Coaxial cable|BNC wire introduction
The BNC wire can be tested for its resistance, voltage, leakage current, open/short circuit and other parameters through the wire testing machine.
1. Baseband coaxial cable
The coaxial cable uses hard copper wire as the core and is covered with a layer of insulating material. This layer of insulating material is surrounded by a densely woven mesh conductor, and the mesh is covered with a layer of protective material. There are two widely used coaxial cables. One is a 50-ohm cable, which is used for digital transmission, and because it is mostly used for baseband transmission, it is also called a baseband coaxial cable; the other is a 75-ohm cable, which is used for analog transmission, which is the broadband coaxial cable to be discussed in the next section.
This structure of coaxial cable makes it have high bandwidth and excellent noise suppression characteristics. The bandwidth of a coaxial cable depends on the cable length. A 1km cable can reach a data transfer rate of 1Gb/s~2Gb/s. Longer cables can also be used, but the transmission rate should be reduced or intermediate amplifiers should be used. At present, coaxial cables are largely replaced by optical fibers, but they are still widely used in cable television and some local area networks.
2. Broadband coaxial cable
A coaxial cable system that uses a limited TV cable for analog signal transmission is called a broadband coaxial cable. The term "broadband" comes from the telephone industry and refers to a frequency band wider than 4kHz. However, in computer networks, "broadband cable" refers to any cable network that uses analog signals for transmission.
Because the broadband network uses standard cable television technology, the usable frequency band is as high as 300MHz (often to 450MHz); due to the use of analog signals, an electronic device needs to be placed at the interface to convert the bit stream entering the network into an analog signal, and Convert the signal output from the network into a bit stream.
Broadband systems are divided into multiple channels, and TV broadcasting usually occupies 6MHz channels. Each channel can be used for analog TV, CD quality sound (1.4Mb/s) or 3Mb/s digital bitstream. TV and data can be mixed on one cable.
One of the main differences between a broadband system and a baseband system is that because the broadband system covers a wide area, it requires an analog amplifier to periodically strengthen the signal. These amplifiers can only transmit signals in one direction. Therefore, if there are amplifiers between computers, packet packets cannot be transmitted backwards between computers. To solve this problem, two types of broadband systems have been developed: dual-cable systems and single-cable systems.
1) Dual cable system
The dual cable system has two identical cables laid side by side. In order to transmit data, the computer transmits the data through the cable 1 to a device at several roots of the cable, that is, the head-end, and then the head-end transmits the signal down the cable through the cable 2. All computers send via cable 1 and receive via cable 2.
2) Single cable system
Another solution is to allocate different frequency bands for internal and external communications on each cable. The low frequency band is used for communication from the computer to the top device, and the signal received by the top device is moved to the high frequency band and broadcast to the computer. In a subsplit system, the 5MHz~30MHz frequency band is used for inward communication, and the 40MHz~300MHz frequency band is used for outward communication. In the midsplit system, the inward frequency band is 5MHz~116MHz, and the outward frequency band is 168MHz~300MHz.
3) There are many ways to use broadband systems. A dedicated permanent channel can be allocated between a pair of computers; other computers can apply to establish a temporary channel through the control channel, and then switch to the requested channel frequency; it can also let all computers share one or a group of channels. Technically speaking, broadband cables are worse than baseband (single channel) cables in sending digital data, but its advantage is that it has been widely installed.
3. Coaxial cable network
Coaxial cable networks can generally be divided into three categories:
· Backbone network.
The backbone line is different from other lines in terms of diameter and attenuation. The former is usually composed of a cable with a protective layer.
· Secondary backbone network.
The diameter of the secondary backbone cable is smaller than that of the backbone cable. When using secondary backbone cables at different building levels, high-gain distributed amplifiers should be used, and the interface between the cable and the user outlet should be considered.
Coaxial cables cannot be spliced, and the parts are connected through low-loss connectors. The connector matches the cable in physical performance. The middle joint and coupler are wrapped with conduit to prevent accidental grounding. If you want the cable to be buried in a place where light cannot be irradiated, it is best to bury the cable in the ground below the freezing point. If you don't want to bury the cable in the ground, it is best to use a pole to erect it. Coaxial cables are marked every 100 meters to facilitate maintenance. If necessary, the cables should be supported every 20 meters. When installing inside the building, consider facilitating maintenance and expansion, and provide pipes where necessary to protect cables
Coaxial cables are generally installed between equipment and equipment. Each user position is equipped with a connector to provide an interface for the user. The installation method of the interface is as follows: (1) Thin cable Cut the thin cable, install BNC headers on both ends, and then connect to both ends of the T-connector. (2) Thick cable Thick cable is generally installed with a tap device similar to a splint, which uses the guide pin on the Tap to penetrate the insulation layer of the cable and directly connect to the conductor. Terminals are provided at both ends of the cable to weaken the reflection of the signal.