The IEEE 802 standard gives a standardized reference model for a local area network (LAN). It corresponds only to the data link layer and the physical layer in the OSI reference model, and divides the data link layer into Logical Link Control (LLC) sublayers. Media Access Control (MAC) sublayer. Among them, the IEEE 802.3 standard defines the CSMA/CD bus medium access control sublayer and physical layer specification, which is also the Ethernet (Ethernet) LAN standard. The two elements that make up an Ethernet network connection are Ethernet cards (with transceivers) and transmission media (connection cables).
2. Transmission mediumThe most commonly used transmission medium is twisted pair, and it is divided into shielded twisted pair (STP) and unshielded twisted pair (UTP). According to transmission characteristics, it can be divided into three categories, five categories, super five categories, six categories, seven categories, etc., as shown in the following table.
* More detailed information can be found in [3][6] or other resources. At present, the most commonly used are the five types of unshielded twisted pair. Because of the attenuation of the signal in the transmission medium, the length of the five types of unshielded twisted pairs cannot exceed 100 meters.
3. RJ-45The EIA/TIA (Electronic Industries AssociaTIon/TelecommunicaTIons Industry AssociaTIon) specified unshielded twisted pairs using the RJ-45 interface (ISO8877). RJ (registered jack) is the standard physical interface for telecommunications equipment such as telephones or computer network equipment.
The picture above shows the RJ-45 plug and socket. There are 4 pairs of wires in the twisted pair, so the interfaces are also corresponding to pins 1~8. Note that when discussing the connection sequence and the corresponding relationship with the signal, the pin number is always used as the standard. When the female and male connectors are connected, they always correspond to one another.
4. Wiring method(1) The schematic diagram of the twisted pair is as follows:
(2) Twisted-pair and crystal head (RJ-45 male) connection method There are two standards, one is T-568B (common), the second is T-568A, as shown below:
(3) Straight Through Cable and Crossover Cable
Due to the existence of the above two standards, both ends of a twisted pair can be connected to the RJ-45 crystal head by any standard, and thus the difference between the parallel line and the cross line appears. The parallel line means that the two ends of the twisted pair are connected to the crystal head by the same standard (usually T-568B) and the internal wiring does not cross. The cross line means that the two ends of the twisted pair adopt different standards (T-568B at one end and T-568A at the other end) to connect with the crystal head, and the 4 pairs of lines should cross inside (including the blue pairs and browns that are not used). Right), as shown in the following table:
Parallel lines and crossover lines are used for interconnection of different occasions and different communication devices. The key is to connect Transmit+ and Receive+ of the two interconnected devices through a twisted pair cable, and Transmit- and Receive-Connect, so that the two devices can work properly. Network communication. In general, parallel lines are used to connect to hubs or switches, and crossover lines are used to connect two peer communications devices directly. This is because the interface definitions of peer communications devices usually use the same standard. A simple example Yes, assuming that the correspondence between the Transmit and Receive signals of the computer Ethernet card and the RJ-45 pins is as follows:
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