1. INTRODUCTION
Like communication between two people can take various forms, communication in the world of computers can mean different with respect to the context. For instance, a function can communicate with the main program through the parameters that it takes as input and the parameters that it gives as an output to the program that invoked it. If we consider a level above this, we can consider a program (or) say a memory management demon (in Unix) that very often communicates with the Operating System, reporting the status of the memory. This is quite different from the previous communication, where the communication was within a program. In terms of a Hardware Computer Network means a minimum of two computers that have a medium in between then that they can use to Communicate between them. This sort of communication that exists upon two computers is quite different from the sort of communications that were discussed previously. In terms of software a network consists of programs that let the two computers communicate through the hardware that is available and let them exchange information reliably. There exists hundreds of ways (protocols and Network structures) to achieve a successful computer network.
The table shown below can help summarize how data moves around in the physical link between two computers. The table also gives you some examples to make yourselves comfortable as to what each kind means.
Successful, trouble-free data communications requires the use of compatible equipment at both ends of a connection. For limited user groups, solutions can be simply agreed upon. In a system that is by definition open, such as the Internet, all-embracing standards are necessary, and must be binding for all participants.
For more than a century, the primary international communication infrastructure has been the telephone system. This system was designed for analog voice transmission and is proving inadequate for modern communication needs such as data transmission, facsimile and video. User demands for these and other services have led to an international undertaking to replace a major portion for the worldwide telephone system by the early part of the twenty-first century. A new system ISDN (Integrated Services Digital Network), has as its primary goal the integration of voice and non-voice services.
The term ISDN was created in 1973 by the world's telecommunications standards group, the Consultative Committee for International Telephony and Telegraphy (CCITT). The scope of ISDN was not defined until 1980 and from 1980 to 1984 CCITT study groups had agreed upon a number of recommendations. Though the fundamental ISDN services have been formally approved there is still much work to be done. So, eighteen years after it's creation, ISDN is just starting to make it's way into some of our larger corporations and ISDN to the home is virtually non-existent. There are many reasons why ISDN has taken so long to evolve. First, design by committee is not the fastest method to stimulate the rapid deployment of technology. Second, once a recommendation has been made everyone in the world needs to agree upon the standard else it goes back to the appropriate committee for revision. Another reason is the fact ISDN must be compatible and interface with telecommunications networks all over the world. Not to mention the replacement of older analog switches to new digital switches. One unfortunate issue with ISDN is that direct user participation in the standards process is still infrequent. It also should be noted that the investment to deploy ISDN is substantial.
2. TECHNOLOGICAL OVERVIEW
ISDN is a public end-to-end digital telecommunications network providing the capability to transmit voice, data, facsimile, telemetry, signaling (or dialing), and slow-motion video, either simultaneously or separately, on a single telephone line, typically existing of newly installed twisted-pair wires or optical fibers.
ISDN uses a concept called channels. In ISDN a channel is a digital stream of information that can be multiplexed with other channels, so many streams of information can be sent over the same piece of wire. The channels used in ISDN are called "B," "D," and "H." The B channel or Bearer channel, is a 64kbps (KiloBits Per Second) digital stream, which is appropriately 8,000 characters, of data or digitized voice. The D channel or Delta Channel has a transfer rate of either 16kbps or 64kbps depending on how ISDN is purchased. The D channel is the signaling channel and only extends as far as the access connection between the user and the Central Office. H channels or Hybrid Channel provide a way to aggregate B channels. They are implemented as:
•H0=384 kbps (6 B channels) •H10=1472 kbps (23 B channels) •H11=1536 kbps (24 B channels) •H12=1920 kbps (30 B channels) - International (E1) only
Standard ISDN makes two access services available to the users, Basic and Primary. Basic services are more frequently called the Basic Rate Interface (BRI). The Basic Rate Interface provides users with two "B" channels and one 16kbps "D" channel commonly called "2B+D." Primary services are typically called the Primary Rate Interface (PRI). The Primary Rate Interface in North America consists of 23B+D, where there are 23B channels and 1 64kbps D channel.
