The frequency spectrum environment of radio communication is: long wave 30-300KHz (10-1km).
Data is usually transmitted between two stations. According to the direction of the data flow, it can be divided into three transmission modes: simplex, half-duplex, and full-duplex. According to the frequency usage mode, it can be divided into: simplex (same frequency, different frequency simplex), duplex (same frequency, different frequency duplex) and half-duplex communication.
One, simplex communication
Simplex communication only supports signal transmission in one direction (forward or reverse), and the signal transmission direction cannot be changed at any time.
1. Simplex communication classification
In order to ensure the correct transmission of the data signal, the receiving end must verify the received data. If the verification error occurs, the signal requesting retransmission is sent through the monitoring channel.
According to the similarities and differences of the receiving and sending frequencies, it can be divided into two types: same frequency simplex (the same frequency is used for receiving and sending) and different frequency simplex (two different frequencies are used for receiving and sending, and the two frequencies are separated by a larger interval: the 150MHz frequency band is 5.7MHz, 450MHz frequency band is 10MHz, 900MHz frequency band is 45MHz).
2. Features of simplex communication
Advantages: simple equipment, power saving and low cost.
Disadvantages: Due to the use of PTT (Push-To-Talk) push-to-talk switch, it is inconvenient to use.
3. Simplex communication application
① Special training occasions, such as military, police, dispatching and other occasions with relatively few and concise calls.
②In the data collection system, such as the collection of meteorological data, the centralized calculation of telephone charges, etc.
Understand the scenario: The communication between the computer and the printer is in simplex mode, because only the computer transmits data to the printer, and there is no data transmission in the opposite direction.
1. Duplex communication concept
Duplex communication refers to the working method in which both parties in communication can transmit information at the same time (without the need to alternately receive and send messages, one party's transceiver can work at the same time), which is different from the latter half-duplex communication. This method is also called It is full-duplex communication. In the full-duplex mode, each end of the communication system is equipped with a transmitter and a receiver, so data can be controlled to be transmitted in both directions at the same time.
2. Duplex communication classification
Duplex communication can be divided into frequency division duplex (FDD: use two different frequencies for transmission and reception, with the same interval as before, also called inter-frequency duplexing) and time division duplex (TDD: use one frequency for transmission and reception, also called same frequency Duplex, this method is mostly used in home cordless phones) two types.
3. Features of duplex communication:
Advantages: easy to use (same as ordinary phones), full-duplex mode does not need to switch the direction, therefore, there is no time delay caused by the switching operation, which is suitable for those interactive applications that cannot have time delay (such as remote monitoring and control systems) ) Is very advantageous. This method requires both communication parties to have a transmitter and a receiver, and at the same time, two data lines are required to transmit data signals. (May need control line and status line, and ground line).
Disadvantages: The equipment is more complicated and consumes a lot of power (especially for mobile stations that use batteries as power sources in a mobile environment, which is even more unfavorable)
One improvement method: the mobile station adopts the simplex mode, and the base station adopts the duplex mode, which is the so-called half-duplex communication.
Theoretically, full-duplex transmission can improve network efficiency, but in fact it is still useful with other related equipment. For example, a twisted-pair network cable must be used for full-duplex transmission, and the hub connected in the middle must also be capable of full-duplex transmission; finally, the network operating system used must also support full-duplex, so that it can be truly Give full play to the power of full-duplex transmission.
For example, a display terminal needs to be connected between the host computer and the keyboard, the host computer uses a serial interface to connect the display terminal, and the display terminal has a keyboard. In this way, the characters entered on the keyboard are sent to the host memory; and the information in the host memory can also be sent to the screen for display. Usually, after typing a character on the keyboard, it will not be displayed first. After the host computer receives the character, it will immediately send it back to the terminal, and then the terminal will display the character. In this way, the return process of the previous character and the input process of the next character are carried out at the same time, that is, it works in full-duplex mode.
1. Half-duplex communication concept
Half-duplex communication allows signals to be transmitted in two directions, but only allows signals to be transmitted in one direction on one channel at a certain time. Therefore, half-duplex communication is actually a simplex communication with switchable directions.
2. Half-duplex application scenarios
This method is suitable for data communication systems such as inquiries, retrieval, and scientific calculations;
The traditional walkie-talkie uses half-duplex communication. Since the two-way radio uses the same frequency for transmission and reception, simultaneous transmission is not allowed. Therefore, after one party has finished speaking, it is necessary to try to inform the other party that the speech is over (for example, add'OVER' after finishing the speech) before the other party knows that it can start speaking.