Telecom Terminology

1) The telephone – How does it work?

The microphone, located in the lower part of the telephone unit, changes the sound of our words to an electric signal. This signal is amplified and sent through the copper wire to the other person at the end of the line. There the signal is amplified again and changed back by the upper part of the telephone from an electrical signal to a sound signal that we can hear and understand.

2) Analogue and Digital Telephones.

An analogue telephone changes the sound of our voice to an analogue signal. An analogue signal is continuous - the louder the sound, the bigger the signal.

A digital telephone changes the sound of our voice to a digital signal. This process is called “Digitalization”. The sound is measured 8000 times per second and each measurement gets a certain value in binary code. This code consists only of “1”’s and “0”’s so the value of each measurement is given in a string of “1”’s and “0”’s. All the measurements are transferred by the copper line to the telephone at the other end of the line. There all the “1”’s and “0”’s will be changed back again to the original sound.

3) Cellular (mobile) telephone.

The name “cellular” comes from the word “cell”. The mobile network is composed of many cells. Each cell covers a surface of several square kilometers, depending on the topography and the amount of mobile telephones in the area. The base station is situated in the centre of the cell and is connected to the main switch of the provider and also to the neighboring base stations. Traffic from all mobile telephones in the area of the cell is transmitted first to the base station, from the base station to the main switch and then to the requested destination.

When the user of a mobile telephone is on the move, he travels from one cell to another. If he is in a car he might move through 10 or more different cells. The base station of each cell detects the place of each mobile phone and also the direction that he is traveling in, so that when a user changes his location and moves to another cell his call is transferred to the next station. This transfer is so swift that the call is not interrupted and the user does not notice any change.

Each base station uses a different frequency for its communication with the mobiles in its area. It is a low power (1-2 watts) frequency and therefore the range is limited to several kilometers, so the same frequency can be used also for cells further away without interrupting and without mixing with each other. This is the reason why a mobile provider can cover an unlimited area using only 4 – 6 different frequencies. This is important because the number of allocated frequencies is limited.

4) DECT – Digital Enhanced Cordless Telecommunication.

DECT is a digital portable telephone. It can be compared to a mobile telephone because it also uses a cell construction. The difference is that the radius of the DECT cell is 25-100 meters while the radius of a mobile cell is 2 – 10 Kilometers. Also the base station of the DECT is of course very simple because it handles only 1 – 5 units while the base station of a mobile phone handles tens of thousands of units.

5) ADSL – Assymetric Digital Subscriber Line.

ADSL is a version of DSL – a technology for data communication with high transmission capacity. Higher than the capacity used in a voice modem. “Assymetric” means different capacity in both directions. The download capacity of an ADSL connection is always higher that its upload capacity. This feature makes ADSL ideal for Internet users who often need to download big files but upload only small files.

Internet providers offer ADSL. For example 2000 MB download and 500 MB upload. The actual capacity depends on the distance from the nearest telecom local switch, the quality of the line and the number of users.

How does ADSL work?

When installing an ADSL line a splitter is added to divide the available frequency in two sections:

25 – 138 KHz for upload traffic

138 – 1104 KHz for download traffic

Each section is divided again to small frequency channels of 4 KHz. Each of these small channels of 4 KHz is tested by the modem for acceptable voice quality. If some channels cannot deliver a good voice quality they are not used and the ADSL capacity is decreased accordingly. This process of testing channel reduces capacity and speed but ensures a good quality.

6) ISDN – Integrated Services Digital Network.

ISDN is a set of telecom protocols necessary to execute a telephone conversation. The transmission of the call is digital and therefore theoretically quicker than analogue transmission. However in practice there is no difference between analogue and digital transmission. ISDN also offers 2 separate channels on each wire and additional services: Conference calls, call through etc. However these additional services are now also available with analogue lines.

ISDN was, for a short time, the successor of the old analogue line but it is now being replaced by ADSL which has much more capacity then ISDN.

7) VoIP – Voice Over Internet Protocol.

VoIP uses a broadband connection (like ADSL) in order to execute a telephone call. With standard, analogue telephony there is a physical connection between the two telephones. For each call a connection is made between one telephone through wires and switches to another telephone. With VoIP, there is no physical connection. The voice of the speaker is digitalized and transferred through the Internet in small “packages”. There are many packages and each package is sent to the receiver through the quickest route that is available at that moment.On the receiver side all the small “packages” are collected, arranged in the correct order and then transformed again to an analogue signal that can be heard by the human ear. Sometimes when it is too busy on the Internet, some packages get lost and then the receiver misses a part of a word so the quality is not perfect. However, with the advance of technology VoIP is improving.