The original telephone network was designed using a series of pulses and tones, or analog signals. Current and future technologies are standardizing on digital communication signals that allow much higher speeds of communications that cannot be handled by current analog voice grade circuits. One fitting achievement is that ISDN will be transparent to the user. The telephone will sound just like any telephone sounds, with a dial tone and the familiar ring. Still, the circuitry and communication mechanism used to make the call will be very different.
2.1. Interfaces and Compatibility
It will be some time until ISDN reaches the far corners of the earth with ISDN switches in all Central Offices. By coupling ISDN Centres with TSAT (T1 rate Small Aperture Terminal), a solution is here today. TSAT employs small low cost antennas and microwave satellite technology to accomplish this task at a low cost. TSAT can also accommodate mixed topologies.
TE - Terminal Equipment NT1 - Network LT - Line Termination
TA - Terminal Adapter NT2 - Termination + - Reference point
The above figure represents an example of an ISDN reference model. As you may know ISDN as an integrated network supports many diverse types of devices at many connection points in the network. In a large network the most common interface that will be used by the customer will be the R interface. The R interface will connect a non-standard ISDN terminal, typically RS-232C, via a Terminal Adapter (TA) to an ISDN S interface. The S interface connects ISDN terminal to network terminating equipment (NT1). Standard ISDN ready terminals can be connected to the S interface, which can support up to eight terminals that access the BRI. Next in the circuit comes the U interface that can run either the BRI or PRI and connects your equipment to the Central Office (CO). In the CO, the V interface is user by the network carrier to connect to the local loop. ISDN standards are complex. The physical attributes of plug and socket specifications, bit rates, signal voltages and private/public network interfaces still are being sorted out by the carriers. Still, it is the signaling software protocols used on the ISDN D channel that will determine which services and features are available over the link.
2.2. ISDN as a back-up
Reliability is important for data networks. This is becoming increasingly true as more and more critical business operations depend upon the distribution of electronic data. ISDN offers new opportunities for making internetworks more reliable because it is very cost effective as a standby for the failure of leased circuits.
2.3. Resilience before ISDN
In the past, the most common method of achieving resilience was triangulation. In the network shown in the figure above, branch offices at Reading and Mysore connect to the central office using kilostream circuits. By installing an additional kilostream between Mysore and Reading, we provide an alternate route as a back up for both the main circuits. If the line to Reading fails, data is re-routed via Mysore to the central site Bangalore and vice versa if the other line fails. It is a characteristic of this system that if the line fails to either site, both sites suffer a degraded service. This is usually considered acceptable given the relative infrequency of line failures.
The real disadvantage of triangulation is the cost involved in providing the third leg of the triangle. For most of the time, this line lies idle, but the rental still has to be paid. Over the life of the system this will amount to many thousands of Rupees outlaid to little effect. What is required is a line that does not cost as long as it's not used - ISDN.
2.4. Resilience with ISDN
In the figure below, the same network is provided with back up on ISDN. Special terminal adapters are used to automate the back-up process. In normal operation the terminal adapters pass data straight through to the kilostream circuits. When a kilostream fails, the terminal adapters automatically set up an ISDN connection and switch the data on to it. The ISDN connection exactly reproduces the kilostream circuit – there is no
degradation of service at
all during back up.
Given that back up is required only relatively rarely, the call charges on ISDN lines will be very small. Rental charges for the three ISDN lines needed in our example will be minimal, compared to the extra line rental involved in the triangulation scheme. The nett result is that, over the life of the system, ISDN back up offers large savings. Since it also provides a higher quality of back up than other methods, ISDN makes supreme sense as a back-up method.
3. SERVICES
There are three major categories of ISDN service defined by the Consultative Committee for International Telephony and Telegraphy (CCITT): Bearer services, Teleservices and supplementary services. These services attempt to define the fundamental mechanism and capability available to users of ISDN. Though an application can use any one of these services independently or all three services simultaneous, it is important to understand the functionality of these services for they are the underlining foundation of ISDN.
