Cable sequence, twisted pair wires, and direct connections

A lan is to connect individual computers or terminals using networks to exchange information and share resources according to certain protocols. Commonly used network cables include twisted pair wires, coaxial cables, and optical fiber cables. Twisted Pair wires can be divided into shielded twisted pair wires (STP) and unshielded twisted pair wires (UTP) based on whether they are added with the screen layer of a wire mesh ). Most local networks use unshielded twisted pair wires (UTP-unshielded twisted pair) as the transmission medium for cabling.
The UTP network cable consists of twisted pair wires of a certain length and RJ45 crystal heads.
Twisted Pair wires are divided into four pairs by eight lines of different colors. The effect of twisted pair is to minimize the impact of electromagnetic radiation and external electromagnetic interference. In EIA/TIA-568 standards, the twisted pair wires are divided:
Category 3, category 4, and Category 5. Three types of cables and five types of cables are most commonly used in the network. Currently, more than six types of cables exist.
Connect the RJ45 crystal head to the RJ45 socket of the network device, such as the NIC or hub. The RJ45 plug-in seat is also divided into three types or five types of electrical characteristics. RJ45 crystal head consists of metal parts and plastic, making the network cable required RJ 45 crystal connector front end has 8 concave monk, referred to as "se" (Position, position ).
There are a total of 8 metal contacts in the groove, referred to as "8C" (contact, contact), so the industry has a "8p8c" alias. Special attention is the RJ45 crystal head pin number. When the metal is facing us, the serial number from left to right is 1 ~ 8. The serial number is very important for network connections and cannot be mistaken.
The wiring standards of EIA/TIA define the line sequence 568a and 568b of the two twisted pair wires.
568a standard:
Green white -- 1, green -- 2, orange white -- 3, blue -- 4, blue white -- 5, orange -- 6, brown white -- 7, brown -- 8
568b standard:
Orange white -- 1, orange -- 2, green white -- 3, blue -- 4, blue white -- 5, green -- 6, brown white -- 7, brown -- 8
("Orange White" refers to light orange, or cables with orange dots or bars on the white line. The same applies to green white, brown white, and blue white ).
The order of the twisted pair and the serial number of the RJ45 head must be one-to-one.
To maintain optimal compatibility, the EIA/TIA 568b standard is widely used to create network cables. Note: Only one network cable standard should be used in the entire network wiring. If the standards are not uniform, several people may be confused when working together. More seriously, once a cable error occurs during the construction process, it is difficult to find and remove the bundled cables. I strongly recommend that the 568b standard be adopted.
In fact, 10 M Ethernet cables only use 1, 2, 3, 6 Core lines to transmit data, that is, 1, 2 for sending, 3, 6 for receiving, in color: orange and orange are used for sending; green and green are used for receiving; 4, 5, 7, and 8 are bidirectional lines.
M and M NICs need to use four pairs, that is, all eight core wires are used to transmit data. Because 10 m NICs can be made in M mode, and the twisted pair wires provide four pairs, the 10 m NICs are generally made in M mode.
In addition, according to the different network devices connected to the two ends of the network cable, the network cable can be divided into two types: Direct Line (parallel line) and cross line. A straight-through line (parallel line) is a network line created according to the 568a or 568b standards described earlier. On the other hand, the line sequence of the crossover line changes a little on the basis of the straight-through line, that is, one and three pairs and two and six pairs at one end of the cable. That is to say, the end of the crossover line remains unchanged (in the straight line order). At the other end, 1 and 3 are called, and 2 and 6 are called.
The line order at both ends of the crossover line is as follows:
One end (unchanged) and the other end (adjust two)
Orange white 1 3 green white
Orange 2 6 green
Green white 3 1 orange white
Blue 4 4 blue
Blue white 5 5 blue white
Green 6 2 orange
Brown White 7 brown white
Brown 8 8 Brown


For connection:
1. Host and switch/hub;
2. router and switch/hub

The crossover line is used for connection:
1. Switch and switch;
2. Host and host;
3. Hub and hub;
4. Hub and switch;
5. direct connection between the host and the router

In practice, it is generally understood as follows:
1. Cross-line connections are used between devices of the same type, and direct-line connections are used between devices of different types;
2. vrouters and PCs belong to DTE devices, and vswitches and hubs belong to DCE devices;
3. RJ45 network connection practices generally include 568a and 568b standards. The same standard is the same as the express Connect line, and different standards are the cross lines.
No matter how the wiring, after the completion of the test with a RJ-45 wire meter, the 8 indicators should flash in turn.