Simple Telecom : Technologies

Telecommunication transmission technology is often lumped into two categories: analog transmission or digital transmission. Figure 4.1 depicts a basic telecommunications transmission system that transfers digital information from one source to an information receiver. The information source is supplied to an end-node that converts the information to a form that can be transmitted through the transmission medium (air, copper, or fiber). The receiving end-node converts the transmission signal into a form that is compatible for the receiver of the information.


Basic Transmission System

Analog
Analog transmission is a process of transferring signals between end-nodes than can have many different signal levels and frequencies. Because analog signals can continuously change to many different levels (voltages) at changing rates (frequencies), the transfer of analog signals (such as an audio signal) requires the transmission medium to have similar transfer characteristics to all parts (levels and frequencies) of the transmission signal. Analog transmission systems must be robust to transfer the signal unaltered for specific voltage levels and frequency components (e.g., high and low frequency).

Figure below shows an analog transmission system. This diagram shows that an audio acoustic (sound) signal is converted by a microphone to an audio electrical signal prior to transmission on a copper line. This audio electrical signal is amplified by an end-node to increase the signal level for transmission on a copper wire. This amplification is necessary to overcome the transmission loss of the copper wire. As the signal progresses down the copper wire, some of the signal energy is converted to heat reducing the signal level. Another amplifier (the receiving end-node) is located at the receiving end to increase the signal to a level suitable for the information receiver (audio speaker).


Analog Transmission System

Digital
Digital transmission is the process of transferring information from node to node in a form that can only have specific levels (usually logic 1 and logic 0). Digital signals have a limited number of different levels (voltages) that represent digital information. Transferring digital signals (such as a computer’s data signal) only requires the transmission medium to transfer two levels without precisely (linearly) transferring levels in between the two levels.

Figure below shows a digital transmission system. This diagram shows a computer that is sending digital data (one equal to +5 volts and zero equal to 0 volts) to an end-node. The end-node is a channel service unit (CSU) and digital service unit (DSU) that converts the levels from the computer to levels suitable for the copper wire transmission medium (logic 1 = +5V and logic 0 = -5V). As the digital signal transfers down the copper wire, some of the energy is converted to heat and some of the frequency components are attenuated resulting in a slightly distorted (rounded) digital pulse arriving at the receiving end-node. Because the receiving CSU/DSU only needs to sense two levels, it is able to re-create the original undistorted digital signal (also known as digital signal regeneration).


Digital Transmission System