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To browse Academia. Skip to main content. Log In Sign Up. Download Free PDF. Introduction to Telecommunications Network Engineering. Meskerem Eshetu. Download PDF. A short summary of this paper. PrefaceTelecommunications is one of the fastest growing business sectors of modern information technologies.
A couple of decades ago, to have a basic understanding of telecommunications, it was enough to know how the telephone network operated. Today, the field of telecommunications encompasses a vast variety of modern technologies and services. Some services, such as the fixed telephone service in developed countries, have become mature, and some have been exploding e.
The deregulation of the telecommunications industry has increased business growth, even though, maybe because, tariffs have decreased. The present telecommunications environment, in which each of us has to make choices, has become complicated.
In the past, there was only one local telephone network operator that we chose to use or not use. Currently, many operators offer us ADSL or cable modem for Internet access and we have many options for telephone service as well.
Telecommunications is a strategically important resource for most modern corporations and its importance continues to increase. Special attention has to be paid to the security aspects and costs of services. The everchanging telecommunications environment provides new options for users, and we should be more aware of telecommunications as a whole to be able to capitalize on the possibilities available today.
The business of telecommunications has been growing rapidly, and many newcomers have found employment in this area. Even if these xv newcomers have a technical background, they may feel that they have a very restricted overall view of the telecommunications network as a whole. The first purpose of this book is to provide an overall view of telecommunications networks to newcomers to the telecommunications business. This kind of general knowledge is useful to the users of telecommunications services, the personnel of operators, and the employees of telecommunications system manufacturers.
The professionals working with these complicated technologies very often have extensive knowledge of one very narrow section of telecommunications, but are not familiar with the hundreds of terms and abbreviations that are used in other telecommunication areas by individuals with whom they need to interact. One purpose of this book is to provide content to some of the most common terms and abbreviations used in different areas of telecommunications.
When I was working as a development department manager at Nokia, I noticed that relatively few books are available that provide a good introduction to data, fixed, and mobile networks. This kind of overview is valuable for people entering a technology area in which all of these technologies are emerging. Most of the books on the market explain telecommunications from only one point of view even though there is no longer any distinct separation of the networks that provide data, speech, and mobile services.
Everyone working in the modern business environment, such as the development engineers, testing personnel, and sales managers, must have a common language if they are to work together efficiently, but not many books supply that common language because they do not provide an overview of telecommunications as a whole. The material included in this book is used in the Telecommunications Networks course for students of information technologies at the Espoo-Vantaa Institute of Technology in Finland.
The goal of this course is to give students a basic understanding of the structure and operation of a global telecommunications network. This course provides an overview of telecommunications; the provision of a deeper understanding about each subject, such as the spectral analysis of signals or detailed knowledge of the operation and functions of mobile networks, is left to dedicated courses.
I have tried not to cover too many aspects of modern telecommunications in this book so as to keep its structure clear. The goal is to lay the basis for later studies of telecommunications for which many good sources are available. Some of them are listed at the end of each chapter. ObjectivesLike the first edition of this book, this second edition is designed to provide answers to the fundamental questions concerning telecommunications networks and services, telecommunications as a business area, and the general trends of technical development.
The Internet has become a very important information source for most of us and we use it daily in the office and at home. Its use for various kinds of commercial service is expanding, and interactive services, including entertainment, are becoming richer. Most new and evolving network technologies for future telecommunications are also based on data communication concepts, especially Internet technology.
Examples of these are packet-switched second and third generation cellular systems. Also the core of the fixed telecommunications network will gradually evolve to packetswitched networks carrying both data and speech traffic.
Here we try to emphasize this development. For future development all opinions and comments concerning the book are welcome. You may send them directly to the author at tarmo.
For those readers who will use this book as training material, please contact the author for additional teachers' instructional material. Telecommunications has been defined as a technology concerned with communicating from a distance, and we can categorize it in various ways.
Figure 1. It includes mechanical communication and electrical communication because telecommunications has evolved from a mechanical to an electrical form using increasingly more sophisticated electrical systems.
This is why many authorities such as the national post, telegraph, and telephone PTT companies are involved in telecommunications using both forms.
Our main concern here is electrical and bidirectional communication, as shown in the upper part of Figure 1. The share of mechanical telecommunications such as conventional mail and press is expected to decrease, whereas electrical, especially bidirectional, communication will increase and take the major share of telecommunications in the future. Hence, major press corporations are interested in electrical telecommunications as a business opportunity.
