The most commonly used communication cables in emergency communication networks are twisted-pair cables. Common twisted-pair cables are analog telephone twisted lines in analog systems and ethernet cables & fiber optical cables in SIP VOIP communication systems.
As its name suggests, the design of twisted pair is composed of several “pairs”, and each pair (referring to two thin copper wires wrapped with insulating material) is twisted with each other at a certain ratio, and the appearance is a bit like Thin hemp rope, and each thread is generally only 1 mm in diameter. Each pair of wires are insulated from each other, which is like the relationship between countries, you can cooperate closely but keep a distance.
Twisted pairs are classified according to the number of pairs, including 1 pair, 2 pairs, 4 pairs, 25 pairs, 50 pairs, etc. 1 pair, and 2 pairs are used for telephone, 4 pairs are used for Ethernet transmission, and more than 25 pairs are telecom cables.
According to whether there is a shielding layer, it is divided into two categories: shielded twisted pair (STP) and unshielded twisted pair (UTP). The performance of STP is good, while the price of UTP is low. UTP is currently the first choice for commercial use.
According to the frequency and anti-interference ability, it can be divided into CAT3, CAT4 types, CAT5 types, super CAT5 types, CAT 6 types, CAT 7 types, etc. Ethernet communication requires more than 3 types. Because interference is ubiquitous, cables cannot be “out of silt without staining.”
The most peculiar part of twisted pair is the twisted pair. Why are the copper wires “twisted” together? Why can’t the line pairs be parallel to each other like a pair of very long chopsticks? It turns out that twisting the pair of wires together serves to resist interference, that is, to prevent “noise” from affecting data.
The “noise” in physics is different from the “noise” in daily life. Many “noises” in physics do not make any sound (from the perspective of a human being, making a sound means that people can hear the sound). They are other electromagnetic interference during transmission, such as placing a mobile phone next to a cable. The electromagnetic wave emitted by the mobile phone will affect the magnetic field of the cable because this electromagnetic wave is also within the frequency range of the copper metal, which will cause damage to the data being transmitted by the copper cable. Twisted pair is a relatively cheap transmission medium to reduce this damage.
It is worth noting that the twisting of copper wires does not make the noise disappear. Instead, the noise is offset by mathematical and physical methods to restore the real data signal. Although the anti-interference ability of twisted-pair cable is strong, it cannot be compared with coaxial cable. Therefore, when transmitting multiple voices over long distances, coaxial cables (or optical cables) are generally chosen instead of twisted pairs. The first wide application of twisted-pair cable occurred in the transmission of voice, which is the wire that connects the telephone-the telephone line. The telephone line generally uses one or two-wire pairs, which are used between the telephone switch and the telephone, and the two ends are connected to the RJ11 crystal head. The RJ11 connector is used to plug into the RJ11 jack of a telephone or PBX. The production method is similar to that of the five-category cable, except that the RJ11 crystal head generally has only 4 metal pieces, and the volume is about half smaller than the RJ45 crystal head. Ethernet also commonly uses twisted-pair cables.
Ethernet twisted pair cables are generally divided into two types: STP and UTP. Shielded twisted pair (STP, Shielded Twisted Pair) has good anti-interference and high performance. When used in remote trunks, the maximum distance can reach more than ten kilometers. This distance has broken through our general “LAN”. High performance determines its high cost, and optical fiber can completely replace it with better performance and lower cost, so STP has not been widely used. Unshielded twisted pair (UTP, Unshielded Twisted Pair), the transmission distance is generally about 100 meters, the most commonly used Super Category 5 cable, can support 100/1000Mbit/s Ethernet connection, is the preferred transmission medium for connecting desktop devices.
As long as the distance between the host and the switch and the road between the routers is within 100 meters, unshielded twisted pair cables of category 5 or more can be used. The twisted pair transmission Ethernet uses 4 wire pairs, and both ends of the cable should be inserted into the RJ45 crystal head.
There are 4 pairs of Category 5 wires, a total of 8 wires, the color of the outer insulating material of each wire is not exactly the same, and their twisting method is four sets of twisting, including orange white and orange, green-white, and green, blue white and blue, brown white and brown. In engineering installation, generally follow the sequence of orange-white, orange, green-white, blue, blue-white, green, brown-white, and brown. The two ends of the cable are arranged and inserted into the RJ45 crystal head, and special tools are required to press.
The RJ45 crystal head is composed of a metal sheet and a plastic jacket. The front end of the RJ45 crystal head required to make the network cable has 8 grooves, called “8P” (Position), and there are 8 metal contacts in the grooves, referred to as 8C (Contact ), so the industry has another name for this “8P8C”. Special attention should be paid to the serial number of the RJ45 crystal headpin. The serial number is very important for the network connection and cannot be reversed.
In many projects, we will also be consulted by customers about whether industrial telephones can use optical fibers. The answer is yes. Our method is a combination of SIP VOIP telephone and optical transceiver to connect to the optical fiber.
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