Communication is the act of sending, conveying, and receiving information.

Communication includes not only conversation and written exchanges but also commands given to machines and data transmission between machines. A communication system can be organized by the roles of the information source, transmitter, channel, receiver, and destination.

1.1.1 Roles of the Components of a Communication System

In a communication system, a message generated by the information source is converted into a signal by the transmitter, sent through the channel to the receiver, and delivered to the destination.

Table 1.1-1 Basic Components of a Communication System and Their Roles

Component	Role
Information source	Generates the information to be conveyed
Transmitter	Converts the message from the information source into a signal suitable for transmission
Channel	Serves as the path that carries the transmitted signal
Receiver	Extracts the original message, or something close to it, from the received signal
Destination	Receives the conveyed information

The channel corresponds to a cable in wired communication and to space in wireless communication.

1.1.2 The Difference Between Messages and Signals

The content to be conveyed, generated by the information source, is called a message. Examples of messages include sequences of characters and sound pressure that changes over time.

On the other hand, what the transmitter converts the message into and sends out in a form suitable for transmission is the transmitted signal. What reaches the receiver through the channel is called the received signal. Distinguishing between messages and signals makes it possible to consider separately what is to be conveyed and the form in which it is carried.

For example, the information source of a telephone system can be regarded as the set of subscribers, and the message is sound pressure that changes over time. When voice is transmitted as an electrical signal, the transmitting side converts the voice into a current waveform and sends it, and the receiving side extracts the voice from the arriving signal and delivers it to the listener.

Table 1.1-2 Example of a Telephone That Transmits Voice as an Electrical Signal

Component of the communication system	Corresponding element
Information source	The set of subscribers
Transmitter	The part that converts voice into an electrical signal
Channel	Telephone lines or other paths that carry the electrical signal
Receiver	The part that extracts voice from the received signal
Destination	The listener

In this example, the voice produced on the transmitting side and the electrical signal traveling through the channel do not take the same physical form. The transmitter and receiver handle the conversion between them.

Classification by Signal Properties

When signals are regarded as stochastic processes, a communication system that handles discrete stochastic processes is called a discrete communication system, and one that handles continuous stochastic processes is called a continuous communication system. Telegraphy is an example of the former, and the telephone is an example of the latter.

1.1.3 Channels and Noise

A signal passing through a channel may be distorted or affected by noise during transmission. For this reason, the received signal is not necessarily the same as the transmitted signal.

The source that generates noise is called a noise source. In a communication system, it is necessary to consider not only the path that conveys information but also the factors that interfere with the signal.

The role of the receiver is not to pass the arriving signal on to the destination as it is, but to recover the original message as closely as possible. In communication, how accurately information can be conveyed is important, along with the speed and cost of sending it.

1.1.4 Structure of Digital Communication Systems

In digital communication, information is handled as a bit sequence and processed for transmission.

In a basic digital communication model that separates the processing on the transmitting and receiving sides, source coding, channel coding, and modulation are placed on the transmitting side, and the corresponding demodulation and decoding are placed on the receiving side.

Table 1.1-3 Main Processes in Digital Communication

Order	Transmitting-side process	Purpose	Corresponding receiving-side process
1	Source coding	Reduces the redundancy of the information source and represents it efficiently	Source decoding
2	Channel coding	Adds redundancy according to rules to prepare for errors caused by noise	Channel decoding
3	Modulation	Converts the coded information into a waveform that can be transmitted over the channel	Demodulation

On the transmitting side, processing is performed in the order of source coding, channel coding, and modulation, and the resulting signal is sent to the channel. On the receiving side, processing is performed in the order of demodulation, channel decoding, and source decoding.

Source Coding and Channel Coding

Source coding is a process that reduces the redundancy contained in the representation of information. Through compression, it aims to represent the data to be transmitted efficiently.

Channel coding is a process that intentionally adds redundancy to counter noise in the channel. On the receiving side, this regularity is used to correct errors.

Although both are called coding, their purposes differ. Source coding makes the representation more efficient, while channel coding prepares for errors during transmission; this is how their roles are divided.

Modulation and Demodulation

The coded bit sequence is converted by modulation into a waveform that can be transmitted over the channel. On the receiving side, the arriving waveform is demodulated, and the subsequent decoding process aims to restore the original message.

1.1.5 Broader Applications of the Communication Model

The senders and receivers of communication are not limited to people. Depending on who generates the information and who receives it, communication can be viewed as follows.

Table 1.1-4 Parties in Communication and Examples

Flow of information	Examples
From person to person	Conveying information through conversation or writing
From person to machine	Commands for operating a machine
From machine to machine	Automatic control and data transmission
From the natural world to people or machines	Observation of physical phenomena

In an experiment, people or machines act on experimental equipment or other targets and receive the response. Such processes can also be regarded as communication in the broad sense of sending and receiving information.

Similarities with Information Processing and Storage Devices

Information processing systems, which take in information, process it, and output the result, have a structure similar to that of communication systems. In this view, the input device corresponds to the transmitter, the main unit that performs the processing corresponds to the channel, and the output device corresponds to the receiver.

Storage devices can also be viewed as consisting of writing, a recording medium, and reading. In a magnetic tape storage device, the writing-side device corresponds to the transmitter, the magnetic tape to the channel, and the reading-side device to the receiver.

In this way, the communication system model can be used to organize the mechanisms for conveying, processing, and recording information.

Summary

A communication system can be organized by the roles of the information source, transmitter, channel, receiver, and destination. Because noise and distortion occur in the channel, the receiver extracts the original message from the arriving signal.

In a basic digital communication model, source coding, channel coding, and modulation are used together with the corresponding receiving-side processes. Distinguishing the purpose of each process is the foundation for understanding how communication systems work.

About This Article

References

• Joho Kogaku no Kiso [Fundamentals of Information Engineering], Toshio Hosono, Corona Publishing
• A Cascade Network for Blind Recognition of LDPC Codes, Xiang Zhang and Wei Zhang
• Design and Analysis of Joint Source-Channel Code System with Fixed-Length Code, Han Bao et al.

(Last accessed: October 18, 2026)


*This article was compiled based on the author’s understanding and practical knowledge, with reference to the above literature.