Significance of TelecommunicationsMany different telecommunications networks have been interconnected into a continuously changing and extremely complicated global system. We look at telecommunications from different points of view in order to understand countries the fixed telephone density, that is, the teledensity, is fewer than 10 telephones per 1, inhabitants; in developed countries in, for instance, North America and Europe, there are around to fixed telephones per 1, inhabitants.
The economic development of developing countries depends on in addition to many other things the availability of efficient telecommunications services. The operations of a modern community are highly dependent on telecommunications. We can hardly imagine our working environment without telecommunications services. The local area network LAN to which our computer is connected is interconnected with the LANs of other sites throughout our company.
This is mandatory so that the various departments can work together efficiently. We communicate daily with people in other organizations with the help of electronic mail, telephones, facsimile, and mobile telephones.
Governmental organizations that provide public services are as dependent on telecommunications services as are private organizations. Telecommunications plays an essential role on many areas of everyday living. Everyday life is dependent on telecommunications. Each of us uses telecommunications services and services that rely on telecommunications daily.
Historical PerspectiveSome of the most important milestones in the development of electrical telecommunications systems according to [1] are discussed in this section. Terms and abbreviations used in the chronology are explained in later chapters of this book. The development and expansion of some telecommunications services is also illustrated in Figure 1 Fully electronic telephone switching system is put into service. Standardization for second generation digital cellular systems is initialized.
StandardizationCommunication networks are designed to serve a wide variety of users who are using equipment from many different vendors. To design and build networks effectively, standards are necessary to achieve interoperability, compatibility, and required performance in a cost-effective manner.
Open standards are needed to enable the interconnection of systems, equipment, and networks from different manufacturers, vendors, and operators. The most important advantages and some other aspects of open telecommunications standards are explained next. Standards lead to economies of scale in manufacturing and engineering. Standards increase the market for products adhering to the standard, which leads to mass production and economies of scale in manufacturing and engineering, very large scale integration VLSI implementations, and other benefits that decrease price and further increase acceptance of the new technology.
This supports the economic development of the community by improving telecommunications services and decreasing their cost. Political interests often lead to different standards in Europe, Japan, and the United States. Standardization is not only a technical matter.
Sometimes opposing political interests make the approval of global standards impossible, and different standards are often adapted for Europe, the United States, and Japan. To protect local industry, Europe does not want to accept American technology and America does not want to accept European technology. We will explain this terminology in Chapter 3. A more recent example is the American decision in the s not to accept European GSM technology as a major digital second generation cellular technology.
International standards are threats to the local industries of large countries but opportunities to the industries of small countries. Major manufacturers in large countries may not support international standardization because it would open their local markets to international competition.
Manufacturers in small countries strongly support global standardization because they are dependent on foreign markets. Their home market is not large enough for expansion and they are looking for new markets for their technology. Standards make the interconnection of systems from different vendors possible.
Standards make international services available. Standardization plays a key role in the provision of international services. Official global standards define, for example, telephone service, ISDN, and facsimile. The standards of some systems may not have official worldwide acceptance, but if the system becomes popular all around the world, a worldwide service may become available. Internet specifications have no official status, and GSM was originally specified for Europe only.
Their specifications have been openly available, which has supported their expansion. In the s, a bolt from one car would not fit another. Currently, bolts are internationally standardized and most often compatible. Standards OrganizationsMany organizations are involved in standardization work. We look at them from two points of view: 1 the players in the telecommunications business Introduction to Telecommunications involved in standardization and 2 the authorities that approve official standards.
Let us look at a list of these parties and their most important interests, that is, why they are involved in standardization work. Telecommunications network operators and manufacturers participate in standardization work. One example of standards made by ETSI is the digital cellular mobile system GSM, which became a major standard for second generation digital mobile communications all around the world.
The deregulation of telecommunications forced national PTTs to become network operators equal to other new operators and they are not allowed to make standards alone any more. American OrganizationsThe U. Some of these organizations are shown in Figure 1. The Institute of Electrical and Electronics Engineers IEEE is one of the largest professional societies in the world and it has produced many important standards for telecommunications.
For example, international standard ISO Many of its standards, such as those for connectors for personal computers, have achieved global acceptance. It is a government organization that regulates wire and radio communications. It has played an important role, for example, in the development of worldwide specifications for radiation and susceptibility of electromagnetic disturbances of telecommunications equipment.