3.1. Bearer Services
Bearer services are the fundamental telecommunications services used to transport user information. Bearer services involve only the lower three levels of the OSI model communication functions. The Bearer services category currently contains three modes of service; circuit mode, packet switched mode and frame relay mode. Circuit mode has two bearer categories; speech and 3.1KHz audio. The 3.1kHz audio service is intended to carry modem traffic up to at least 9.6kbps, which includes Group III fax capabilities. Packet mode services include traditional X.25 and other packet modes where as frame relay mode is targeted at emerging applications such as high-speed LAN-WAN interconnections and image transfers. Most customers will use ISDN Bearer services for 64kbps data transfers. A user may choose any set of higher layer protocols on top of the Bearer services; thereby producing any application or service required to accomplish the task.
3.2. Teleservices
Teleservices describe a service for a specific application, but ISDN Teleservices are currently now well developed. In OSI modeling of the network, you should not be required to have specific knowledge of the upper layer protocols. In contrast, Teleservices are premised on the network knowing what application is being run. Typical examples of Teleservices are telephony, teletex, videotex and message handling.
3.3. Supplementary Services
Supplementary services could be better known as value added services. Supplementary services cannot be stand-alone; they work in conjunction with Bearer or Teleservices.
The most common example of a Supplementary service would be Caller Identification (Caller ID), also called Calling Number Identification (CNI) and Automatic Number Identification (ANI). Caller ID allows the number of the calling party to be forwarded to the caller of the receiving party. In practice many early implementations and service standards for Supplementary services are limited to the ISDN Basic Rate Interface.
4. APPLICATIONS
This section will attempt to discuss the applications that are most likely to succeed The first difference a user will notice when using ISDN is it's ability for creating an instantaneous connection. The average setup time for an ISDN connection will drop to 300-400 milliseconds from 5 seconds for touch tone, 7 seconds for rotary pulse dialing and approximately 20 seconds for current modem connections.
Digital circuitry also will make the telephone sound much clearer, audio telephone transmission in high-fidelity sound quality. This means there will be high quality phone conversations for handsets, speakerphones and teleconferencing systems. We have divided ISDN applications into 12 areas that will have the most impact in the future. They are as follows:
4.1. ENHANCED PHONE SERVICES
Consumer base telephone services will widely expand with ISDN. Almost everything that you can do at work with your PBX based telephone, you will be able to do with your home telephone. With ISDN your telephone number is issued for life; no matter where you move in the world, your ISDN number travels with you. ISDN also allows sub-routing so incoming calls can be sent to one of several home telephones.
4.2. CUSTOMER PREMISE EQUIPMENT
Customer Premise Equipment (CPE) is equipment traditionally used by companies to communicate to the outside world, such as PBXs, telephones gateways, multiplexors, concentrators and on-line computer terminals. Everyone will have a device in his or her home or office that combines the computer, telephone and television.
4.3. HIGH SPEED DATA TRANSFERS
High-speed data transfers retrieve large amounts of information from distant databases. ISDN will link PBX to PBX, PBXs to LANs, LANs to WANs and remote PCs to LANs. Still there is the issue of using ISDN as a backbone. If you have low data transfer rates why add another topology, just use ISDN as a backbone. For companies already using high-speed data transfers such as T1, ISDN will offer the luxury of high speed transfers without having to reserving bandwidth from a T1 system.
4.4. TELEX, TELETEX, TELEFAX
Telex will provide faster text communications without a modem. Communications may be up to three times faster than any current implementation. Teletex services will provide standard character sets and communications protocols for the electronic transfer of technical documentation and contracts using a rich selection of graphics and control characters. Teletex was designed to replace older telex. Also telefax (Group 4 facsimile), will be a standard for transmitting graphic and text data at 200-400 pixels in black and white, at a rate of 64kbps. Current Group 3 fax machines transmit data at 9.6kbps.
4.5. TELEMESSAGING
Electronic Messaging Systems (EMS) for both single system Electronic Mail (E-Mail) and for network E-Mail will speed mail service. Equal access providers will offer additional store and forward capabilities to telemessage services.
4.6. VIDEOTEX
Videotex will allow on-line information providers to provide on-line access to databases, bulletin board, reference libraries, for movie information and catalog shopping. With a video screen available, on-line electronic directories and electronic reverse directories will allow you to use your telephone line as a telephone book or if someone calls, you will be able to look up a person's name from their telephone number.
