Selection Of Coaxial Cable, Twisted Pair And Optical Fiber

1、 Coaxial cable

Coaxial cable is a cable with a central copper conductor wrapped by layers of insulated wires. It is characterized by good anti-interference ability, stable transmission data and low price, so it is widely used, such as closed-circuit television line.

However, according to the analysis of the characteristics of the coaxial cable, when the signal is transmitted in the coaxial cable, the attenuation is related to the transmission distance and the frequency of the signal itself. Generally speaking, the higher the signal frequency, the greater the attenuation. Therefore, coaxial cable is only suitable for short-range transmission of image signals. When the transmission distance reaches about 200 meters, the image quality will decline significantly, especially the color will become dim and have a sense of distortion.

In practical engineering, in order to extend the transmission distance, coaxial amplifier should be used. The coaxial amplifier can amplify the video signal to a certain extent, and can compensate different frequency components through equalization adjustment, so as to minimize the distortion of the video signal output by the receiver. However, coaxial amplifiers cannot be cascaded indefinitely. Generally, in a point-to-point system, coaxial amplifiers can only be cascaded 2 to 3, otherwise the video transmission quality cannot be guaranteed, and it is difficult to adjust. Therefore, when using coaxial cable in the monitoring system, in order to ensure better image quality, the transmission distance is generally limited to about four or five hundred meters.

2、 Twisted pair

Common twisted pair wires include class 3 wire, class 5 wire, super class 5 wire, class 6 wire and class 7 wire.

1. Class V cable: this kind of cable increases the winding density and is covered with a high-quality insulating material with a transmission rate of 100MHz. It is used for voice transmission and data transmission with a transmission rate of 100Mbps. It is mainly used for 100Base-T and 10Base-T networks. This is a common Ethernet cable.

2. Super class 5 line: super class 5 has small attenuation and less crosstalk, higher ratio of attenuation to crosstalk, signal-to-noise ratio and smaller delay error, and its performance has been greatly improved. The transmission rate of super class 5 line is 250mbps.

3. Class VI line: the transmission frequency of this kind of cable is 1MHz ~ 250MHz. The comprehensive attenuation crosstalk ratio (PS-ACR) of class VI wiring system should have a large margin at 200MHz, which provides twice the bandwidth of super class v. The transmission performance of Cat 6 wiring is much higher than that of cat 5 standard, which is suitable for applications with transmission rate higher than 1Gbps.

4. Cat 6 wire: Cat 6 wire is an improved version of Cat 6 wire, which is also ANSI / EIA / tia-568b An unshielded twisted pair cable specified in class 2 and ISO 6 / class E standards, which is mainly used in Gigabit networks. In terms of transmission frequency, it is the same as class VI line, which is also 200 ~ 250 MHz, and the transmission speed can also reach 1000 Mbps, but it has been greatly improved in terms of crosstalk, attenuation and signal-to-noise ratio.

5. Class VII line: this line is a twisted pair in ISO class 7 / class F standard. It is mainly to adapt to the application and development of 10 Gigabit Ethernet technology. However, it is no longer an unshielded twisted pair, but a shielded twisted pair, so its transmission frequency can reach at least 500 MHz, more than twice that of Cat 6 line and Super Cat 6 line, and the transmission rate can reach 10 Gbps.

Twisted pair is so widely used because it has many advantages, such as strong anti-interference ability, long transmission distance, easy wiring, low price and so on. Due to the large attenuation of twisted pair signals, the frequency of signals cannot be too high when the transmission distance is long, while high-speed signals such as Ethernet can only be limited to within 100m.

3、 Optical fiber

Optical fiber transmits signals in the form of optical pulses, so the material is also mainly glass or plexiglass. The utility model is composed of a fiber core, a cladding and a protective sleeve.

The structure of optical fiber is very similar to that of coaxial cable. The center is an optical fiber made of glass or transparent plastic, surrounded by protective materials. Multiple optical fibers can be combined into one optical cable as needed.

According to the different modes of optical signal generation, optical fiber can be divided into single-mode optical fiber and multimode optical fiber (the so-called "mode" refers to a beam of light entering the optical fiber at a certain angle). Multimode optical fiber is generally used for network connection in the same office building or relatively close area. The single-mode optical fiber has higher quality and longer transmission distance. It is usually used to connect between office buildings or geographically dispersed networks. If optical fiber cable is used as the network transmission medium, optical transceiver and other equipment need to be added, so the cost investment is greater and it is less used in general applications.

The characteristic of optical fiber is that it transmits optical signals, so it is not interfered by external electromagnetic signals, and the attenuation speed of signals is very slow. Therefore, the transmission distance of signals is much farther than the above various network cables transmitting electrical signals, and it is especially suitable for places with bad electromagnetic environment.