The modern telecommunication systems as one group for the greater part the electrical communication system or optical communication system with point-to-point configurations.
Although that system has ability to carry electric signal and optic, carried information is not always on both that signal formed. Example, human voice signal, first needs to convert to electric signal or optic, before carried by a communication system. Transducers are needed for converting. Using optical source to need electric signal as input, and optic detector will produce electric signal as output. In this case, original signal first converted to electric signal and then to optical signal on moment to go into optical communication system and in this optical signal receiver is converting back to electric signal before original signal reproduce. A media need to carry that signal. This media known as channel. Example, the outer space, cable (copper or fiber optic), etc. To be connected with satellite on the electric communication system. These channels, generally to have the character of loss and easy or inclining get noise from outside that will reduce carried signal information value. Different channel will to show different characteristic and influenced by disturbance with different degree. So, transmitted information signal via channel must exactly match with specification used or condition of channel, so that basic losses effect from channel and disturbance can protect constantly on accept level. And the signal can to reach destination with accept level (still to comply with receiver equipment specification).
Channel characteristic and signal from telecommunication system individually on one telecommunication network very varies. Example, communication system between users and switching system commonly using two wire copper cable as channel, even though communication between switching system usually using coaxial cable, fiber optic, or along space (radio frequency wave) as channel. Equipments type that used as terminal will to ascertain characteristic of carried signal between terminal end and switching system. Example, the electrical characteristic from teleprompter signal, is totally different from telephone signal. Services type on telecommunication network increased and growth. Severally to posses specification and specific used, and operated without hanging each other. Example:
Telegraph network
Telex network
Telephone network
Data network
Telephone network application it self now already growth, not only for processing telephone information (voice) but also data, example application facsimile, Internet, etc. So that to become blur different between telephone network and data network.
If we see from terminal equipment side, telephone equipment right now not only just for processing voice information, but also data information even picture/graphic. To fulfill users flexibilities, telephone at this time already mobility, so that to make easier telephone service users to conversation or data exchange anywhere and anytime.
The telecommunication network form is more and more complex, so that management and also maintenance will be very expensive, especially for multiple networks. So that to emerge an idea for designed single network that can handling all service. If all information or specific signal form from that services can convert to a similar form (standard) on digital domain, then a network that have ability to transports digital signal will handling multitude of service. This idea right is now already to bring forth the evolution to Integrated Services Digital Network (ISDN).
