Network Cable is a medium that connects a network device (such as a computer) to another network device to transmit information, and is the basic component of a network. In our common local area network, there are many types of network cables. Under normal circumstances, a typical local area network generally does not use many different types of network cables to connect network devices. In large networks or wide area networks, different types of network cables are used to connect different types of networks together. Among the many types of network cables, which one to use depends on the network topology, network structure standards, and transmission speed.
Twisted Pair
Twisted pair (Twisted Pair) is divided into shielded (Shielded Twisted Pair, STP) and unshielded (Unshielded Twisted Pair, UTP).
shield
The so-called shielding means that the signal line inside the network cable is wrapped with a layer of metal mesh, and the shielding layer is the insulating sheath, which can effectively isolate the interference of external electromagnetic signals.
Origin of the name
UTP is currently the most frequently used network cable in local area networks. This kind of network cable is wrapped with eight signal wires in a plastic insulating sheath. They come in eight colors. They are: white orange, orange, white green, blue, white blue, green, white brown, brown (568B line sequence). Although there are eight lines, only four are actually useful. They are: white orange, orange, white green, green. Every two of them are twisted into a pair, forming a total of four pairs, and the twisted pair is also named.
Intertwined purpose
The purpose of twisted pairs twisted with each other is to use the electromagnetic field generated by the current in the copper wire to cancel the interference of adjacent lines and reduce the interference from the outside world. The number of times each pair of wires are twisted in each inch of length determines the anti-interference ability and the quality of communication. The tighter the winding, the higher the communication quality, which can support higher network data transmission rates. Of course, its cost is also Higher. The International Electrotechnical Commission and the International Telecommunications Commission EIA/TIA? Electronic Industry Association/Telecommunication Industry Association have established the international standard for UTP cables and divided them into 5 categories (Categories or CAT for short) according to the field of use. Each category of cable manufacturers The type of insulation will be marked on the insulation cover, such as CAT-5 or Categories-5, we need to pay attention when purchasing.
Use length
CAT-3 and CAT-5 are the most commonly used types in computer networks. Without adding other network connection devices (such as hubs), the maximum allowable length of a single segment of CAT-3 and CAT-5 is 100 meters. The type 100Base-TX network does not exceed 220 meters. The so-called CAT-5 cable that I usually talk about is just the CAT-5 standard that the manufacturer unilaterally improves to ensure communication quality, and it has not been recognized by EIA/TIA.
RJ-45 connector
The UTP network cable uses the RJ-45 crystal plug for connection. The RJ45 connector is a plastic connector that can only be inserted in a fixed direction and automatically prevents falling off. Each signal wire inside the network cable needs to use a special crimping tool to make it and RJ-45 The contact points are tightly connected. Depending on the network speed and network structure standards, the connection methods of the contact points and the network cable are also different.
Scope of application
UTP cable is suitable for 10Base-T, 100Base-T, 100Base-TX standard star topology network.
STP uses a metal shielding layer to reduce external electromagnetic interference (EMI). When the shielding layer is properly grounded, the received electromagnetic interference signal can be turned into a current signal, which is opposite to the interference signal current formed in the twisted pair. . As long as the two currents are symmetrical, they can be cancelled without bringing noise to the receiving end. However, when the shielding layer is discontinuous or the shielding layer current is asymmetric, the shielding effect will be reduced or even completely lost, resulting in noise. STP cables can prevent electromagnetic radiation and interference only when the complete end-to-end link is completely shielded and properly grounded. To minimize the noise and improve the signal-to-noise ratio, this kind of anti-interference and anti-radiation ability is the so-called electromagnetic compatibility (EMC).
Disadvantages of STP cables
The attenuation increases during high-frequency transmission. If there is no good shielding effect, the balance will be reduced and crosstalk noise will also be caused. The shielding effect depends on the shielding material, shielding layer density, electromagnetic interference signal type, frequency, distance from noise source to shielding layer, shielding continuity and grounding structure used.
STP is generally used in environments prone to electromagnetic interference and radio frequency interference.
Application occasion
Intelligent building/smart community monitoring
Office building/hospital/prison/school/warehouse and other venue monitoring
Highway toll station video monitoring system
Remote video surveillance distributed front-end collection and transmission system
Coaxial cable
Introduction
Coaxial cable refers to a cable with two concentric conductors, and the conductor and shielding layer share the same axis. It is another kind of wire widely used in computer networks. Because it wraps the insulating material outside the main line, and there is a layer of woven shielded metal wire outside the insulating material, it can well block external electromagnetic interference and improve communication quality.
advantage
It can support high-bandwidth communication on relatively long lines without repeaters, and its shortcomings are also obvious: First, the volume is large, and the diameter of the thin cable is 3/8 inches thick, which takes up a lot of space in the cable duct; It is unable to withstand tangles, pressures and severe bends, which will damage the cable structure and prevent signal transmission; the final cost is high, and all these shortcomings can be overcome by twisted pair. Therefore, in the existing LAN environment, It has basically been replaced by the Ethernet physical layer specification based on twisted pair.
classification
Coaxial cables are divided into thin cables (RG-58) and thick cables (RG-11).
The diameter of the thin cable is 0.26 cm, the maximum transmission distance is 185 meters, and it is used with a 50Ω terminal resistance.
The diameter of the thick cable (RG-11) is 1.27 cm, and the maximum transmission distance is 500 meters. Because the diameter is quite thick, it has poor flexibility and is not suitable for installation in a narrow indoor environment. Moreover, the manufacturing method of the RG-11 connector is relatively more complicated and cannot be directly connected to a computer. It needs to be connected through a switch Turn the device into an AUI connector, and then connect it to the computer. Because the thick cable is stronger and the maximum transmission distance is longer than the thin cable, the main purpose of the thick cable is to play the role of the network backbone, connecting several networks formed by thin cables. The impedance of the thick cable is 75Ω.
