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The common telephone is a device connected to the outside world by a pair of wires. It consists of a handset and its cradle with a signalling device, consisting of either a dial or push buttons. The handset is made up of two electro acoustic transducers, the earpiece or receiver and the mouthpiece or transmitter. There is also a side tone circuit that allows some of the transmitted energy to be fed back to the receiver.
The transmitter or mouthpiece converts acoustic energy into electric energy by means of a carbon granule transmitter. The transmitter requires a direct-current potential. We call this the talk battery, and in modern telephone systems it is supplied over the line (central battery) from the switching center and has been standardized at 48 V dc. Current from the battery flows through the carbon granules or grains when the telephone is lifted from its cradle or goes “off hook”.
The typical receiver consist of a diaphragm of magnetic material, often soft iron alloy, placed in a steady magnetic field supplied by a permanent magnet, and a varying magnetic field caused by voice currents flowing through the voice coils. Such voice currents are alternating (ac) in nature and originate at the far end telephone transmitter. The telephone receiver, as a converter of electrical energy to acoustic energy, has a comparatively low efficiency on the order of 2-3%.
Sidetone is the sound of the talker’s voice heard in his (or her) own receiver. Sidetone level must be controlled. When the level is high, the natural human reaction is for the talker to lower his or her voice. Thus by regulating sidetone, talker levels can be regulated. If too much sidetone is fed back to the receiver, the output level of the transmitter is reduced as a result of the talker lowering his or her voice, thereby reducing the level (voice volume) at the distant receiver and deteriorating performance.
The public switched telecommunication network (PSTN) is immense. It consists of hundreds of smaller networks interconnected. There are “fixed” and “mobile” counterparts. They may or may not have common ownership. In certain areas of the world the wired and wireless portions of the network compete. One may also serve as a backup for the other upon failure. It is estimated that by 2009 there will be as many wireless telephones as wired telephones, about 5 x 10 handsets worldwide of each variety.
These networks, whether mobile or fixed, have traditionally been based on speech operations. Meanwhile, another network type has lately gained great importance in the scheme of things. This is the enterprise network. Such a network supports the business enterprise. It can just as well support the government “enterprise” as a private business. Its most common configuration is local area network (LAN) and is optimized for data communications. The enterprise network also has a long-distance counterpart. Since its inception around 1987, it has taken on a very large life of its own, having been opened to the public worldwide. It is the internet. Its appeal is universal, serving its original intent as a resource-sharing medium extending way beyond the boundaries of universities and now including a universal messaging service called email (electronic mail).
The basic underpinning of the industry was telephone service. That has now changed. The greater portion of the traffic carried today is data traffic, and all traffic is in a digital format of one form or another. We include wireless/cellular and “broadband” as adjuncts of the PSTN.
Telecommunication engineering has traditionally been broken down into two basic segments: transmission and switching. This division was most apparent in conventional telephony. Transmission deals with the delivery of a quality electrical signal from point X to point Y.
Контрольна робота N 2
Ex.1. Underline the correct alternatives.
1. Of all the methods available in/for/at separating signals from noise the most widely used is that involving filtering.
2. The principle of/upon/with sharing the same line between a number of terminals is shown in Fig.6.
3. Before/by/atintroducing these new elements more needs to be said about the practical uses of flowcharts.
4. In/upon at handling a telephone call an exchange performs three essential functions.
5. The first satellites were seen as a way about/in /of communicating with people who lived in isolated areas of the word.