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FMF measurement technology is overviewed. Projected performance requirements are leading to formidable cost and energy efficiency challenges. Hybrid and integrated photonic technologies are currently being developed to reduce assembly complexity and to reduce the numbers of individually packaged parts. This chapter provides an overview of the important challenges that photonics currently face, identifies the various optical technologies that are being considered for use at the different interconnection levels, and presents examples of demonstrated state-of-the-art optical interconnection systems.

Finally, the prospects and potential of these technologies in the near future are discussed. The authors cover important background concepts such as SONET, coding device technology, andWOM components as well as projecting the trends in telecommunications for the 21st century.

One of the hottest subjects of today's technology Includes the most up-to-date research available in optical fiber telecommunications Projects the trends in telecommunications for the 21st century. Cox Publisher : Elsevier Inc. As wireless services and technologies evolve to multi-gigabit radio access networks, their speed is increased but the wireless coverage of a single access point is inevitably reduced dramatically.

In this chapter, we introduce the radio-over-fiber technique and its challenge of handling optical millimeter-wave generation, transmission, and converged multi-band system. By exploring real-world, system implementation and characterization, the unique features and versatile applications of radio-over-fiber technologies are investigated and reviewed to reach next-generation converged optical and wireless access networks.

The material is well-presented and designed for undergraduate and postgraduate students pursuing courses in Electrical Engineering, and Electronics and Telecommunication Engineering.

The book offers a completely accessible and in-depth knowledge of the principles and applications of optical fiber communication OFC. It deals with materials, devices, components, and systems of OFC. The coverage includes key concepts such as properties of light, evolution and elements of OFC, its benefits, along with applications in optical LAN and communication links.

The attenuation loss of different types, dispersion mechanism, photon sources LED and lasers , detectors PIN and avalanche , analog and digital transmitter and receiver systems, connectorization, OADM, and amplifiers are described.

It also contains solved numerical problems for better understanding of topics. Popular Books. Twelve Days of Christmas by Debbie Macomber. Pandemia by Alex Berenson. 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 regu- lates 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 electro- mechanical for example, connectors , environmental, and safety aspects.

The organizations shown in Figure 1. 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. Unofficial forums are more flexible and can produce necessary standards on a shorter timescale than can official organizations.

Their specifications are often used as a basis for later official standards. The Telemanagement Forum TMF is an organization of system manu- facturers that works to speed the development of network management stan- dards.

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 intellec- tual property rights IPRs.

One company involved in development of a stan- dard 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 manu- facturers may not be able to implement the standard in an economically fea- sible 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]. The national PTTs were once the only national telecommunications operators in most countries. For political reasons domestic manufacturers were preferred as suppliers of the systems needed in the network. Competi- tion 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 telecommu- nications 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 devel- opment of free markets heavily.

In Europe the European Union has paid much attention to the deregu- lation 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 ana- log mobile telephone standards, did not even support a multioperator envi- ronment.

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 ter- minal equipment, such as mobile telephones, and the fees for calls.

These developments have demonstrated how dangerous it is for manu- facturers to be too dependent on a single domestic customer. Many telecom- munications 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 inter- ested, 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 communica- tions, 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 wide- band access to the Internet over ordinary telephone lines. Cable TV opera- tors 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. This requires telecommunications network operators, including cellular net- work operators, to change their strategies from telephone and data transmis- sion 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 chap- ters, 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. Explain why you think so because this is a matter of opinion. Explain both political and business interests.

References [1] Carlson, A. Hanzo, Mobile Radio Communications, 2nd ed. The operation of a conventional telephone, which is easy to understand, is used to clarify how telephone con- nections are built up in the network. We look at subscriber signaling over the subscriber loop of the telephone network. The same kind of signaling is needed in modern telecommunications networks, such as ISDN and cellular networks.

We start with this simple service to lay a foundation for under- standing more complicated types of service in later chapters. In this chapter we divide the network into layers and briefly describe different network technologies that are needed to provide various kinds of service. Some of these, such as mobile and data networks, are discussed in more detail later in this book.

The last topic of this chapter is an introduction to the theory of traffic engineering; that is, how much capacity we should build into the network in order to provide a sufficient grade of service for the customers. These users of public networks, for example, a telephone network, are called subscribers. We will see that the telecommunications net- work consists of many different networks providing different services, such as data, fixed, or cellular telephony service.

These different networks are dis- cussed in later chapters. In the following section we introduce the basic func- tions that are needed in all networks no matter what services they provide.

The three technologies needed for communication through the net- work are 1 transmission, 2 , switching, and 3 signaling. Each of these technologies requires specialists for their engineering, operation, and maintenance. Transmission systems use four basic media for information transfer from one point to another: 1.

Copper cables, such as those used in LANs and telephone sub- scriber lines; 2. Optical fiber cables, such as high-data-rate transmission in telecom- munications 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 intercon- nect 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. 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 ongo- ing calls is much smaller than the number of telephones Figure 2. 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 subscrib- ers—and thousands of them may have calls ongoing at the same time. 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. Sig- naling 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 hun- dreds of signaling messages between exchanges in international and national networks. Signaling in a subscriber loop is discussed in Section 2.