Its task is to adapt the global standard to the American environment [4]. ITU-T works for the standards of public telecommunications networks e. Many parties participate in their work, but only national authorities may vote. ISO has done important work in the area of data communications and protocols, and IEC in the area of electromechanical for example, connectors , environmental, and safety aspects. The organizations shown in Figure 1. Other OrganizationsMany organizations other than those just mentioned are working with standards.
Some of these are active in ITU-T and ISO, and many international standards are based on or may even be copies of the initial work of these groups. We introduce some of these as examples of standards organizations without official status see Figure 1. Its goal is to define a third generation cellular system that will receive worldwide acceptance and ensure compatibility among equipment from different vendors. Unofficial forums are more flexible and can produce necessary standards on a shorter timescale than Upstream And Downstream Problems Formula Js can official organizations.
Their specifications are often used as a basis for later official standards. The Telemanagement Forum TMF is an organization of system manufacturers that works to speed the development of network management standards. With the help of these standards, telecommunications network operators will be able to control and supervise their multivendor networks efficiently from the same management center.
The organizations mentioned here are just examples; many other such organizations and cooperative units exist. New groups appear and some organizations disappear every year. One important problem in standardization is the question of intellectual property rights IPRs. One company involved in development of a standard may have a patent or copyright for a method or function that is essential for implementation of the standardized system.
In such a case, other manufacturers may not be able to implement the standard in an economically feasible manner without interfering with a patent or copyright. There are no fixed rules regarding how to solve this problem, but very often the patent or copyright owner agrees to license the patent or copyright for a standardized system under fair terms [5].
Development of the Telecommunications BusinessIn the past, telecommunications has been a protected business area. The national PTTs were once the only national telecommunications operators in most countries.
They had control over standardization in international standardization bodies and a monopoly in providing telecommunications services in their home country. For political reasons domestic manufacturers were preferred as suppliers of the systems needed in the network. Competition was not allowed, and the development of services and networks was slow in many countries. During the latter part of the s the deregulation of the telecommunications business started in Europe and proceeded rapidly in many other areas of the world.
Competitive telecommunications services are important for the development of an economy, and governments supported the development of free markets heavily.
In Europe the European Union has paid much attention to the deregulation of the telecommunications business. New operators have obtained licenses to provide local and long-distance telephone and data services and mobile telecommunications services.
Previously many standards, such as analog mobile telephone standards, did not even support a multioperator environment. The initial requirement of the digital mobile telecommunications system GSM in Europe was the support of multiple networks in the same geographical area. The deregulation of the telecommunications business has reduced tariffs on long-distance calls and mobile calls to a small fraction of the tariffs paid in the mids.
The reduction of fees has further increased the demand for services, which has prompted reductions in the price of terminal equipment, such as mobile telephones, and the fees for calls. These developments have demonstrated how dangerous it is for manufacturers to be too dependent on a single domestic customer. Many telecommunications manufacturers that were independent in the past do not exist as independent suppliers anymore.
This process still continues. At the same time, new small manufacturers are appearing. Their window of opportunity is to produce special equipment, in which the largest vendors are not interested, or systems for brand new rapidly growing services. Plain old telephone service POTS will still be important in the future, but mobile and data communications grow most rapidly in volume.
The two main directions of this development are in the areas of voice communications, which will become mobile, and data communications, which will become wideband, high-data-rate communications. Because of deregulation, subscribers can choose which network operator they want to use to get wideband access to the Internet over ordinary telephone lines. Cable TV operators are also providing similar services in competitive terms. The provision of developing multimedia services in the future will be especially interesting.
The expansion of the Internet, with its improving capability to transmit voice in addition to data, presents a new challenge to the public telecommunications network operators. Wideband access to homes will be used for telephone calls in addition to Internet surfing. This requires telecommunications network operators, including cellular network operators, to change their strategies from telephone and data transmission to complete service and information content provision. These services will contain Internet portals and location-based services, such as information on the nearest fast-food restaurant, in cellular networks.
For the future development of the telecommunications business, we must pay attention to customer services that technology can provide, not technology itself. Many good technologies, which we explain in later chapters, have not been successful because ordinary subscribers have not viewed them as attractive.
On the other hand, some services, such as the WWW, have grown very rapidly. We have to keep in mind that only attractive services make new technologies successful.
