To the Reader				< Previous • Next >

Introduction
Book structure
Division into chapters
Network model and theme of Volume 2

Introduction

The global telecommunications network - the largest and most complex technical system that man has created - makes up a substantial part of a country's infrastructure and is vital to the development of that country.

Development in the field of telecommunications has been very rapid in recent years. By telecommunications we mean here all processes that render it possible to transfer voice, data and video with the help of some form of electromagnetic system, including optical transfer methods.

This rapid development points to new demands on knowledge and competence for everyone who takes an active part in modern telecommunications. This field's growing complexity also calls for a structured approach, especially concerning the principles of telecommunications. For anyone willing to tackle writing books to broaden users' knowledge of this subject, a clear structure is particularly important, so that the books are easy to understand and easy to navigate through.

The prime focus of our books is on principles and context rather than on profound technical descriptions. It is also important to point out that the contents are not associated with any particular technical system.

To the uninitiated, the telecommunications world may seem to be rather intangible: some familiar, commonplace phenomena and an abstract system that for the most part works without anyone needing to think about how. See Figure 0.1. One might know about some types of equipment connected to the telecommunications network, but not very much more than that.

Figure 0.1 Telecommunications network

People who want to learn more realise fairly soon that telecommunications networks are mainly composed of "nodes" (exchanges) and "links", and that "terminals" are connected to the network. See Figure 0.2.

Figure 0.2 Main components of a telecommunications network

But still many things are unclear, not only in networks themselves but also in the complicated interworking between different types of network.

Book structure

To be able to unravel the mystery surrounding this subject, we will start by describing our book structure and its purpose. A well-thought-out structure is a good aid to the reader who is looking for information on a certain part of a subject. Furthermore, it makes it easier to transfer information to databases and to search in them. As the books are designed to be used for different types of competence development in parts of the telecommunications field - signalling, broadband, access, and so forth - we have come to the conclusion that a strictly logical structure also has a certain pedagogical value.

Understanding Telecommunications consists of eight parts: Part A is in Volume 1, and Parts B - H are in Volume 2. All the parts are divided into ten chapters, each of which deals with a specific topic (see Figure 0.3). In Volume 2, To the Reader is followed by an introductory part - The telecommunications services market - that describes current and forecast trends in the field of teleservices.

Conclusion, at the end of Volume 2, includes a two-page table showing the characteristics of the different networks described in Part B - Part H.

Figure 0.3 Division into parts and chapters

Part A - General review

To understand the structuring of telecommunications networks it is important to understand the demands that basic services place on the network. Voice, data and video transmission belong to this category of services. As far as voice is concerned we may say that the telephone network is tailor-made to be able to provide the telephony service at a reasonable cost.

The remarkably rapid development in the field of telecommunications is characterised by many new bearer networks coming into being alongside the telephone network. This depends to a great extent on the growth of data communication services which are far more richly varied than telephony. A future field of growth is multimedia services, usually including video. Another reason for the proliferation of new bearer networks is that it has become so popular to make services mobile.

Operators also aim at integrating many services into the same network to reduce the number of networks in the long term. But as long as the old networks still exist, the short-term effect will be a larger number of networks to handle.

This technical development is reflected in our book structure: the different tailor-made - fixed, mobile and integrated - networks are described in separate Parts of Volume 2.

Another question of structure applies to public networks as opposed to business networks. However, the field of business networks will not be explicitly dealt with in the first edition of our books.

Many features are common to two or more bearer networks - cables, buildings and other types of infrastructure. In the future we may even have a switching technique that can be developed to form part of the infrastructure ( ATM technique). To avoid tiring repetition, we have collected common aspects like these in Part A, which is a general review and a sort of frame for the different Parts that make up Volume 2.

To take into account the development of increasingly integrated telecommunications networks - a trend sometimes described as "bearer networks migrating together" - Part A must also deal with tomorrow's situation. In the descriptions of the specialised networks in Parts B - G the main emphasis is instead on the current situation, before migration.

Parts B-H - Network-specific topics

Below follows a brief presentation of the network-specific topics dealt with in the different Parts of Volume 2.

Telephony and data communication - A comparison

The demands that users put on telephony and data communication are different in many ways. Telephony can manage relatively poor transmission quality because of the redundancy built into natural language and speech, and because people engaged in a telephone conversation can easily overcome temporary disturbances and interruptions by repeating what they have just said. On the other hand we find it difficult to accept delays in a telephone connection. Delay must be kept below a certain value so as not to be experienced as irritating, and variations can be inconvenient even if the total delay is moderate.

