Please explain what twisted pair is.

　　Twisted pair is a commonly used communication tranSMission medium composed of two metal wires (usually copper wires) that are insulated from each other and twisted together at a certain density. The following is a detailed explanation of twisted pair:

　　First, structural characteristics

　　Wire material: Twisted pair wires are usually composed of two 22-26 insulated copper wires, which are wrapped in an insulating protective layer.

　　Twisting method: Two wires are twisted together according to a certain density. This twisting method helps to reduce signal interference. The radio waves radiated by each wire during transmission are cancelled by the radio waves emitted by the other wire, thus effectively reducing the degree of signal interference.

　　Cable structure: multiple pairs of twisted pairs can be collectively wrapped in an insulated cable jacket to form a twisted pair cable. This structure makes the twisted pair more robust and durable, suitable for a variety of communication environments.

　　Second, performance parameters

　　Attenuation: Attenuation refers to the degree to which the signal gradually weakens during transmission. The attenuation of a twisted pair varies with frequency, so the attenuation should be measured at all frequencies within the application range.

　　Near-end crosstalk (NEXT): Near-end crosstalk refers to the signal coupling from one pair of wires to another in an unshielded twisted pair link. As the signal frequency increases, the measurement of near-end crosstalk becomes more difficult.

　　DC resistance: The DC loop resistance consumes a part of the signal and converts it into heat. The DC resistance of a twisted pair refers to the sum of the resistance of a pair of wires, and its value should not be too large, otherwise it will affect the quality of signal transmission.

　　Characteristic impedance: Characteristic impedance includes resistance, inductance impedance and capacitance impedance with a frequency of 1 to 100 MHz, which is related to the distance between a pair of wires and the electrical performance of the insulator. The characteristic impedance of twisted pair cable is usually 100 ohms, 120 ohms and 150 ohms.

　　Attenuation Crosstalk Ratio (ACR): In some frequency ranges, the proportional relationship between crosstalk and attenuation is another important parameter reflecting cable performance. A larger ACR value indicates stronger anti-interference ability.

　　III. Application and classification

　　Application: Twisted pair is widely used in network communication, telephone and other fields due to its low cost and strong anti-interference ability.

　　Classification: Twisted Pairs can be divided into various types according to their transmission performance, such as Category 3 wires, Category 5 wires, Super Category 5 wires, and the latest Category 6 wires. Different types of twisted pairs are suitable for different transmission requirements.

　　IV. Advantages and disadvantages

　　Advantages:

　　Low cost, easy to install and maintain.

　　It has strong anti-interference ability and can withstand some electromagnetic wave interference from the outside world.

　　The transmission distance is moderate, suitable for most local area network environments.

　　Disadvantages:

　　The transmission speed is relatively slow and cannot meet the demand for high-speed data transmission (but high-performance twisted pairs such as Category 6 twisted pair can support gigabit networks).

　　The transmission distance is limited, and the signal quality will decrease after a certain distance.

　　To sum up, twisted pair is an important communication transmission medium with the advantages of low cost and strong anti-interference ability. When choosing twisted pair, the appropriate type should be selected according to the specific transmission requirements and environment.