How much do you know about the “wireless” mechanism in wireless communications? (Part 2)
Communication technology is still advancing, especially through wireless technologies such as 5G, the Internet of Things, and digitization.
The first part of this article briefly reviewed the historical evolution of wireless communications and introduced the basic composition and applications of wireless communication systems. Here we introduce you to the data transmission direction, communication protocols, and basic knowledge about radio waves:
Table of contents
01. Introduction: History and Evolution of Wireless Communications
02. What is wireless communication?
03. Application examples of wireless communication
04. Basic components of wireless communication systems
05. Wireless communication methods
06. Data transmission direction
07. What is a communication protocol?
08. Appendix: Introduction to radio waves and their frequencies
Data transmission direction: bidirectional (full duplex/half duplex) and unidirectional
Whether it is wireless or wired, the transmission form where data is transmitted is an important specification for communication applications. This form can be divided into two-way transmission and one-way transmission. Bidirectional transmission can be further divided into full-duplex transmission and half-duplex transmission. Below, we explain the various transfer forms.
Bidirectional transmission is a transmission form that is considered to account for a considerable proportion of current digital communication equipment. There are two methods of full-duplex transmission and half-duplex transmission.
It should be noted here that terms such as "download/upload" and "downlink/uplink" are often heard in bidirectional transmission. If the terminal or PC you are using is regarded as device A, receiving data from device B is called downloading, and conversely, sending data from device A to device B is called uploading. In addition, if the data transmission itself is not important, but the connection between device A and device B, such as the connection between the terminal and the base station, is very important, it is called downlink/uplink.
Full duplex transmission
As shown in the figure above, this is a transmission form that can conduct data communication and conversation in both directions (duplex) from device A to device B and from device B to device A at the same time. Additionally, when signals for reception and transmission (sometimes called "Rx" and "Tx" respectively) are assigned different frequencies, the electronic component that separates them is called a duplexer (shown in orange).
Typical examples of bidirectional transmission include landlines, mobile phones, smartphones, etc. (direct direction between terminals).
If it is a form in which device A and device B transmit data by switching between sending and receiving, this form is different from full-duplex transmission because data cannot be transmitted in both directions at the same time, which is called half-duplex transmission ( Half-duplex transmission), as shown in Figure 8-2a and 8-2b:
half duplex transmission
An application example of half-duplex transmission is a transceiver.
One-way transmission is a form of transmission in which data is transmitted from device A to device B in only one direction (one-way) between device A (sender) and device B (receiver), as shown in the figure below:
Examples of one-way transmission are AM/FM radio broadcasts, remote controls using radio waves, etc.
What is a communication protocol?
So far, we have given an overview of wireless communications from a hardware perspective. On the other hand, software is also important in setting up telecommunications, whether wireless or wired. That is the communication convention known as a communication protocol or protocol.
A protocol is a set of steps and rules used in data communication (digital communication) systems, including computers, to transmit data to each other without errors between different systems. The figure below shows an example of the role of data transfer-related protocols.
The role of communication protocols using full-duplex communication as an example
In order to transmit data reliably, protocols for multiple roles and functions need to be specified, such as control steps, data structures and interfaces, etc., with a wide range and a huge amount of data. If the protocol is fixed to one model, it will be difficult to cope with version upgrades such as adding content to the protocol or changing the protocol. To make upgrading easier, roles and functions were assigned to each protocol and a hierarchy was created for these protocols. This systematically organized protocol structure is called a protocol stack (sometimes also called a protocol suite or network architecture).
The protocol stack has been modeled according to international standards and is called the OSI Basic Reference Model (table below).
OSI Reference Model Protocol Stack
In reality, this model is not used, but a protocol stack with appropriate specifications (high data transmission efficiency, high data transmission reliability, etc.) is used for each purpose. The following table takes the TCP/IP model used by Internet standards and the protocol stack in the Bluetooth® LE model as examples to show the correspondence with the OSI basic reference model. Descriptions of the layers in the TCP/IP model and Bluetooth® LE model have been omitted, but you can see that there are fewer layers and some layers are omitted.
