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1 DEFINITION AND CONCEPT
Telecommunication deals with the service of providing electrical communication at a distance. The service is supported by an industry that depends on a large body of increasingly specialized scientists, engineers, and craftspeople. The service may be private or open to public correspondence (i.e., access). Examples of the latter are government-owned telephone companies, often called administrations or private corporations, that sell their services publicly.
1.1 Telecommunication Networks
The public switched telecommunication network (PSTN) is immense. It consists of hundreds of smaller networks interconnected. There are "fixed" and "mobile" counterparts. They may or may not have common ownership. In certain areas of the world the wired and wireless portions of the network compete. One may also serve as a backup for the other upon failure. It is estimated that by 2005 there will be as many wireless telephones as wired telephones, about 5 × [10.sup.9] handsets worldwide of each variety.
These networks, whether mobile or fixed, have traditionally been based on speech operations. Meanwhile, another network type has lately gained great importance in the scheme ofthings. This is the enterprise network. Such a network supports the business enterprise. It can just as well support the government "enterprise" as a private business. Its most common configuration is a local area network (LAN) and is optimized for data communications, The enterprise network also has a long-distance counterpart, called a WAN or wide area network. The U.S. Department of Defense developed a special breed of WAN where the original concept was for resource sharing among U.S. and allied universities. Since its inception around 1987, it has taken on a very large life of its own, having been opened to the public worldwide. It is the internet. Its appeal is universal, serving its original intent as a resource-sharing medium extending way beyond the boundaries of universities and now including a universal messaging service called email (electronic mail).
Some may argue that telecommunications with all its possible facets is the world's largest business. We do not take sides on this issue. What we do wish to do is to impart to the reader a technical knowledge and appreciation of telecommunication networks from a system viewpoint. By system we mean how one discipline can interact with another to reach a certain end objective. If we do it right, that interaction will be synergistic and will work for us; if not, it may work against us in reaching our goal.
Therefore, a primary concern of this book is to describe the development of the PSTN and enterprise network and discuss why they are built the way they are and how they are evolving. The basic underpinning of the industry was telephone service. That has now changed. The greater portion of the traffic carried today is data traffic, and all traffic is in a digital format of one form or another. We include wireless/cellular and "broadband" as adjuncts of the PSTN.
Telecommunication engineering has traditionally been broken down into two basic segments: transmission and switching. This division was most apparent in conventional telephony. Transmission deals with the delivery of a quality electrical signal from point X to point Y . Let us say that switching connects X to Y, rather than to Z. When the first edition of this book was published, transmission and switching were two very distinct disciplines. Today, that distinction has disappeared, particularly in the enterprise network. As we proceed through the development of this text, we must deal with both disciplines and show in later chapters how the dividing line separating them has completely disappeared.
2 THE SIMPLE TELEPHONE CONNECTION
The common telephone as we know it today is a device connected to the outside world by a pair of wires. It consists of a handset and its cradle with a signaling device, consisting of either a dial or push buttons. The handset is made up of two electroacoustic transducers, the earpiece or receiver and the mouthpiece or transmitter. There is also a sidetone circuit that allows some of the transmitted energy to be fed back to the receiver.
The transmitter or mouthpiece converts acoustic energy into electric energy by means of a carbon granule transmitter. The transmitter requires a direct-current (dc) potential, usually on the order of 3-5 V, across its electrodes. We call this the talk battery, and in modern telephone systems it is supplied over the line (central battery) from the switching center and has been standardized at -48 V dc. Current from the battery flows through the carbon granules or grains when the telephone is lifted from its cradle or goes "off hook." When sound impinges on the diaphragm of the transmitter, variations of air pressure are transferred to the carbon, and the resistance of the electrical path through the carbon changes in proportion to the pressure. A pulsating direct current results.
The typical receiver consists of a diaphragm of magnetic material, often soft iron alloy, placed in a steady magnetic field supplied by a permanent magnet, and a varying magnetic field caused by voice currents flowing through the voice coils. Such voice currents are alternating (ac) in nature and originate at the farend telephone transmitter. These currents cause the magnetic field of the receiver to alternately increase and decrease, making the diaphragm move and respond to the variations. Thus an acoustic pressure wave is set up, more or less exactly reproducing the original sound wave from the distant telephone transmitter. The telephone receiver, as a converter of electrical energy to acoustic energy, has a comparatively low efficiency, on the order of 2-3%.
