Signal attenuation is an important feature of communication transmission. The degree of signal attenuation is not only an important index to evaluate the quality of communication, but also has a direct impact on the expansion and upgrading of communication system and the relay distance in the layout of communication transmission cables.
Signal attenuation is an important feature of communication transmission. The degree of signal attenuation is not only an important index to evaluate the quality of communication, but also has a direct impact on the expansion and upgrading of communication system and the relay distance in the layout of communication transmission cables. In order to ensure the quality of communication signal, the transmission factors at both ends of communication transmission should be changed. Transmission factor refers to the ratio of the normalized complex wave amplitude of the transmitted wave at one port or cross section of the transmission line to the incident wave at another port or cross section.
Communication is the transmission of information, which refers to the transmission and exchange of information from one place to another. Its purpose is to transmit messages. However, with the development of social productivity, people have higher and higher requirements for message delivery.
In the field of communication, communication transmission is generally divided into wired communication and wireless communication. Due to various reasons, such as attenuation, the quality of communication signals varies greatly. Transmission factor is one of the indexes to detect the quality of communication signal. Transmission factor refers to the ratio of the normalized complex wave amplitude of the transmitted wave at one port or cross section of the transmission line to the incident wave at another port or cross section. Wired communication and wireless communication have different ways to change the transmission factor. Wireless communication is generally to replace equipment, such as using antenna with larger transmission factor. In general, the attenuator is used in the communication.
Classification of communication transmission
Classification by transmission medium
Cable communication: refers to the communication in the form of wire, cable, optical cable, waveguide, nano material and other forms of transmission media. Its characteristics are that the media can be seen and touched (open line communication, cable communication, optical cable communication)
Wireless communication: refers to the transmission medium can not see, touch (such as electromagnetic wave) a form of communication
(微波通信、 短波通信、 移动通信、 卫星通信、 散射通信)
(microwave communication, short wave communication, mobile communication, satellite communication, scattering communication)
Classification by signals transmitted in the channel
模拟信号：凡信号的某一参量(如连续波的振幅、频率、 相位， 脉冲波的振幅、宽度、位置等)可以取无限多个数值， 且直接与消息相对应的，模拟信号有时也称连续信号， 这个连续是指信号的某一参量可以连续变化数字信号：凡信号的某一参量只能取有限个数值，并且常常不直接与消息相对应的，也称离散信号
Analog signal: when a certain parameter of the signal (such as amplitude, frequency and phase of continuous wave, amplitude, width, position of pulse wave, etc.) can take infinite number of values and directly correspond to the message, analog signal is sometimes also called continuous signal, This continuity means that a certain parameter of the signal can continuously change digital signal: where a certain parameter of the signal can only take a finite number of values, and often does not directly correspond to the message, also known as discrete signal
Classification by working frequency band
Short wave communication
Classification by modulation mode
Baseband transmission: refers to the signal without modulation and directly sent to the channel for transmission
Frequency band transmission: refers to the signal after modulation and then sent to the channel for transmission, with corresponding demodulation measures at the receiving end
According to the division of labor and data transmission direction of both sides of communication
For point-to-point communication, according to the direction of message transmission, the communication mode can be divided into simplex communication, half duplex communication and full duplex communication.
所谓单工通信，是指消息只能单方向进行传输的一种通信工作方式。单工通信的例子很多，如广播、遥控、无线寻呼等。这里，信号（消息）只从广播发射台、遥控器和无线寻呼中心分别传到收音机、遥控对象和 BP 机上。
The so-called simplex communication refers to a communication mode in which messages can only be transmitted in one direction. There are many examples of simplex communication, such as broadcasting, remote control, wireless paging, etc. Here, the signal (message) is only transmitted to the radio, the remote control object and the BP machine respectively from the broadcasting transmitting station, the remote controller and the wireless paging center.
The so-called half duplex communication mode refers to the working mode that both sides of communication can receive and receive messages, but can not receive and send messages at the same time. Walkie talkie, transceiver and so on are all this way of communication. The so-called full duplex communication refers to the working mode in which both sides of communication can simultaneously transmit messages in two directions. Both sides can send and receive messages at the same time. Obviously, the channel of full duplex communication must be bidirectional. There are many examples of full duplex communication in our life, such as ordinary telephone, mobile phone, etc.