Data traffic, by contrast, is relatively insensitive to delay (within reasonable limits) whereas poor transmission quality can cause bit errors and a garbled message. Information must arrive at the receiving end in exactly the same shape that it was in when transmitted; otherwise, it is practically worthless. Sometimes large files or the entire contents of mass storage devices must be retransmitted when a fault has occurred. Because reliable transmission is so important, mechanisms have been introduced which will guarantee error-free transfer of information. But security has a price: complex routines and an extensive need of processor capacity.

Another difference is that telephony generates a continuous stream of information, while data traffic is more intermittent. During a telephone call, about 40% of the capacity is used for transfer of "useful" information, while a line for data traffic might not be made very good use of even when traffic is formally in progress.

Networks carrying telephony (with a few minor exceptions) are circuit-switched networks that set up a connection between subscribers who want to come into contact with each other. The connection remains established throughout the call, irrespective of whether subscribers speak with each other or not, and there is no chance of any other subscriber using that particular accessible capacity.

A special technique, called packet switching, has been developed to expand the degree of utilisation in data networks when traffic sources (computers) are only sending scattered information. Each data traffic packet has an address that controls the switching process in the exchanges (nodes).

Figure 0.4 Networks for telephony and data communication

For the sake of completeness we will add here that circuit-switched networks in certain cases are well suited for data traffic, especially for the transfer of large amounts of data on sections with good transmission quality.

The public switched telephone network (PSTN) is described in greater detail in Part B. Two types of dedicated data network (X.25 and frame relay) are described in Part F.

Integration of services

Every network operator naturally wants to minimise the number of networks needed to provide, in an economically viable way, the services demanded by the public and by business subscribers. Besides, bearer networks ought to be reasonably homogeneous in technical respects. These have been realistic requirements in the case of bearer networks recently introduced but impossible for telephony networks that are more than 100 years old. There is a special boundary line between analog and digital technique.

The desire to create service integration as well as a more homogeneous technique brought about the integrated services digital network (ISDN). After a slow start during the 1980s, this network is now growing considerably, partly at the expense of a reduced expansion of telephone and data networks. ISDN was specified for bandwidth requirements in the range called narrowband but to a growing extent handles wideband as well (up to 2 Mbit/s).

ISDN - described in Part C - is principally a circuit-switched network but can also handle packet traffic. This traffic is directed to special packet handlers or to the packet networks we describe in Part F.

For broadband services above 2 Mbit/s, an elaborated packet technique called asynchronous transfer mode (ATM) has captured the interest of the telecommunications world. At the imminent turn of the century, telecommunications systems of this type ought to be common. For public ATM networks the term broadband ISDN (B-ISDN) is sometimes used, and our present ISDN is therefore referred to as narrowband ISDN (N-ISDN).

B-ISDN is a "truly integrated network" designed to handle all services in the same way. ATM and B-ISDN are described in Part G.

Figure 0.5 Networks for integrated services

Mobile networks

The continuing popularity of mobile telecommunications is obviously due to the fact that there are evident needs for this facility in society, not only in cars, on trains and on ships but also in the home and in the workplace. Mobile telephony is sometimes considered as a "special access" to the telephone network, because the normal call is made between a fixed and a mobile telephone. But bearing in mind the rapidly growing significance of mobility, it will soon be provided as a standard rather than a special case.

Because public land mobile networks (PLMNs) often have their own network operators, these networks thoroughly deserve their own part in our books - Part D . (See Figure 0.6)

Figure 0.6 Mobile telecommunications

Internetworking

The Internet, intended for internetworking between computers, has grown faster than any other network. Part H covers this subject.

The signalling network

More and more processor capacity is needed in the telecommunications networks. The reasons for this are numerous.

• The number of connections is expanding.
• Setting up a connection requires increasing processor capacity. Mobile telecommunications is a typical example, ISDN another.
• Processor capacity is distributed in the networks (which requires increased communication and, hence, more processor capacity).
• The number of supplementary services in the networks is growing.

Naturally, the rapidly growing and increasingly distributed processor capacity requires expanded processor communication. In the digital circuit-switched networks there is in most cases a special data network - conforming to a standard known as signalling system No. 7 (SS7) - for this extremely important communication.

This network, which only in exceptional cases carries traffic between subscribers, is very small in terms of capacity, compared with telephone and mobile networks, but dependability requirements are extremely high. It is usually called the signalling network and is dealt with in Part E.

