplementary Services : Supplementary services are those that provide additional functionality to the bearer services and teleservices. These services include reverse teleservices. These services include reverse charging, call waiting and message handling.

ISDN - Architecture :

Figure shows the block diagram of ISDN. ISDN supports a new physical connector for users, a digital subscriber loop (link from end user to central or end office), and modifications to all central office equipment. The area which most attention has been paid by standards organisations is that of user access. A common physical interface has been defined to provide, in essence, a DTE - DCE connection. The same interface should be usable for the telephone, computer terminal and videotex terminal.

Protocols are needed for the exchange of control information between user device and the network. Provisions must be made for high-speed interfaces to a digital PBX or a LAN.

The subscriber loop consists of twisted pair links between the subscriber and the central office carrying 4kHz analog signals.

The digital central office connects many ISDN subscriber loop signals to the IDN (Integrated Digital Network). It provides access to both the circuit - switched, packet switched networks, dedicated lines and time - shared, transaction oriented computer services.

Block Diagram of ISDN

ISDN — Channels

To allow flexibility, digital pipes between customers and the ISDN office (Subscriber loops) are organised into multiple channels of different sizes. The ISDN standard defines three channel types, each with different transmission rates. They are:

1. Bearer (B) channel - 64 kbps

2. Data (D) channel - 16 or 64 kbps

3. Hybrid (H) channel - 384 (H 0), 1536 (H 11) and 1920 (H 12) kbps.

B Channels : The B channels is the basic user channel. It can carry any type of digital data in full - duplex mode as long as the required data rate doesn't exceed 64 kbps. It carries transmissions end-to-end. It is not designed to demultiplex a stream midway in order to separate and divert transmissions to more than one recipient.

D Channels : The D channel serves two purposes - to carry signalling information to control circuit - switched calls on associated B channels at the user interface and for packet - switching or low - speed telemetry at times when no signalling information is waiting.

H Channels : H channels are provided for user information at higher bit rates. The user may employ such a channel as a high - speed trunk, or the channel may be subdivided according to the user's own TDM scheme. Some of the applications of H channels include fast facsimile, video, high - speed data, high - quality audio, teleconferencing and multiple - information streams at lower data rates.