Problem 1. We will see that the telecommunications network consists of many different networks providing different services, such as data, fixed, or cellular telephony service. These different Upstream And Downstream Problems Formula In Excel networks are discussed in later chapters. In the following section we introduce the basic functions that are needed in all networks no matter what services they provide.
Problems and Review QuestionsThe three technologies needed for communication through the network are 1 transmission, 2 , switching, and 3 signaling. Each of these technologies requires specialists for their engineering, operation, and maintenance. TransmissionTransmission is the process of transporting information between end points of a system or a network. Transmission systems use four basic media for information transfer from one point to another Copper cables, such as those used in LANs and telephone subscriber lines; 2.
Optical fiber cables, such as high-data-rate transmission in telecommunications networks; 3. Radio waves, such as cellular telephones and satellite transmission; 4. Free-space optics, such as infrared remote controllers. In a telecommunications network, the transmission systems interconnect exchanges and, taken together, these transmission systems are called the transmission or transport network.
Note that the number of speech channels which is one measure of transmission capacity needed between exchanges is much smaller than the number of subscribers because only a small fraction of them have calls connected at the same time. We discuss transmission in more detail in Chapter 4. SwitchingIn principle, all telephones could still be connected to each other by cables as they were in the very beginning of the history of telephony. However, as the number of telephones grew, operators soon noticed that it was necessary to switch signals from one wire to another.
Then only a few cable connections were needed between exchanges because the number of simultaneously ongoing calls is much smaller than the number of telephones Figure 2. The first switches were not automatic so switching was done manually using a switchboard. Strowger developed the first automatic switch exchange in At that time, switching had to be controlled by the telephone user with the help of pulses generated by a dial.
For many decades exchanges were a complex series of electromechanical selectors, but during the last few decades they have developed into software-controlled digital exchanges. Modern exchanges usually have quite a large capacity-tens of thousands subscribers-and thousands of them may have calls ongoing at the same time. SignalingSignaling is the mechanism that allows network entities customer premises or network switches to establish, maintain, and terminate sessions in a network.
Signaling is carried out with the help of specific signals or messages that indicate to the other end what is requested of it by this connection. Signaling is naturally needed between exchanges as well because most calls have to be connected via more than just one exchange. Many different signaling systems are used for the interconnection of different exchanges.
Signaling is an extremely complex matter in a telecommunications network. In approximately 10 seconds he is able to receive calls directed to him. Information transferred for this function is carried in hundreds of signaling messages between exchanges in international and national networks. Signaling in a subscriber loop is discussed in Section 2. Operation of a Conventional TelephoneThe ordinary home telephone receives the electrical power that it needs for operation from the local exchange via two copper wires.
This subscriber line, which carries speech signals as well, is a twisted pair called a local loop. The principle of the power supply coming from the exchange site makes basic telephone service independent of the local electric power network. Local exchanges have a large-capacity battery that keeps the exchange and subscriber sets operational for a few hours if the supply of electricity is cut off.
This is essential because the operation of the telephone network is especially important in emergency situations when the electric power supply may be down. Figure 2. Elements of the figure and operation of the subscriber loop are explained later in this chapter.
Originally telephone microphones were so-called carbon microphones that had diaphragms with small containers of carbon grains and they operated as variable resistors supplied with battery voltage from an exchange site see the subscriber loop on the left-hand side of Figure 2. When sound waves pressed the carbon grains more tightly, loop resistance decreased and current slightly increased. The variable air pressure generated a variable, alternating current to the subscriber loop.
This variable current contained voice information. The basic operating principle of the subscriber loop is still the same today, although modern telephones include more sophisticated and better quality microphones. EarphoneAlternating current, generated by the microphone, is converted back into voice at the other end of the connection. The earphone has a diaphragm with a piece of magnet inside a coil.
The coil is supplied by alternating current produced by the microphone at the remote end of the connection. The current generates a variable magnetic field that moves the diaphragm that produces sound waves close to the original sound at the transmitting end see the subscriber loop on the right-hand side of Figure 2.
Signaling FunctionsThe microphone generates the electrical current that carries voice information, and the earphone produces the voice at the receiving end of the speech circuit. The telephone network provides a dialed-up or circuit-switched service that enables the subscriber to initiate and terminate calls. The subscriber dials the number to which she wants to be connected. This requires additional information transfer over the subscriber loop and from the exchange to other exchanges on the connection, and this transfer of additional information is called signaling.
