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Chapter 2: Recent Advances in Telecommunication Services and Systems

One of the problems caused by a wide geographical spread of people is the demand for faster and more efficient communication systems. The continuous evolution of telecommunications methods and media, from the existing analogue to a future integrated broadband digital network, will help to meet this demand.

Conventional telephony is the simplest form of a telecommunications network which facilitates voice communication. Such telephone networks consist of copper cables, with a bandwidth of approximately 3.1 kHz (300 to 3400 Hz), connecting the subscriber to local switching centres which are in turn connected to larger centres by cable or radio transmission, and so on. As the desired subscriber number is dialled, an end-to-end connection is established between the two communicating partners by progressively closing the appropriate switches in the switching centres (circuit switching). International agreements on standards allow national networks to be connected together so as to create a global telephone network.

In addition to the telephone network described above, various other independent/single-service telecommunications networks have also been developed. An example of these is the telex network, which is very similar in structure to the telephone network. It links up telex machines, which provide printed messages rather than speech communication. Another type of independent network is the cable television system, which differs from the others in that it usually distributes information from a central point outwardly to users, without any switching, and with no backward transmission, so end-to-end connection is not possible.

Additional capabilities have been built on top of the analogue telephone network, for example, fax (facsimile) machines which use modulated tones to transmit black and white copies of documents or drawings. Many other services have also grown up on the ordinary telephone system, for example, those which allow a user to remotely interrogate a computer system via a modem for applications such as videotex.

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TABLE 1. Table 2.1: The channels of the ISDN
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CHANNEL CAPACITY TRANSMISSION CAPABILITY

B-channel 64 kbit/s Voice & Data

D-channel 16 kbit/s Signalling information between the

terminal & the network

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The second generation of telecommunications use digital telephony (usually Pulse Code Modulation). Direct connection to a computer for data transmission can be achieved, without the need for a modem, as in the case of the analogue telephony, alleviating the noise that is usually introduced in the analogue signal, digital telephony enables the transmission of high quality speech over indefinite distances. Higher speech bandwidth (e.g. 7 kHz) can also be provided.

This growth in the number of different networks and services provided may lead to a loss of the operational efficiency, particularly if dedicated equipment is used for each service. This would mean duplication of equipment, lack of flexibility for users who may find that they need many different terminals for different purposes, and ad-hoc solutions to the growing number of service interworking problems that are expected to arise. In order to address these and other issues, the CCITT (International Consultative Committee for the Telephony and Telegraphy) has published a set of recommendations for the Integrated Services Digital Network (ISDN). As its name suggests, the ISDN aims to integrate all the various networks into one, and also provide the basis for the interworking of the various services that can be provided within such a network.

The basic service provided by the ISDN is comprised of two independent "B" channels of 64 kbit/s each, and the "D" channel of 16 kbit/s (Table 2.1).

The "D" channel is used for signalling purposes during call set-up, but may subsequently be used as a data channel. The "B" channels may be used to make two independent calls, or for some purposes linked together to provide a single 128 kbit/s channel. In its simplest use, a "B" channel provides a digital circuit capable of transmitting high quality speech. The real power of ISDN is that the "B" channel can also be used to send still or moving images, data and FAX at speeds far greater than the ordinary telephone network, and also provide new services, such as, teletex, textfax, interactive videotex, and others.

The main advantage of the ISDN is the integration aspect. Different services are transmitted and switched by just one network, reducing the cost for control and maintenance. Since the basic ISDN service provides two information channels, two different types of information can be mixed (multimodal communication). For example, the communicating partners may use speech communication and simultaneously exchange text or graphic documents.

Some of the services which can be provided by the ISDN are the following:

• Teletex is an extension of the well known telex service. It supports the full character set (telex has only lower case characters) which on an analogue telephone network are transmitted at 2400 bit/s. This transmission speed is about 50 times faster than telex. ISDN teletex will use a whole 64 kbit/s channel and thus the transmission speed is much faster (for example, one page of text is transmitted in less than one second).
  
• Textfax is a new service which combines teletex and fax. Documents with text and graphics information are transmitted in a mixed form (facsimile and character coding) with higher transmission speed and resolution.
  
• Interactive videotex was developed on the basis of the UK Prestel service. Videotex information (text and graphics elements) is transmitted via a telephone line and displayed on a television set. Since the bit rate in its original version is extremely low (1200 bit/s), according to the restrictions of the analogue telephone channel and the demand for a low cost modem, the transmission time is up to one minute, which is obviously not adequate. In an ISDN version a full 64 kbit/s channel can be used and the transmission time will be of the order of seconds.
  