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 tele- phone 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 impor- tant 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 con- tained 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.

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 cur- rent generates a variable magnetic field that moves the diaphragm that pro- duces sound waves close to the original sound at the transmitting end see the subscriber loop on the right-hand side of Figure 2. The telephone network provides a dialed-up or circuit-switched serv- ice that enables the subscriber to initiate and terminate calls. The subscriber dials the number to which she wants to be connected.

This requires addi- tional information transfer over the subscriber loop and from the exchange to other exchanges on the connection, and this transfer of additional informa- tion is called signaling.

The basic subscriber signaling phases are described in the following section. Modern electronic tele- phones 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.

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 acti- vates 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 con- trols 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. The ringing voltage is often about 70V ac with a Hz frequency, which is high enough to activate the bell on any telephone. 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 establish- ment 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.

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 high- resolution 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 break- ing 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 approxi- mately 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.

Typi- cally it is from 20 to 50 mA, high enough to control old generation electro- mechanical switches that used pulses to control directly the rotating switches of the switching matrix of an exchange. 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 —3, Hz and can thus be transmitted through the network from end to end, when the speech connection is estab- lished. 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 digi- tal one. These services, for example, call transfer, are not avail- able with telephones that use pulse dialing. We use tone dialing also to control value-added services. Value-added services are services that we can use via the telephone network but that are usually provided by another service provider, not the telecommunications network operator.

One example of value added services is telebanking. Tones are transmitted on the same frequency band as voice, and during a call we are able to dial digits to transmit, for example, our discount number and security codes to the telebanking machine. The worst disadvantage of a fixed subscriber telephone is still the poor man—machine interface that makes new services difficult to use.

Some tele- phones that have displays are more user friendly, but subscribers still have to memorize command sequences to use the new services offered by a modern telephone network. The local loop, which connects a tele- phone to a local exchange is a two-wire 2W circuit that carries the signals in both transmission directions Figure 2.

The Telecommunications Network: An Overview 29 loop. 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 cir- cuits. 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 trans- former 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 trans- former 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.

Main signal paths Example of currents generated by the signal from far end receive pair Figure 2. The ISDN basic rate interface uses bidirectional Kbps data trans- mission on a 2W circuit ordinary subscriber loop. 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 micro- phone generates an alternating current in the telephone set of the figure. In mobile telephone networks, each telephone set or subscriber card has a unique identification number. The numbering is hierarchical, and it has an internationally standard- ized country code at the highest level.

In the follow- ing sections, we explain the fields of the telephone number shown in Figure 2. It tells the network that the connection is to be routed via an international telephone exchange to another country. The international pre- fix may differ from country to country, but it is gradually becoming harmo- nized. 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 codes are not needed for national calls because their purpose is to make the subscriber identification unique in the world. A tele- phone 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 sin- gle digit to four digits some small areas have an even longer code.

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 com- ing 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 long- distance operator numbers are for Finnet and for Song Networks. To connect to a certain sub- scriber, 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.

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 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 competi- tion 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 telecommunica- tions 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, long- distance, and international calls.

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.

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 switch- ing 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 inte- grated 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 pro- gram control SPC exchange Figure 2. This principle is similar to computer communications where data packets are transmitted between computers.

Every 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.

Usually the call is routed via many exchanges and the signaling information needs to be transmitted from one exchange to another. 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 pri- mary PCM frame as explained later in Chapter 4.

The main phases of signal- ing 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 sig- nal 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. Signal- ing 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 estab- lished 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.

The Telecommunications Network: An Overview 37 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 sub- scriber 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 mes- sage to the next one, and each receiving exchange acknowledges it with a release guard RLG signal. The RLG message indicates to the receiving exchange that the connection has been cleared and the channel released by the other exchange.

It also ensures that both exchanges have cleared the cir- cuit to make it available for a new call. Hence, switching offices are still often referred to as central offices.

As telephone density grew and subscribers desired longer distance con- nections, 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 structure of the telephone number, explained in Section 2. The routing plan includes the numbering plan and network configuration. 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 sys- tems.

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 sub- scribers connected to a certain switch.

The Telecommunications Network: An Overview 39 2. This includes the deletion of certain digits and automatic alternate routing. Number conversion may also be needed when, for example, the emer- gency 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 struc- ture is called an intelligent network IN and is described in Section 2. In the example of Figure 2. When exchanges in the A call is routed to Helsinki area To other regions and The number is analyzed by 0 9 to subscriber The international exchange then analyzes the country code and selects an outgoing route to Sweden. Another example in Figure 2. 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 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 hier- archical 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 low- level switch, as illustrated in Figure 2.

This may be reasonable, for exam- ple, 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. In this section we have described the switching hierarchy of the tele- phone network and the telephone call routing principle through the exchanges in this hierarchy.

In modern networks the actual implementation may be different from this strictly hierarchical routing principle we described. Local telephone exchanges may analyze the whole telephone number, bypass the switching hierarchy, and route the call directly if the des- tination is a subscriber of a neighbor local exchange.

Also, some sets of the telephone numbers have no fixed connection to the physical location of a subscriber loop.