Business networks/private networks

As we mentioned earlier, business networks are not explicitly dealt with in our books. The trend is towards business networks that employ the same techniques as public networks: ATM technique, multimedia traffic and mobile communication. A large part of the contents in our books is therefore applicable to business networks as well.

Figure 0.7 Business networks/private networks

Division into chapters

Principles

The division into chapters has been made with a view to identifying

• what the networks produce; that is, user services , often illustrated with a picture of a terminal;
• the different building blocks and functions of the networks;
• the network operators' main processes; and
• interworking between networks.

Each one of these topics has been given a chapter of its own in the seven Parts of Volume 2.

Chapter contents

One could be led to believe that there is a fixed connection between different services and different bearer networks, especially in the context of what we mentioned about tailor-made bearer networks. However, that is not the case. Many bearer networks have a standardised interface that permits many different services to be provided over the same bearer network. It is therefore natural in Chapter 1 in Volume 1 to provide overall information on user services, and then in Chapter 1 of each Part of Volume 2 to give an example of terminal designed for connection to specific bearer networks.

Terminals link networks and services together and, hence, are considered to consist of two main entities:

• one entity related to the service (transfer of voice, data and video with the respective "subservices"); and
• one entity related to a bearer network; in principle corresponding to one of the Parts of Volume 2.

In Chapter 2 we deal with standardisation, a field that is very important for us to be able to communicate internationally. To achieve standardisation we must apply standardised reference models to the greatest extent possible. Here we have also covered techniques that are important to many of the main functions in the networks - analog/digital conversion (A/D conversion), and the three main transfer modes: circuit mode, packet mode including frame relay, and cell mode.

Switching and switch control are described in Chapter 3 . By switching we mean here the technique and equipment used to control the network and establish contact with the person we wish to reach, which we normally do by dialling his telephone number. (In modern English usage, "dial" is an accepted term for "pressing keys to call a number".) This requires that we associate the called party with a geographical point in the telecommunications network (normally his telephone jack), in the vicinity of which there is a terminal. Consequently, the number in the telephone directory corresponds to a telephone jack. Similar numbering schemes apply to other networks (telex numbers, data networks numbers, and the like). If connection points (exchanges) must be passed before the caller is connected to the called party, this is known as routing the call.

For a telecommunications network to function and deliver the services we are willing to pay for, it must be able to span distances which in extreme cases can be up to 12,500 miles; that is, on the other side of the earth. The technique used to transfer services over both long and short distances is called transmission and is dealt with in a chapter of its own (Chapter 4).

Using this kind of technique we build transmission networks. For different reasons they are divided into access and trunk networks - terms that are explained in detail in the important, "futuristic" Chapter 5. Principal terms in this field have already been mentioned in Chapters 3 and 4.

In Chapter 3 we describe basic, "unintelligent" switching and routing. People might wonder whether "intelligent" routing would enable them to be independent of a geographical point and instead direct a call straight to a particular person. Or why not inform the network that you can be reached at another number and have the call rerouted, or that you want your call to be directed to your car?

As we all know, much of this is already possible. The key term would consequently be "intelligent routing", but many more intelligent network (IN) services can be included here. The intelligent functions and services are described in Chapter 6. Normally, the IN functions are centrally deployed in the network, but Chapter 6 also deals with distributed supplementary services, and information services.

A fundamental, linking function in the traffic machine is signalling, which in effect is data communication between nodes and between terminals and nodes. In Chapter 7 of Volume 1 we provide an overview of different signalling systems. The specific signalling for each bearer network is described in Chapter 7 in the respective Parts of Volume 2.

Network management and network planning are part of the network operator's main processes. Unlike network planning, network management requires a large amount of equipment associated with the traffic functions. The management network and the corresponding processes for control, modification and maintenance are described in Chapter 8.

There is only one function in the network that we have not mentioned so far: interworking between networks . This is an important function, because the same service may be implemented in many networks. The introduction of integrated networks and the growing number of operators notably contribute to the need for intercommunication. Interworking between networks is described in Chapter 9.

Chapter 10 has two main purposes:

• to describe the network planning process; and
• to link the previous chapters.

Here we describe areas associated with the network development plan, such as network architecture, fundamental technical plans and network dimensioning.

Network model and theme of Volume 2

In Volume 2, each Part starts with a reference model of the bearer network described in that Part. The model shows network functions and characteristic properties of the bearer network presented.

The specific theme of Volume 2 - the expanding services market and the competing bearer networks - is illustrated in Figure 0.8.

Figure 0.8 Theme of Volume 2

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