The basic subscriber signaling phases are described in the following section. Signaling to the Exchange from the TelephoneTelephone exchanges supply dc voltage to subscriber loops, and telephone sets use this supplied voltage for operation. Modern electronic telephones would not necessarily need this if they could take their power from a power socket at home.
However, getting the power supply from the exchange is still an important feature because it ensures that the telephone network operates even in emergency situations when the power network may be down. Setup and Release of a CallEach telephone has a switch that indicates an on-or off-hook condition. When the hook is raised, the switch is closed and an approximately 50 mA of current starts flowing. This is detected by a relay giving information to the control unit in the exchange Figure 2.
The control unit is an efficient and reliable computer or a set of computers in the telephone exchange. It activates signaling circuits, which then receive dialed digits from subscriber A. We call a subscriber who initiates a call subscriber A and a subscriber who receives a call subscriber B. The control unit in the telephone exchange controls the switching matrix that connects the speech circuit through to the called subscriber B.
Connection is made according to the numbers dialed by subscriber A. When the call is being routed to subscriber B, the telephone exchange supplies to the subscriber loop a ringing voltage and the bell of subscriber B's telephone starts ringing. The ringing voltage is often about 70V ac with a Hz frequency, which is high enough to activate the bell on any telephone.
The ringing voltage is switched off immediately when an off-hook condition is detected on the loop of subscriber B, and then an end-to-end speech circuit is connected and the conversation may start.
When the exchange detects the off-hook condition of a subscriber loop, it informs us with a dial tone that we hear when we raise the hook that it is ready to receive digits. After dialing it keeps us informed about whether the circuit establishment is successful by sending us a ringing tone when the telephone at the other end rings.
When subscriber B answers, the exchange switches off both the ringing signal and the ringing tone and connects the circuit. At the end of the conversation, an on-hook condition is detected by the exchange and the speech circuit is released. In next sections we explain in more detail one of the subscriber signaling phases, the transmission of dialed digits from a subscriber's telephone to the local exchange.
Rotary DialingThe telephone set has a switch that is open in the on-hook condition and closed when the hook is off. This indicates to the telephone exchange when a call is to be initiated and when it has to prepare to receive dialed digits. We call this principle rotary or pulse dialing. In rotary dialing a local loop is closed and opened according to the dialed digits, and the number of current pulses is detected by the exchange.
This signaling method is also known as loop disconnect signaling. The main disadvantages of this method are that it is slow and expensive due to highresolution mechanics and it does not support supplementary services such as call forwarding.
The local-loop interfaces in telephone exchanges have to support this old technology though it has been gradually replaced by tone dialing. When a digit is to be dialed, the dialing plate with finger holes is rotated clockwise to the end and released.
While homing, the switch is breaking the line current periodically and the number of these periods indicates the dialed digit. For example, digit 1 has one period, 2 has two periods, and 0 has 10 periods or cycles. Mechanics make the homing speed approximately constant and each period is about ms long with a ms break Figure 2. This method for the transmission of digits has also been used for signaling between exchanges and then it is known as loop disconnect signaling.
The value of the loop current differs slightly from country to country and it is also dependent on line length and supply voltage, for example. Typically it is from 20 to 50 mA, high enough to control old generation electromechanical switches that used pulses to control directly the rotating switches of the switching matrix of an exchange.
Tone DialingCurrently telephones include electronic circuits that make possible the implementation of better means for signaling. Digital exchanges do not require high-power pulses to drive the selectors as old electromechanical switches did.
However, subscriber lines are still, and will be, supplied by a or V battery so that telephones continue to operate independent of the electric power supply. Modern telephones usually have 12 push buttons keys A to D of Figure 2. One of the frequencies is from the upper frequency band and the other from the lower band.
All frequencies are inside the voice frequency band , Hz and can thus be transmitted through the network from end to end, when the speech connection is established. This signaling principle is known as dual-tone multifrequency DTMF signaling. Tones are detected at the subscriber interface of the telephone exchange and, if necessary, signaled further to the other exchanges through which the connection is to be established.
All digital local exchanges have a capability to use either pulse or tone dialing on a subscriber loop. The subscriber is able to select with a switch on his telephone which type of dialing is to be used.
Tone dialing should always be selected if the local exchange is a modern digital one. Subscriber loops are and will remain two-wire circuits, because they are one of the biggest investments of the fixed telephone network. Early telephone connections through the network were two-wire circuits. Longer connections attenuate the speech signal and amplifiers are needed on the line.
