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The spectrum environment of wireless communication is: long wave 30-300KHz 10-1km.
Data is usually transmitted between two stations (point-to-point), and can be divided into three transmission modes according to the direction of data flow: simplex, half duplex, and full duplex. According to frequency usage, it can be specifically divided into: simplex (same frequency, different frequency simplex), duplex (same frequency, different frequency duplex), and half duplex communication.
1、 Simplex communication
Simplex communication only supports signal transmission in one direction (forward or reverse), and the direction of signal transmission cannot be changed at any time.
1. Classification of simplex communication
To ensure the correct transmission of data signals, the receiving end needs to verify the received data. If the verification error occurs, a request for retransmission is sent through the monitoring channel.
According to the similarities and differences in transmission and reception frequencies, it can be divided into two categories: same frequency simplex (using the same frequency for transmission and reception) and different frequency simplex (using two different frequencies with a large interval between them: 5.7MHz in the 150MHz frequency band, 10MHz in the 450MHz frequency band, and 45MHz in the 900MHz frequency band).
2. The characteristics of simplex communication
Advantages: Simple equipment (shared circuits or components for transmitting and receiving), power-saving, and low cost.
Disadvantage: Due to the use of PTT (Push To Talk) switch, it is inconvenient to use (not accustomed to).
3. Simplex communication applications
① Special training occasions, such as military, police, dispatch, etc., where there are relatively few and concise calls.
② In data collection systems, such as meteorological data collection, centralized calculation of telephone bills, etc.
Understanding the scenario: The communication between the EG computer and the printer is in simplex mode, as only the computer transmits data to the printer, and there is no data transmission in the opposite direction.
2、 Duplex communication (also known as full duplex communication)
1. Duplex communication concept
Duplex communication refers to the working mode in which both parties can transmit information simultaneously (without the need for alternating reception and transmission, with one partys reception and transmission functions working simultaneously), which is different from half duplex communication and is also known as full duplex communication. In full duplex mode, each end of the communication system is equipped with a transmitter and receiver, so data can be controlled to be transmitted in both directions simultaneously.
2. Duplex communication classification
Duplex communication can be divided into two types: frequency division duplex (FDD: transmitting and receiving using two different frequencies with the same spacing, also known as cross frequency duplex) and time division duplex (TDD: transmitting and receiving using one frequency, also known as co frequency duplex, which is commonly used in home cordless phones).
3. Working principle of duplex communication
4. The characteristics of duplex communication:
Advantages: Easy to use (similar to regular telephones), full duplex mode does not require directional switching, therefore, there is no time delay caused by switching operations, which is very beneficial for interactive applications that cannot have time delay (such as remote monitoring and control systems). This method requires both communication parties to have transmitters and receivers, and also requires two data cables to transmit data signals. (Control lines, status lines, and ground wires may also be required).
Disadvantages: The equipment is more complex and consumes a lot of power (especially for mobile devices that use batteries as power sources in mobile environments)
Improvement method: The mobile station adopts simplex mode, while the base station adopts duplex mode, which is called half duplex communication.
In theory, full duplex transmission can improve network efficiency, but in reality, it is still useful when combined with other related devices. For example, twisted pair network cables must be used to achieve full duplex transmission, and the hub (HUB) connected in the middle must also be able to achieve full duplex transmission; Afterwards, the network operating system used must also support full duplex in order to truly unleash the power of full duplex transmission.
For example, a display terminal needs to be connected between the computer host and the keyboard. The computer host is connected to the display terminal through a serial interface, and the display terminal comes with a keyboard. In this way, the characters entered on the keyboard are sent to the host memory; And the information from the host memory can also be sent to the screen for display. Usually, after entering one character into the keyboard, it is not displayed for the time being. Once the computer host receives the character, it immediately sends it back to the terminal, which then displays the character. In this way, the feedback process of the previous character and the input process of the following character are carried out simultaneously, working in full duplex mode.
3、 Half duplex communication
1. Half duplex communication concept
Half duplex communication allows signals to be transmitted in two directions, but at a certain moment, signals are only allowed to be transmitted unidirectionally on one channel.
Therefore, half duplex communication is actually a direction switchable simplex communication.
2. Half duplex communication principle
3. Half duplex application scenarios
This method is suitable for data communication systems such as inquiry, retrieval, and scientific computing;
Traditional walkie talkies use half duplex communication methods. Due to the use of the same frequency for transmitting and receiving on walkie talkies, simultaneous operation is not allowed. Therefore, after one party finishes speaking, they need to try to inform the other party that the speech is over (for example, adding Over after speaking), so that the other party knows that they can start speaking.