OSI reference model, TCP/IP model and Bluetooth® LE protocol stack
In addition, the mechanism of 4G LTE and 5G communication is through base stations, so their communication protocols are more complex than TCP/IP and Bluetooth® LE models.
Attachment: About radio waves and their frequencies
Electric waves, like motion and heat, are a form of energy, also known as electromagnetic waves (in fact, light is also a type of electromagnetic waves). According to Japan’s Radio Wave Law and the Wireless Communications Regulations attached to the International Telecommunications Convention, radio waves are defined as electromagnetic waves with a frequency of 3000GHz or lower.
These waves are emitted from wireless devices, but actually visualizing them is not easy. Therefore, we will explain the generation and propagation of radio waves by using the phenomenon that occurs when sinusoidal alternating current passes through conductor rods such as metal, so that you can understand it more easily.
Schematic diagram of radio wave propagation
The image above shows how the radio waves are traveling at this time. In fact, electric waves propagate in three dimensions, but here we focus on the waves propagating in a direction perpendicular to the conductor to show how they propagate. The electric and magnetic fields remain at right angles (orthogonal) to each other. Changes in the magnetic field create an electric field, and changes in the electric field create a magnetic field. This effect repeats itself and propagates as sinusoidal vibrations. The main properties of radio waves are as follows:
・Electric waves are transverse waves in which the amplitude (intensity) of the electric and magnetic fields changes perpendicularly relative to the direction of travel, and the electric and magnetic fields also propagate perpendicularly to each other;
・The propagation speed of radio waves is the same as the speed of light;
・ Radio waves have no medium (the air vibrates and propagates in the form of waves, and the sound is felt when it reaches the human ear. At this time, the air is called the medium).
Although the above-mentioned properties of radio waves without a medium are far from our daily perception, it is currently believed that radio waves propagating even in a vacuum like outer space are generated by the vibration of electric and magnetic fields in space itself.
By the way, at the beginning we pointed out that radio waves are electromagnetic waves with a frequency of 3000 GHz or lower. The frequency f (Hz) can be calculated by f = c/λ, where the radio wave wavelength λ (m) is as shown in the previous figure, and c (3×108 m/s) is the speed of light. Electromagnetic waves are divided into several types based on frequency and wavelength (picture below):
Classification of electromagnetic waves by frequency and wavelength
When beginners understand radio waves, they especially need to understand the concepts of electric fields and magnetic fields.
The electric field is the action space of electric force, and the magnetic field is the action space of magnetic force. The figure below is a schematic diagram using arrow lines to indicate the range of the electric field generated by applying a voltage and the range of the magnetic field generated around the magnet.
Schematic diagram of the coverage of electric and magnetic fields
Beginners often see a schematic diagram of radio wave propagation (picture below) that is a simple combination of the phenomena of changing a magnetic field to produce an electric field and changing an electric field to produce a magnetic field. If you want to study radio waves seriously because it is related to antennas, etc., please do not understand it based on this diagram!
Intuitive radio wave propagation diagram
The correct way to understand it is to understand it according to the schematic diagram of radio wave propagation in this section - that is, the schematic diagram of the vectors (electric field vector and magnetic field vector) after the intensity of the electric field and magnetic field are represented by arrows (derived from Maxwell's equations). If understood from a conscious diagram, the radio waves used in BS broadcasting are described as circularly polarized radio waves that propagate in a spiral shape while the electric field vector in the radio wave propagation diagram rotates to the left or right.
The electric field vector and the magnetic field vector in the radio wave propagation diagram are both vectors that represent rightward rotation and rotation speed (in this sense, they are also called "rotation vectors"). Neither represents the direction in which something like an object is moving, like a velocity vector does. Both lead to Maxwell's equations - the fundamental equations related to electromagnetic fields, expressing all relationships between electricity and magnetism. Coulomb's law, which beginners learn about electromagnetism, can also be derived from this equation.
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