In two-wire circuits, amplification of a signal may cause oscillation or ringing if the output signal of an amplifier loops back to the input circuit of another transmission direction Figure 2. The operating principle of electronics in the network is unidirectional and inside the network we use two wires for each direction, or four-wire 4W connections. Four-wire connections are also much easier to maintain than 2W connections because transmission directions are independent from each other and potential oscillation, as shown in Figure 2.
A transformer consists of coils of wires wrapped around an iron object. When an alternating current flows through one coil, it produces a magnetic field in the iron core. This magnetic field generates current to the wires of other coils around the same iron core. Two separate transformers are needed in the hybrid and both of them consist of three similar, tightly coupled windings. In each transformer an alternating current in one coil generates alternating current to all other coils of the same transformer.
Spots of coils indicate the direction of the current flow polarity of the coil. In Figure 2. These currents have opposite directions in transformer T1; they, or actually their magnetic fields in the iron core, cancel each other, and the signal from the receive pair is not connected to the transmit pair, or at least it is much attenuated.
In practice, the balance is not ideal and attenuated signal is connected back, which is heard as an echo from the far end of the telephone circuit if two-way propagation delay of the circuit is long enough.
Dashed lines in Figure 2. Satellite connections have long propagation delays because of the long propagation distances. The round-trip delays of these connections are longer than 50 to ms, causing a disturbing echo. Hence, in the case of these connections, we have to use special equipment known as echo cancellers in the network to eliminate the echo. There the transmission directions are separated with the help of digital signal processing technology.
Many applications use the transformer circuit described earlier together with digital signal processing technology to improve performance. The reader can imagine what happens when the microphone generates an alternating current in the telephone set of the figure.
Telephone NumberingAn international telephone connection from any telephone to any other telephone is made possible by unique identification of each subscriber socket in the world. In mobile telephone networks, each telephone set or subscriber card has a unique identification number. The numbering is hierarchical, and it has an internationally standardized country code at the highest level. This makes national numbering schemes independent from each other.
In the following sections, we explain the fields of the telephone number shown in Figure 2. International PrefixAn international prefix or international access number is used for international calls. It tells the network that the connection is to be routed via an international telephone exchange to another country. The international prefix may differ from country to country, but it is gradually becoming harmonized.
For example, all of Europe uses 00; elsewhere it may be different. If many operators are providing international telephone service, a subscriber may select from among different operators by using an operator prefix instead of 00, for example, in Finland a user would dial for Oy Finnet International. Country CodeThe country code contains one to four numbers that define the country of subscriber B. Country codes are not needed for national calls because their purpose is to make the subscriber identification unique in the world.
A telephone number that includes the country code is called an international number and it has a maximum length of 12 digits. Because there are a few hundred countries in the world, many country codes have been defined by the ITU and the length of them varies from a single digit to four digits some small areas have an even longer code Germany, 44 for the United Kingdom, 52 for Mexico, for Finland, and for Jamaica.
Trunk Code, Trunk Prefix, or Area CodeThe trunk code defines the area inside the country where the call is to be routed. The first digit is a long-distance call identification and other numbers identify the area. The first digit is not needed in the case of an international call because that type of call is always routed via the long-distance level of the destination network. In the case of cellular service, the trunk code is used to identify the home network of the subscriber instead of the location.
With the help of this network code, a call is routed to the home network, which then determines the location of the subscriber and routes the call to the destination.
The trunk code and the subscriber number together create a unique identification for a subscriber at the national level. This is called a national number and its maximum length is 10 digits.
Trunk codes start with a 0 in Europe, but the 0 is not used in calls coming from abroad. In countries where multiple operators provide long-distance telephone service, the subscriber may select an operator by dialing an operator prefix in front of the trunk code.
In Finland, two examples of the longdistance operator numbers are for Finnet and for Song Networks. Subscriber NumberThe subscriber number in a fixed telephone network is a unique identification of the subscriber inside a geographical area.
To connect to a certain subscriber, the same number is dialed anywhere in the area. Because of the numbering hierarchy, the subscriber part of the telephone number of one subscriber may be the same as that of another subscriber in another area. If provision of local telephone service is deregulated as is the goal in Europe , a subscriber is able to choose a network operator for local calls by dialing a local operator prefix in front of the subscriber number.
Operator NumbersAs the telecommunications business is deregulated, new service providers are beginning to enter on the market. Then in addition to the numbers just described, a subscriber will need to dial additional digits to select a service provider network operator.
