The Telephone Network

   A telephone network is made up by a number of linked elements by which we can send or receive voice or other sounds from different locations. Although the land phone was the first to appear, the invention of the mobile phone has supposed a worldwide revolution in telephony.

·    Digital or analogical?

Telephone communication is transmitted via two different systems: analogical and digital transmission. Both land telephony and mobile telephony can use this two systems.

Ø In analogical telephony voice is directly turned into electric impulses that are send by a transmitter. The emissary terminal generates this impulses, and the receptor terminal turns then again into sound. There are different systems of analogical transmission:

     -Telephoning Basic Net (TBN): a chain of circuits carries the electric impulses from the emissary to the receptor. Nodes are used to link the different telephones with other nodes and telephones.

    -First Generation Mobile Telephony: it uses analogical systems of transmission, providing a better covering than any other mobile telephony.

Ø In digital telephony voice is transmitted with a higher quality, and better services can be offered. In this system, voice is codified.

The main systems of digital transmission are:

-RDSI Land Technology.

-Second Generation Mobile Telephony: it transmits voice at a higher speed.

         - Third Generation Mobile Telephony: it allows Internet connection, files download, multimedia messages, etc. 

·    Telephony Services and Security:

  As time goes on, telephony offers more and more services than the simple voice communication. Among this services we should mention the SMS (Short Message Service) -used to send messages of less than 160 letters-,  the EMS (Enhanded Multimedia Service) -used to send messages with images or little melodies-, the MMS (Multimedia Messaging Service) -used to send messages with no  limit of words-, Internet Navigation, chats...

At the same time that telephony offers different functions, it provides more security to the users. Since the appearance of the digital transmission, in which the information is codified, calls have become safer. Besides, the data we want to send can be encrypted by using pairs of codes. Finally, they exist several elements created in order to protect mobile phones: the PIN (Personal Identification Code),  PIN2 and PUK codes, the SIM target (Subscriber Identity Module), etc.