Integrated Service Digital Network (ISDN)

IDSN stands for Integrated Service Digital Network. IDSN is a set of digital transmission protocols defined by CCITT ( the consultative committee for International Telephone and Telegraphy)., These protocols are accepted virtually by all the world’s communication carriers as the standard. IDSN is a high speed, fully digital telephone service. Just as compact discs have made recorded music digital, IDSN upgrades today’s analog telephone network to a digital system.

For more than a century, the primarily international telecommunication infrastructure has been the public circuit switched telephone system. This system was designed for analog need. In view of higher user demand for an end to end digital services, the world’s telephone companies and PTTs got together in 1984 under the auspices of CCITT and agree to build a new, fully digitial, circuit-switched telephone system by the early part of 21st century. This new system, called IDSN stands for Integrated service digital Network – the name for digital telephone service that works over existing copper telephone wiring.

IDSN has it primary goal the integration of voice and non-voice services. It is already available in many locations and its use is growing slowly.

The key IDSN service will continue to be voice, although many enhanced feature will be added. One IDSN feature is telephones with multiple buttons for instant call setup to arbitrary telephone anywhere in the world. Another visible service of IDSN includes the telephones that display the caller’s telephone number name and addresses on a display while ringing. A more sophisticated feature allows the telephone to be connected to a computer, so that the caller’s data base record is displayed on the screen as the call comes in. Other advance voice services include call forwarding and conference call world wide. Advanced non-voice service are remote electricity meter reading and on line medical, burglar and smoke alarms that automatically call the hospital, police, or fire departments respectively and give their addresses to speed up response.

Likewise there are several types of service of IDSN, but the most appropriate type for individual computer users and the type that this site focuses on is the IDSN Rate Interface ( BRI).

Basic rate IDSN divides the telephone line into 3 digital channels two “B” channels and one “D” channel, each of which can be used simultaneously. The B channels are used to transmit data, at the rate of 64 k and 56 k depending on your telephone company. The “D” channel does the administrative work, such as setting up and tearing down the call and communicating with the telephone network. With two B channels, you can make two calls simultaneously.

The key idea behind ISDN is that of the digital bit pipe a conceptual pipe between the customer and the carrier through which bits flow. Whether the bits originated from a digital telephone, a digital terminal, a digital facsimile machine or some other device is irrelevant. At those matters is that bits can flow through the pipe in both directions.

The digital bit pipe can, normally does, support multiple independent channel by time division multiplexing of the bit stream. The exact format of the bit stream and its multiplexing is a carefully defined part of the interface specification for the digital but pipe.

Two principle standards for the bit pipe have been developing a low bandwidth standard for home use and a higher bandwidth standard for business use that supports multiple channels that are identical to the home use channels. Further more business may have multiple but pipes if they need additional capacity beyond what the standard business pipe can provide.

In the following figure (a) the normal configuration for a home or small business is illustrated. The carrier places a network terminating devices, NTI on the customer’s premises and connects it to the ISDN exchange on the carrier’s office, several kilometers away, using the twisted pair that was previously used to connect to the telephone. The NTI box has a connector on it into which a passive bus cable can be inspected. Upto eight ISDN telephones, terminals alarms, and other devices can be connected to the cable, similar to the way devices are connected to a LAN from the customer’s point of view, the network boundary is the connector on NTI.

For large business, the model of figure (a) as shown above is inadequate because it is common to have more telephone conversations going on simultaneously than the bus can handle. Therefore, the model of figure (b) as shown below is used. In this model, we find a device, NT2 called a PBX (Private Branch Exchange) connected to NT1 and providing the real interface for telephones, terminals and other equipments.

As ISDN PBX is not very different conceptually from an ISDN switch, although it is usually smaller and cannot handle as many conversations at the same time. CCITT defined four reference points, called R, S, T and V between the various devices. The U reference point is the connection between the ISDN exchange in the carrier’s office and NT1. AT present it is a two-wire copper twisted pair but at some time in the future it may be replaced by fibre optics. The T reference point is what the connector on NT1 provides to the customer. The S reference point is the interface between the ISDN PBX and the ISDN terminals. The R reference point is the connection between the terminal adapter and non-ISDN terminals.

Thus ISDN its various services truly represent the next generation of the world’s telephone service for all forms of tele-communications, including voice.

B- IDSN

The original version of IDSN employs baseband transmission. When CCITT figured out that narrowband IDSN was not going to set the world on fire, it tried to think of a new service that might. The result was broadband IDSN (B- IDSN) basically a digital virtual circuit for moving fixed size this data (cells) from source to destination at 155 mbps. Since this data rate is even enough for (uncompressed) HDTV, It is likely to satisfy even the biggest bandwidth hogs for atleast a few years.

Where as narrowboard IDSN was a timid first step into the digital age, broadband IDSN is a bold step/leap into the unknown. The benefits are enormous such as a bandwidth increase over narrowband IDSN by a factor of 2500.

Broadband IDSN or B- IDSN is based on ATM technology, a packet switching technology. B- IDSN cannot be sent over existing twisted pair wiring for any susbstantial distance. B- IDSN requires fibre optic cables and is not widely available. Further more space division switches and time-division switches cannot be used with B- IDSN transmission. They will all have to be replaced by new switches based on different principles and running at much higher speeds.

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