• Videotelephony is a service that is not possible to provide on an existing analogue network, but which the ISDN can. It provides simultaneous speech and video transmission, between two individuals.
  

The ultimate goal of current research and development efforts in the telecommunications area is the Broadband Integrated Services Digital Network (B-ISDN). It is based on new transmission media (optical fibres etc.) in combination with high speed electronic switching systems. This will extend the integration provided by the narrowband ISDN, to include the switching, signalling, and transfer facilities, minimising the need to build service-specific transmission switching systems to support a broad mix of services.

A new transmission and switching technique termed Asynchronous Transfer Mode (ATM) is proposed as the basis for the realisation of the B-ISDN. The greatest advantage of the ATM is that it can dynamically allocate a capacity much greater than the average required by a specific channel. For example, in the case of a rapid change in picture content, additional video channel capacity can be allocated dynamically. Capacities in the Gbit/s range can also be achieved by multiplexing, using the Synchronous Digital Hierarchy (SDH) (Table 2.2).

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TABLE 2. Table 2.2: Two transmission modes of B-ISDN
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TECHNIQUE CAPACITY TRANSMISSION CAPABILITY

ASYNCHRONOUS TRANSFER 150 Mbit/s Simultaneous video, data and

MODE (ATM) voice transmission

SYNCHRONOUS DIGITAL 155.2 Mbit/s ATM channels, Local (10-50

HIERARCHY (SDH) Mbit/s) and Wide (50-560

Mbit/s) area network

channels

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One of the key aspects related to the B-ISDN is the provision of new telecommunications services with impressive facilities like full motion colour video transmission, high quality digital television, advanced radio with digital transmission and signal processing in addition to the narrowband ISDN provided services. As B-ISDN is intended to cover a wide range of services varying in bandwidth, burstiness and session length, particularly attention is being paid during the current design phase of the network to provide built in flexibility in order to meet these diverse requirements.

Four broad categories of services can be identified:

• Conversational services refer to end-to-end real time exchange of information between users or between the user and the server at times determined by the user. The information is generated by the sending user or the server and is directed toward one or more individual communicating partners at the receiving end. Representative conversational services are the audio telephone, the videotelephone, multimedia telephone, multimedia conference etc.
  
• Messaging services provide user-to-user communication via storage units. Examples of functions included are store & forward, mailbox and/or message handling (e.g. information editing, processing and conversion). Some typical messaging services are text mail, multimedia mail etc.
  
• Retrieval services offer users the possibility of retrieving and manipulating information stored in information centres (data bases, film libraries etc.).
  
• Distribution (or broadcast) services provide a continuous flow of information, which is distributed from a central source to a large number of users. Typical examples of distribution services are the Television Broadcast Service, Document Broadcast, etc.
  
• 
  

By using one or a combination of these services a variety of applications can be implemented. Applications are foreseen in the following broad areas:

• commercial (e.g. teleshopping, publishing, translation and ticket reservation);
  
• education (e.g. tele-university, computer assisted education and film library);
  
• entertainment (e.g. tele-bingo, participative radio and television);
  
• financial (e.g. cash transactions and home banking);
  
• government/general public communication (e.g. general public poll and government information); and,
  
• automation (e.g. CAD/CAM, telemetry and alarms, telecontrol, surveillance and medical care).
  

An important aspect that should be considered in service design and implementation related to the available capabilities, quality and cost, is the environment in which it is to be applied. Four environments have been considered as representative.

• Home or residential including services/applications relevant to personal communication (digital telephone, videophone etc.), entertainment (High Definition Television, video etc.), health supervision (telesurveillance, telemedicine etc.), education (tele-university, computer assisted education etc.) as well as domestic tasks (teleshopping, payments etc.).
  
• Office or business covering premises where business activities are carried out requiring high quality services such as, multimedia data base access and archiving, multimedia mail, multimedia document handling, multimedia conference etc.
  
• Industrial, concerned with all aspects related to the creation of "products" including research, design and engineering, production, testing and packaging. It is obvious that special purpose services and applications are needed in these environments such as, CAD/CAM, telemetry, telecontrol etc., which require accuracy, speed, reliability and are costly due to their restricted use.
  
• Public, addressing daily life needs through the provision of services/applications such as public communications, government information retrieval, ticket reservation, cash transactions etc.
  

Another sector of interest for the B-ISDN is mobile communication which allows people to communicate with each other from any location. In addition to mobile and cordless telephone currently in use, existing market needs demand more powerful mobile terminals that access services and applications such as videotelephone, video conferencing, group working, information retrieval, vehicle navigation support, remote control and others.

GRIFFTITHS, J.M. (1990). ISDN explained. London: John Wiley and sons.

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