As explained earlier, a subscriber may choose a service provider for local calls, long-distance calls, and international calls. The national telecommunications authority defines the operator numbers used. The national telecommunications authority also defines how calls dialed without an operator number are charged. If the subscriber does not specify the international and long-distance network operators by operator prefix, the network is chosen randomly or according to other rules specified by the national telecommunications authority.
The creation of real competition in fixed telecommunications service provision has been successful in many countries. One problem with this situation is that additional dialing of operator prefixes at all levels is required, and another is that the fees for fixed telephone service are too low to make subscribers interested in taking the time to choose a service provider.
For business users, for which monitoring the costs of telecommunications is essential, competition will certainly reduce those costs. To avoid the problem of additional dialing, a business or residential subscriber may make a service agreement with one of the network operators for local, longdistance, and international calls. Switching and SignalingTo build the requested connection from one subscriber to another, the network has switching equipment that selects the required connection.
These switching systems are called exchanges. The subscriber identifies the required connection with signaling information dialing that is transmitted over the subscriber line. In the network, signaling is needed to transmit the control information of a specific call and circuits from one exchange to another.
Telephone ExchangeThe main task of the telephone or ISDN exchange is to build up a physical connection between subscriber A, the one who initiates the call, and subscriber B according to signaling information dialed by subscriber A. The speech channel is connected from the time when the circuit was established to the time when the call is cleared. This principle is called the circuit switching concept and is different from packet switching, which has been used in data networks.
In the past, the switching matrix was electromechanical and controlled directly by pulses from a telephone.
Later, the control functions were integrated into a common control unit. Currently, the common control unit is an efficient and reliable computer or a multiprocessor system, including large amounts of real-time software. This kind of exchange is called a stored program control SPC exchange Figure 2.
The Telecommunications Network: An OverviewEvery exchange between subscribers A and B connects a speech circuit according to signaling information that is received from a subscriber or from the previous exchange.
If the exchange is not the local exchange of subscriber B, it transmits signaling information to the next exchange that connects the circuit further. SignalingThe control unit of the local exchange receives the subscriber signaling, such as dialed digits, from the subscriber line and makes consequent actions according to its program.
Usually the call is routed via many exchanges and the signaling information needs to be transmitted from one exchange to another. CASWhen a call is connected from a local exchange to the next exchange, a speech channel is reserved between exchanges for this call.
At the same time another channel is reserved only for signaling purposes and each speech path has its own dedicated signaling channel while the call is connected.
This channel can be, for example, a signaling channel in time slot 16 of the primary PCM frame as explained later in Chapter 4. The main phases of signaling between exchanges are shown in Figure 2. First the speech channel and the related signaling channel are seized from exchange A to exchange B. Then the telephone number of subscriber B is transmitted to exchange B, which activates the ringing signal.
When subscriber B answers, the speech connection is switched on and the conversation may start. Exchange A responds with a clear-forward CLF signal when subscriber A hangs up or when the time constant expires. The call is then disconnected by both exchanges. Many different signaling systems are used for CAS and some of them include additional signals that are not present in Figure 2.
Signals that carry signaling information indicated in Figure 2. CAS is still used in telephone networks, but it is gradually being replaced with a more efficient standardized method known as CCS.
It is based on the principles of computer communications in which data frames containing information are exchanged between computers only when required. Signaling frames contain, for example, information about the connection to which the message belongs, the address of the destination exchange, dialed digits, and information about whether subscriber B has answered.
In most cases only one data channel between two exchanges is required to serve all established calls. This is usually one Kbps time slot of a primary 2-or 1. Establishing a call requires the same signaling information as indicated in Figure 2.
The dialed digits are transmitted from subscriber A to the local exchange, as explained in Section 2. When a set of digits is received by exchange A, it analyzes the dialed digits to determine to which direction it should route the call. From this information it looks up an address of the exchange to which it should send the signaling message for call connection. Then the exchange builds a data packet that contains the address of exchange B.
This signaling message, called the initial address message IAM , is then sent to exchange B. When all the digits that identify subscriber B are received by exchange B, it acknowledges this with an address complete message ACM , to confirm that all digits have been successfully received. This message also contains information about whether the call is to be charged or not and if the subscriber is free or not. Exchange B transmits the ringing tone to subscriber A and the ringing signal to subscriber B, and telephone B rings.
