"Hub" in English. "Hub" means "center". The main function of the hub is to regenerate, shape and amplify the received signal, so as to expand the transmission distance of the network, and at the same time concentrate all nodes on the node centered on it. It works in the first layer of the OSI (open system interconnect) reference model, that is, the "physical layer". Hub, like network card, network cable and other transmission media, belongs to the basic equipment in LAN and adopts CSMA/CD (a detection protocol) access mode.

Hub belongs to the bottom equipment of pure hardware network, and basically does not have the ability of "intelligent memory" and "learning" similar to switches. It also has no MAC address table owned by the switch, so it sends data by broadcast, not as a target. That is to say, when it wants to send data to a node, it does not send data directly to the destination node, but sends data packets to all nodes connected to the hub, as shown in the figure.

This way of sending data by broadcasting has two shortcomings: (1) User data packets are sent to all nodes, which easily brings about unsafe factors in data communication, and some people with ulterior motives can easily intercept other people's data packets illegally; (2) Because all data packets are sent to all nodes at the same time, the above-mentioned * * * bandwidth sharing mode is more likely to cause network congestion and further reduce the network execution efficiency. (3) Non-duplex transmission leads to low network communication efficiency. At the same time, each port of the hub can only carry out data communication in one direction, and it can't carry out bidirectional duplex transmission like a switch, so the network execution efficiency is low and it can't meet the communication needs of a larger network.

Crystal head

The term name of the crystal head is RJ45 connector, which is a transparent plug for connecting a network card or a hub, and is used for connecting both ends of a twisted pair.

Unshielded Twisted Pair

English name is Twist-Pair. It is the most commonly used transmission medium in integrated wiring engineering.

Twisted pair is a pair of insulated metal wires twisted together to resist some external electromagnetic interference. Winding two insulated copper wires together at a certain density can reduce the degree of signal interference, and the radio waves radiated by each wire will be cancelled by the radio waves emitted by the other wire. The name "twisted pair" also comes from this. Twisted pair is generally formed by winding two insulated copper wires of No.22-No.26.. In practical use, twisted pairs are wrapped together in an insulated cable sleeve. There are four typical twisted pairs, and there are more twisted pairs in a cable sleeve. These are called twisted pair cables. In twisted-pair cable (also called twisted-pair cable), different pairs have different twist lengths. Generally speaking, the twisting length ranges from 38. 1cm to 14cm, and it is twisted counterclockwise. The twist length of adjacent pairs is greater than 12.7cm. Generally, the denser twisted pairs, the stronger the anti-interference ability. Compared with other transmission media, twisted pair is limited in transmission distance, channel width and data transmission speed, but its price is relatively low.

Twisted-pair cables are common in Category 3, Category 5 and Super Category 5 cables, as well as the latest Category 6 cables. The former has a thin line diameter, while the latter has a thick line diameter. These models are as follows:

1) Class I line: mainly used for voice transmission (before the early 1980s, Class I standard was mainly used for telephone cables), which is different from data transmission.

2) Class II line: the transmission frequency is 1MHZ, which is used for voice transmission and data transmission, and the highest transmission rate is 4Mbps. It is common in old token networks that use 4MBPS standard token passing protocol.

Class III cable: refers to the cable specified in the current ANSI and EIA/TIA568 standards. The transmission frequency of this cable is 16MHz, which is mainly used for voice transmission and data transmission, and the highest transmission rate is 10Mbps.

4) Class 4)IV cable: The transmission frequency of this cable is 20MHz, which is used for voice transmission and data transmission, and the highest transmission rate is 16Mbps, which is mainly used for token-based LAN and10base-t/100base-t. ..

5) Category 5 cable: this kind of cable is covered with high-quality insulating material, and the transmission rate is 100MHz, which is used for voice transmission and data transmission. The highest transmission rate is 10Mbps, which is mainly used in 100BASE-T and 100 base-t networks. This is the most commonly used Ethernet cable.

6) Super Category 5 line: Super Category 5 line has small attenuation, less crosstalk, higher attenuation crosstalk ratio (ACR) and structural return loss, smaller time delay and greatly improved performance. Category 5 cables are mainly used for Gigabit Ethernet (1000Mbps).

7) Category 6 cable: the transmission frequency of this cable is 1 MHz ~ 250 MHz. The PS-ACR of Category 6 cabling system should have a large margin at 200MHz, which can provide 2 times the bandwidth of Category 5. The transmission performance of Category 6 cabling is much higher than Category 5 standard, and it is most suitable for applications with transmission rate higher than 1Gbps. An important difference between Class 6 and Class 5 is that it improves the performance of crosstalk and return loss, which is very important for the new generation of full-duplex high-speed network applications. The basic link model is cancelled in the six standards, and the wiring standard adopts star topology. The required wiring distance is: the permanent link length cannot exceed 90m, and the channel length cannot exceed1200mm.

At present, twisted pair can be divided into unshielded twisted pair (UTP) and shielded twisted pair (STP). The outer layer of shielded twisted pair cable is wrapped with aluminum platinum to reduce radiation, but it can't completely eliminate radiation. Shielded twisted pair is relatively expensive and more difficult to install than unshielded twisted pair. Unshielded twisted pair has the following advantages:

(1) No shielding sheath, small diameter, saving occupied space;

(2) light weight, easy bending and easy installation;

(3) minimize or eliminate crosstalk;

(4) it is flame retardant;

(5) It has independence and flexibility, and is suitable for structured integrated wiring.

In these two categories, there are 100 ohm cables, double stranded cables, large logarithmic cables and 150 ohm shielded cables.