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.
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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.
Introduction to Telecommunication Systems
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.
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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.
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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.