Telecommunication

 Since that historical moment when Alexander G. Bell made the very first phone call, the telephone has developed on a large scale. Today the normal telephone that we use in our homes is facing extinction. The only reason for some of us to use a telephone is for connecting to the Internet. Even things like sending faxes and are being taken over by cell phones and you can connect to the Internet with a cell phone these days. You may disagree, but the signs are there. Maybe there won't be a single telephone connected to a wire in any house anywhere within the next few years.
 
 

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THE TELEPHONE

Today electret microphones are being used. The material in an electret can, once energised, provide a permanent electrical field in space. The movement of an electrode in an electrostatic field causes a change in voltage between the moving electrode and the stationary electrode behind the electret. Electret microphones rely on transistors for amplification. They are lightweight, small and fairly cheap. The receiver is still made from a permanent magnet wound with wire. The diaphragm, however, is now made from aluminium attached to a piece of iron. The design has been improved but it is still the same efficient device.

The original bell as alerter has now been replaced by digital alerters. Finding a suitable replacement was a surprisingly difficult task. It had to be an attention getting, yet pleasing sound and not too expensive. There are still people who prefer the sound of a bell but, in order to be effective, a bell has to be a certain size. This is why electronic alerters are now used in smaller telephones. In future the actuation of the alerter can change with the replacement of the bell. Currently actuation is done by applying 90 volts 20 Hz ac to the line. A lower voltage is more compatible with transistorised phones.

There are two forms of dialling in the telephone system: dial pulse and multifrequency tone ("Touch Tone"). The old rotary dial was a very clever mechanical invention and worked in a very complex way. This dial, however, isn't used that much anymore because of high repair costs and the rotary dial process is slow. The availability, inexpensiveness and reliable amplification made possible by the transistor opened the gate for the design of a dialling system that transmits low power tones instead of the high power dial pulses used by the rotary dialling system.

The newer, and probably the only way we use today, is the multifrequency dial. Each button can send two tones. A "2 out of 7" coding scheme is used. One tone corresponds with the row and the second tone with the column of a 12-button array. In other words, 4 rows and 3 columns are 7 tones.

The Touch-Tone was introduced as an optional premium cost service. For this reason the exchange has to have the ability to receive both pulses and multitone dialling. If you buy a telephone, you may have a line on which multifrequency signals cannot be accepted by the telephone company. For this reason, push button telephones have a switch with which you can determine if your phone should send pulses or tones.

An invisible, yet very important part of the telephone, is the antisidetone network. We humans continuously monitor our voices while speaking and change our volume and intonation accordingly. This is called sidetone. The transmitter and receiver of each set, as well as the line, were all connected to each other in early phones. Because of this the user could hear his own voice very loudly. This happens because the carbon microphone amplifies the sound at the same time that it turns the sound from acoustic to electric. Not only was this unpleasant, but it also caused the user to speak softer, which in turn made it more difficult for the listener to hear.

Along with other components, the original antisidetone network also contained a transformer. The antisidetone network basically had the ability to transfer the energy from the transmitter to the line without letting any of this energy through to the receiver. Now you can no longer shout in your own ear. Yet there is a small amount of speech energy allowed into the receiver. This prevents the connection of sounding unpleasantly "dead". New telephone designs use transistors (lighter, smaller, cheaper) in integrated circuits to replace the transformer. The other parts on the circuits function as a volume control to compensate for various lengths of wire between customers and the exchange. If this part weren't there, customers very far from the exchange would receive too little volume while someone close to the exchange would receive very loud volumes.

Now that we know how the telephone itself works, let's look at how you are connected to the person you call. When lifting the handset from its base an electrical switch called the switchhook is closed. This causes the flow of an electric current over the caller's line (called a loop between the caller's location and the building with the automatic exchange - part of the switching system). The current is a dc current which doesn't change direction but its intensity and amplitude can vary. The current is detected by the exchange which sends back a dialling tone. The dialling tone is a precise combination of two notes so that it is absolutely unmistakable by people and machines. The caller now enters a sequence of numbers unique to one other telephone. When the first digit is entered, the exchange equipment removes the dialling tone. After the last digit the exchange determines if the called person is in the same exchange or in a different exchange. Bursts of ringing current (20 Hz ac) is applied to the called party's line if it is in the same exchange. Both of the subscribers' telephones have ringers that respond to the ringing current, hopefully making a sound that can draw lots of attention. When the called party picks up the handset, a dc current, detected by the exchange, flows over the called party's line. The ringing is stopped and a connection that can be used for talking is established between the two parties.