4.7. TELECONFERENCING
Teleconferencing is a method by which images of individuals, at different locations in the world can simultaneously transmit voice and data to each other without needing to travel the distance. One great benefit of teleconferencing is group teleconferences because many people can communicate over the telephone line at one time. Teleconferencing in the short term will be limited to slow-motion video pictures, but as video compression techniques improve, full motion video will enter our world. Videophones will enter our homes and life will never be the same. Another interesting related aspect of teleconferencing is sharing computer screens of information. This allows the ability to edit diagrams on the same computer screens from anywhere in the world while simultaneously talking to one another about the work.
4.8. TELECONTROL, TELEPOLLING
The term telecontrol is the ability to automatically control temperature, humidity or any remote device at different locations throughout the world. Before you leave work you could call your home and turn on the air conditioner or heater. Telecontrol will save millions of rupees with its energy management capabilities. On the other hand, telepolling refers to the ability to check on-line security services such as burglar alarms, fire alarms and will automatically read public utility meters. The utility man will no longer need to come to your home and read your meter. In the future when customers call a utility company for service their names and address will be available within seconds because employees can match their phone number with their account records. The utility company will also automatically know that you are having a problem. Customers reporting problems will be notified that the problem has already been reported and is being attended too.
4.9. TELEFINANCING
Services such as telebanking, teleaccounting and teletax will be provided to you in your home. On-line services will be provided by banks and investment analysis services. Just-in-time inventory control systems using Electronic Point Of Sale (EPOS) systems will now have distributors and retailers linked together to provide universal financial system access for easier management and tighter control of goods and resources.
4.10. ON-LINE SERVICES
Companies will use teletraining to train employees from a far with remote slide shows and multimedia presentations. A teledoctor will provide on-line outpatient monitoring in your home and surgeons will transmit 3-D pictures to specialists throughout the world while they are performing surgery. Telecommuting will allow individuals to work at home providing all the above available services while reducing the cost and travel time.
4.11. Remote Video Surveillance
Remote surveillance using ISDN allows temporary connections to be made to cameras and other sensors at remote sites. The very fast call set-up time of ISDN allows a central operator to switch between the various cameras checking the sites. The Call Line Identity (CLI) feature automatically can be used to inform the control centre when an alarm has been triggered. An innovative and cost effective example of this application is found in a large retail chain which uses its ISDN lines during the day to transmit data but at night switches the application to remote security surveillance of its property. The technology can also used for the remote monitoring of inaccessible machines and equipment.
4.12. Audio Transmission
ISDN is widely used in the broadcasting industry with the BBC probably being one of the
largest users of ISDN in Europe. Audio transmission over ISDN telephony is of sufficiently high quality to be directly broadcast ‘on-air’. This has proved particularly useful in radio broadcasting for the delivery of foreign news reporting, sports commentary and radio interviews. Local radio stations often subscribe to independent news, whether or travel information providers, which deliver hourly reports to the local studio via ISDN. Some radio stations are also planning to use ISDN to broadcast live performances from musicians’ studios.
5. CONCLUSION
ISDN is an underlying technology, which provides cost-effective networking through the public telephone networks. Prior to ISDN end-users used desperate networks to communicate different types of information. ISDN is an important step forward in the adaptation of the network to handle the increasing global demand for computer to computer data communications. ISDN brings us closer to the goal of a ubiquitous multi-service network, integrating voice, data, video and image.
As stated in this paper the advantages of ISDN such as speed, multiple device connectivity and signaling, it is still difficult to network properly. It still doesn’t work well. Standardization effects are just getting started, and there are too few Internet Service Providers around that offer ISDN access. International equipment compatibility issues still pose a major hurdle in the effective implementation of ISDN.
Service availability, standards, falling ISDN service prices, government regulations, and applications that were not economical a few years back have carved out a role for ISDN in today’s digital communications market.
6. ACKNOWLEDGEMENTS
The authors of this paper thank Dr. M. S. Shiva kumar, Head, Department of Computer Science & Engineering of The National Institute of Engineering, Mysore for his valuable guidance in writing this paper.
7. REFERENCES