When subscriber B lifts the handset, an answer signal charge ANC is sent in order to activate charging. Exchange B switches off the ringing signal and ringing tone. Then both exchanges connect the speech channel through so the conversation can start. Exchange A responses with CLF signal. All exchanges on the line transmit the CLF message to the next one, and each receiving exchange acknowledges it with a release guard RLG signal.
The RLG message indicates to the receiving exchange Upstream And Downstream Problems Formula Video that the connection has been cleared and the channel released by the other exchange. It also ensures that both exchanges have cleared the circuit to make it available for a new call. Switching HierarchyDuring the early years of the telephone, the switching office or exchange was located at a central point in a service area and it provided switched connections for all subscribers in that area. Hence, switching offices are still often referred to as central offices.
As telephone density grew and subscribers desired longer distance connections, it became necessary to interconnect the individual service areas with trunks between the central offices. With further traffic growth, new switches were needed to interconnect central offices and a second level of switching, trunk or transit exchanges, evolved.
Currently national networks have several switching levels. The actual implementation of the hierarchy and the number and names of switching levels differ from country to country. The hierarchical structure of the network helps operators manage the network and it makes the basic principle of telephone call routing straightforward; the call is routed up in the hierarchy by each exchange if the destination subscriber is not located below this exchange.
The structure of the telephone number, explained in Section 2. Telephone Call RoutingCalls that are carried by the network are routed according to a plan, a set of rules. The routing plan includes the numbering plan and network configuration. Numbering PlanThe global rules for the highest-level numbering, country codes, and overall numbering maximum length and so on are given by ITU-T.
The national telecommunications authority coordinates the national numbering plan. It defines, for example, trunk or area codes and operator prefixes used inside the country.
It also defines nationwide service numbers e. These service numbers are defined to be the same wherever the call is originated and they require additional intelligence from switching systems. Their routing principle is explained later in Section 2. At the regional level, the numbering plan includes digits allocated to certain switching offices, exchanges, and the subscriber numbers for subscribers connected to a certain switch. From the received signaling information dialed digits , a switching system must be able to interpret the address information, determine the route to or toward the destination, and manipulate the codes in order to advance the call properly.
This includes the deletion of certain digits and automatic alternate routing. Number conversion may also be needed when, for example, the emergency call dialed with a nationwide short emergency number has to be routed to a regional center that has a different physical telephone number.
Some of this intelligence for routing may be stored in a centralized control system from which the exchanges request routing information. This modern network structure is called an intelligent network IN and is described in Section 2. Route Selection GuidelinesThe basic routing principle is hierarchical: If the destination does not belong to the subscribers of the switch or of the switches under it, the call is routed upward; otherwise, it is routed to the port toward that destination Figure 2.
In the example of Figure 2. When exchanges in the The number is analyzed by each exchange on the way and routed further. Regional center routes calls to subscriber numbers 2xxxx and 1xxxx to the left-hand and 4xxxx to the right-hand sides of the branch. A call is routed to Helsinki area 0 9 to subscriber The international exchange then analyzes the country code and selects an outgoing route to Sweden. Another example in Figure 2.
A subscriber in another region dialed "09 " for a long-distance call to Helsinki. The first digit "0" tells the exchanges that this is a long-distance call and is to be routed to the regional exchange. The regional center is connected to other regional centers and then routes this call, with the help of other regional centers, to Helsinki according to the next digit, "9.
Operator has defined in his numbering plan that the subscriber numbers 2xxxx and 1xxxx are placed on the left-hand branch from the regional center. The primary center then checks the following numbers, "," and notices that this is not "my subscriber" but the destination subscriber is located "below me" and routes the call to the corresponding lower-level exchange, in this example, the local exchange.
The local exchange selects the subscriber loop of the telephone number and connects a ringing signal to the subscriber. However, modern exchanges can do more than the simple strictly hierarchical routing just introduced. If there is a sufficient volume of traffic, calls may pass by a hierarchy level or may be connected directly to another lowlevel switch, as illustrated in Figure 2. This may be reasonable, for example, if the local exchanges of subscribers A and B are on the opposite sides of the regional border.
The telecommunications operator is free to define the detailed actual routing to optimize the usage of the network. Our cheap essay writing service employs only writers who have outstanding writing skills. The quality of all custom papers written by our team is important to us; that is why we are so attentive to the application process and employ only those writers who can produce great essays and other kinds of written assignments.
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