A connection between the exchange and the called person's exchange is made through the network when the called party is in another exchange. Part of this process is that the caller's exchange has to tell the other exchange who the called party is. The called exchange then handles the ringing, answering and telling the calling exchange and billing machinery when the call is finished. Actually, in telephone terminology, a call is finished when the called party lifts the handset and not when the conversation is over. When one of the parties hang up the switch hook is opened which stops the flow of dc current over the line. The exchange now initiates the process of taking down the connection and again notifying the billing equipment. In your local area you can either be charged with flat rate or message rate. With flat rate service you are allowed an unlimited number of calls within a fixed fee. With message rate service you are charged according to the duration of the call and the distance between you and the person you call. Long distance calls are always charged at message rate.

In early phones as well as at the local circuit the current was generated by a battery. The local circuit had a battery, a transmitter connected to one winding of a transformer. The other winding was connected to the line. Operators at the local circuit had to make the connections manually with switchboards. As telephone systems grew, manual switching became too slow and took a lot of labour. In order to do switching automatically a series of devices had to be developed. Now an electronic device sends a number of successive pulses of current or a sequence of audible tones corresponding to the number dialled which can be interpreted automatically by electronic equipment at the central switching station. The call is then routed to the called party.

Solid-state technology enables these central exchanges to process calls at a millionth of a second. This means that large numbers of calls can be handled at the same time. The caller's voice is converted into digital signal pulses. These pulses are sent through the network by high-capacity systems that exchange calls by using computerised mathematical switching operations. Operating instructions for the system are stored in computer memory. When a defect comes into the system, a backup unit immediately starts handling calls. Computer techniques can be used to handle telephone calls, data messages and visual signals, enabling the system to make speed calls, by quickly determining the most efficient route.

In Britain, the United States and many other countries, there are no more manual switchboards and all subscribers are served by automatic exchanges. A line current relay in a line circuit replaces the switchboard light. The cords are replaced by a crosspoint switch and the key is replaced by other relays. The dial is used to indicate the number being dialled. Incoming registers store the number and then pass it to the central computer which operates the crosspoint switch array to complete the call or send it to a higher switch for further processing.

At first carrying the telephone calls over long distances was a problem but since the first telephone cables, there have been major developments and every few years replacements are made. For making overseas calls, the first submarine telephone cables were laid in1956. Carrier-current telephony was the first improvement on the usage of normal telephone cables. By using frequencies above the voice range, ranging from 4000 Hz to several million hertz, as many as 13 200 phone calls can be carried over one medium. This technique is also being used to send telephone messages over normal distribution lines without having a bad effect on the regular service. Because systems are growing bigger and more complex, solid-state amplifiers, called repeaters are used to amplify signals at regular intervals. In 1936 the coaxial cable was developed. The modern coaxial cable has copper tubes with each a copper wire exactly in the middle. They have the same centre, or in other words, they are coaxial. Cables with 22 tubes arranged in tight rings and covered with lead and polyethylene can carry 132 000 messages at the same time.

Replacing the coaxial cable is optical glass fibre. Messages are coded into pulses of light and sent over great distances with these slender fibres. Fibre cables carry up to 50 fibre pairs, each with 4000 voice circuits. Carrying of much larger volumes of information is now possible with laser technology. This is possible because the laser exploits the visible part of the electromagnetic spectrum where the frequency is much higher than that of radio. For most transmissions, the LED or light-emitting diode is appropriate. The TAT 8 fibre-optic cable can carry more than double the transatlantic circuits that were available in the 1980's. It can carry 50 000 conversations simultaneously. High-speed transmission of computer data is also possible through these cables. In 1992 the TAT 9 was introduced and can carry over 75 000 conversations at the same time.

Another way of transmission is by using microwave relay. Microwaves are relayed between stations. Because there has to be clear sight between sending and receiving stations, stations are usually placed 40 km from each other. Microwave relay channels can carry about 600 conversations at once.

The first global telephone relay network was completed in 1969 with a series of satellites in stationary orbits (35 880 km above the earth) which are powered by solar energy cells. They amplify and retransmit calls sent from an earth antenna and send them to a distant ground station. Using satellite and terrestrial facilities together, enables us to send calls between continents just as well as in a domestic area. Because transmission has been digitised, satellites can relay up to 33 000 conversations at once including several television channels. Two satellites have to be used for a call between New York and Hong Kong for example. Yet this is far cheaper to install and maintain per channel than using submarine coaxial cables. This is why we use satellite connections as far as possible for long distances.

  One major shortcoming that satellites have is the distance between and the limited speed of radio waves causes major delays in conversations. That is why sometimes a call would be sent by satellite only in one direction and use a ground transmission in the other direction. After you have finished speaking the other party usually takes more than a second to respond, a very annoying thing after a while. Now a combination of coaxial cable, optical-fibre, microwave and satellite paths link major cities in the world. The capacity of each city depends on its age and territory covered. Generally they go from coaxial cable to microwave to satellite to optical-fibre. Constant engineering improvement has improved the capacity of each system since they were introduced.