Sidetone is the sound of the talker's voice heard in his (or her) own receiver. Sidetone level must be controlled. When the level is high, the natural human reaction is for the talker to lower his or her voice. Thus by regulating sidetone, talker levels can be regulated. If too much sidetone is fed back to the receiver, the output level of the transmitter is reduced as a result of the talker lowering his or her voice, thereby reducing the level (voice volume) at the distant receiver and deteriorating performance.
To develop our discussion, let us connect two telephone handsets by a pair of wires, and at middistance between the handsets a battery is connected to provide that all-important talk battery. Such a connection is shown diagrammatically in Figure 1.1. Distance D is the overall separation of the two handsets and is the sum of distances [d.sub.1] and [d.sub.2]; [d.sub.1] and [d.sub.2] are the distances from each handset to the central battery supply. The exercise is to extend the distance D to determine limiting factors given a fixed battery voltage, say, 48 V dc. We find that there are two limiting factors to the extension of the wire pair between the handsets. These are the IR drop, limiting the voltage across the handset transmitter, and the attenuation. For 19-gauge wire, the limiting distance is about 30 km, depending on the efficiency of the handsets. If the limiting characteristic is attenuation and we desire to extend the pair farther, amplifiers could be used in the line. If the battery voltage is limiting, then the battery voltage could be increased. With the telephone system depicted in Figure 1.1, only two people can communicate. As soon as we add a third person, some difficulties begin to arise. The simplest approach would be to provide each person with two handsets. Thus party A would have one set to talk to B, another to talk to C, and so forth. Or the sets could be hooked up in parallel. Now suppose A wants to talk to C and doesn't wish to bother B. Then A must have some method of selectively alerting C. As stations are added to the system, the alerting problem becomes quite complex. Of course, the proper name for this selection and alerting is signaling. If we allow that the pair of wires through which current flows is a loop, we are dealing with loops. Let us also call the holder of a telephone station a subscriber. The loops connecting them are subscriber loops.
Let us now look at an eight-subscriber system, each subscriber connected directly to every other subscriber. This is shown in Figure 1.2. When we connect each and every station with every other one in the system, this is called a mesh connection, or sometimes full mesh. Without the use of amplifiers and with 19-gauge copper wire size, the limiting distance is 30 km. Thus any connecting segment of the octagon may be no greater than 30 km. The only way we can justify a mesh connection of subscribers economically is when each and every subscriber wishes to communicate with every other subscriber in the network for virtually the entire day (full period). As we know, however, most telephone subscribers do not use their telephones on a full-time basis. The telephone is used at what appear to be random intervals throughout the day. Furthermore, the ordinary subscriber or telephone user will normally talk to only one other subscriber at a time. He/she will not need to talk to all other subscribers simultaneously.
If more subscribers are added and the network is extended beyond about 30 km, it is obvious that transmission costs will spiral, because that is what we are dealing with exclusively here-transmission. We are connecting each and every subscriber together with wire transmission means, requiring many amplifiers and talk batteries. Thus it would seem wiser to share these facilities in some way and cut down on the transmission costs. We now discuss this when switch and switching enter the picture. Let us define a switch as a device that connects inlets to outlets. The inlet may be a calling subscriber line, and the outlet may be the line of a called subscriber. The techniques of switching and the switch as a concept are widely discussed later in this text. Switching devices and how they work are covered in Chapters 3 and 9. Consider Figure 1.3, which shows our subscribers connected in a star network with a switch at the center. All the switch really does in this case is to reduce the transmission cost outlay. Actually, this switch reduces the number of links between subscribers, which really is a form of concentration. Later in our discussion it becomes evident that switching is used to concentrate traffic, thus reducing the cost of transmission facilities.
3 SOURCES AND SINKS
Traffic is a term that quantifies usage. A subscriber uses the telephone when he/she wishes to talk to somebody. We can make the same statement for a telex (teleprinter service) subscriber or a data-service subscriber. But let us stay with the telephone.
A network is a means of connecting subscribers. We have seen two simple network configurations, the mesh and star connections, in Figures 1.2 and 1.3. When talking about networks, we often talk of sources and sinks. A call is initiated at a traffic source and received at a traffic sink. Nodal points or nodes in a network are the switches.