When the signal propagates in the transmission medium, a part of the energy will be converted into heat energy or absorbed by the transmission medium, resulting in the continuous weakening of the signal strength. This phenomenon is called attenuation. In electronic equipment, in order to prevent the input stage from limiting or blocking due to excessive signal, an attenuator is added artificially.
Causes of attenuation
1. The loss from the optical fiber itself includes the inherent absorption loss of the optical fiber material itself, the absorption loss of impurities in the material (especially the loss caused by the OH – component remaining in the fiber), Rayleigh scattering loss and scattering loss caused by the imperfect structure of the optical fiber.
Intrinsic absorption: the inherent absorption of optical fiber basic materials (such as SiO2), which is not caused by impurities or defects. Therefore, intrinsic absorption basically determines the lower limit of absorption loss of a certain material.
Impurity absorption: additional absorption loss (dust, metal ions, etc.) caused by impure optical fiber materials.
Scattering loss: due to the defects or inhomogeneity of the material, shape and refractive index distribution of the optical fiber, the conducted light in the optical fiber is scattered, resulting in the loss.
2. Because the optical fiber is made into optical cable through bundle, the additional loss of optical fiber is caused by laying, connecting and coupling the optical fiber in various environments. It includes bending loss, micro bending loss, connection loss in optical fiber line and coupling loss between optical devices.
Methods of changing transmission factor
中继器（RP repeater）是工作在物理层上的连接设备。适用于完全相同的两类网络的互连，主要功能是通过对数据信号的重新发送或者转发，来扩大网络传输的距离。 中继器是对信号进行再生和还原的网络设备：OSI 模型的物理层设备。
A repeater is a connected device that works on the physical layer. It is suitable for the interconnection of two identical networks. Its main function is to extend the distance of network transmission by retransmission or forwarding data signals. Repeater is a network device that regenerates and restores signals: physical layer device of OSI model.
中继器是局域网环境下用来延长网络距离的最简单最廉价的网络互联设备，操作在 OSI 的物理层，中继器对在线路上的信号具有放大再生的功能，用于扩展局域网网段的长度（仅用于连接相同的局域网网段）。
Repeater is the simplest and cheapest network interconnection device used to extend the network distance in LAN environment. It operates in the physical layer of OSI. It has the function of amplifying and regenerating the signal on the line, and is used to extend the length of LAN segment (only used to connect the same LAN segment).
RP repeater is a kind of device connecting network lines, which is usually used to forward physical signals between two network nodes. The repeater mainly completes the function of the physical layer, which is responsible for transmitting information by bit on the physical layer of two nodes, completing the functions of signal replication, adjustment and amplification, so as to extend the length of the network. Due to the existence of loss, the signal power transmitted on the line will gradually decay, and when it is attenuated to a certain extent, it will cause signal distortion, which will lead to receiving error. Repeater is designed to solve this problem. It completes the connection of the physical circuit, amplifies the attenuated signal and keeps the same as the original data. In general, both ends of the repeater are connected to the same media, but some repeaters can also complete the transfer of different media. Theoretically speaking, the use of repeaters is unlimited, so the network can be extended indefinitely. In fact, this is impossible because the delay range of the signal is specified in the network standards, and the repeater can only work effectively within the specified range, otherwise it will cause network failure.
Attenuator is an electronic component that provides attenuation, Widely used in electronic equipment, its main purposes are: (1) adjusting the size of the signal in the circuit; (2) in the comparative measurement circuit, it can be used to read the attenuation value of the network under test; (3) to improve the impedance matching. If some circuits require a relatively stable load impedance, an attenuator can be inserted between the circuit and the actual load impedance, which can slow down The change of impulse impedance.
Attenuator is a circuit used to introduce a predetermined attenuation in a specified frequency range. It is generally indicated by the decibel number of the attenuation introduced and the ohmic number of the characteristic impedance. Attenuator is widely used in cable TV system to meet the level requirements of multi port. Such as amplifier input, output level control, branch attenuation control. There are two kinds of attenuators: passive attenuator and active attenuator. The active attenuator is combined with other thermal elements to form a variable attenuator, which is used in the automatic gain or slope control circuit in the amplifier. Passive attenuator has fixed attenuator and adjustable attenuator.