4 TELEPHONE NETWORKS: INTRODUCTORY TERMINOLOGY
From our discussion we can say that a telephone network can be regarded as a systematic development of interconnecting transmission media arranged so that one telephone user can talk to any other within that network. The evolving layout of the network is primarily a function of economics. For example, subscribers share common transmission facilities; switches permit this sharing by concentration.
Consider a very simplified example. Two towns are separated by, say, 20 miles, and each town has 100 telephone subscribers. Logically, most of the telephone activity (the traffic) will be among the subscribers of the first town and among those of the second town. There will be some traffic, but considerably less, from one town to the other. In this example let each town have its own switch. With the fairly low traffic volume from one town to the other, perhaps only six lines would be required to interconnect the switch of the first town to that of the second. If no more than six people want to talk simultaneously between the two towns, a number as low as six can be selected. Economics has mandated that we install the minimum number of connecting telephone lines from the first town to the second to serve the calling needs between the two towns. The telephone lines connecting one telephone switch or exchange with another are called trunks in North America and junctions in Europe. The telephone lines connecting a subscriber to the switch or exchange that serves the subscriber are called lines, subscriber lines, or loops. Concentration is a line-to-trunk ratio. In the simple case above, it was 100 lines to six trunks (or junctions), or about a 16 : 1 ratio.
A telephone subscriber looking into the network is served by a local exchange. This means that the subscriber's telephone line is connected to the network via the local exchange or central office, in North American parlance. A local exchange has a serving area, which is the geographical area in which the exchange is located; all subscribers in that area are served by that exchange.
The term local area, as opposed to toll area, is that geographical area containing a number of local exchanges and inside which any subscriber can call any other subscriber without incurring tolls (extra charges for a call). Toll calls and long-distance calls are synonymous. For instance, a local call in North America, where telephones have detailed billing, shows up on the bill as a time-metered call or is covered by a flat monthly rate. Toll calls in North America appear as separate detailed entries on the telephone bill. This is not so in most European countries and in those countries following European practice. In these countries there is no detailed billing on direct-distance-dialed (subscriber-trunk-dialed) calls. All such subscriber-dialed calls, even international ones, are just metered, and the subscriber pays for the meter steps used per billing period, which is often one or two months. In European practice a long-distance call, a toll call if you will, is one involving the dialing of additional digits (e.g., more than six or seven digits).
Let us call a network a grouping of interworking telephone exchanges. As the discussion proceeds, the differences between local networks and national networks are shown. Two other types of network are also discussed.
Excerpted from Telecommunication System Engineering by Roger L. Freeman Copyright © 2004 by Roger L. Freeman. Excerpted by permission.
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|Ch. 1||Basic telephony||1|
|Ch. 2||Local networks||41|
|Ch. 3||Switching in an analog environment||73|
|Ch. 4||Signaling for analog telephone networks||111|
|Ch. 5||Introduction to transmission for telephony||139|
|Ch. 6||Long-distance networks||157|
|Ch. 7||The design of long-distance links||185|
|Ch. 8||Digital transmission systems||261|
|Ch. 9||Digital switching and networks||317|
|Ch. 10||Introduction to data communications||365|
|Ch. 11||Data networks and their operation||409|
|Ch. 12||Voice-over IP||483|
|Ch. 13||Local area networks||501|
|Ch. 14||Integrated services digital networks||565|
|Ch. 15||Speeding things up with frame relay||603|
|Ch. 16||The asynchronous transfer mode (ATM) and broadband ISDN||631|
|Ch. 17||CCITT signaling system no. 7||681|
|Ch. 18||Wireless and cellular/mobile radio||737|
|Ch. 19||Last-mile broadband connectivity and wireless local loop (WLL)||805|
|Ch. 20||Optical networking||835|
|Ch. 21||Network management||871|
Posted February 21, 2002
This book is very well written to explain the complex world of telecommunications, to include everything from Routers to microwave links. The target reader for this book is an engineer or technical manager that needs detailed information on telecommunications. Mr. Freeman does an excellent job of covering Microwave Line-of-Sight communications, digital transmission and LAN connections. Mr. Freeman starts off with some basic concepts of analog and digital theory and takes the reader on a journey through the different types of networks ¿ along the way he explains how each layer of the OSI model is used as in ISDN, Digital Switching, Routers, etc. I would recommend this book to anyone who works with networks or phone systems.
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