As an Internet fan and an Internet enthusiast, why don’t we unveil the veil of the Internet to see what’s inside the Internet? OK, let's talk about the hub.
Basic concepts
Among the networks we often come into contact with, most of them are the simplest small LANs in the network, and their composition is simple. Its composition is generally composed of computers, network cards, network cables, and hubs. In such a small network, the hub has the greatest contribution, and all computers in the network pass on it to transmit information.
What is a hub? The current Ethernet topology mainly uses unshielded twisted pair as the transmission medium, and the maximum distance of each network segment is 100 meters. If more than 100 meters, repeaters need to be used to expand the topological distance of the network. This is because after a long distance transmission, the signal will cause signal attenuation. The function of the repeater is to sort out the signal that has been attenuated and become incomplete, and then regenerate the complete signal before continuing to transmit. Repeater is the predecessor of ordinary hubs. A hub is a multi-port repeater. Taking the hub in (Figure 1) as an example, the hub generally has a BNC connector, an AUI connector and a number of RJ45 interfaces of 4, 8, and 16. Foreigners call ordinary hubs HUB, and the original meaning of E text has the meaning of "center". It seems that hubs are indeed very important. Let's go along with the flow and call it HUB.
External structure of HUB
Let's take (Figure 1) as an example to see what they really look like.
The above figure shows a common 10BASE-T shared hub of TP-Link. It has a BNC port, an AUI port (Figure 2) and 16 RJ45 ports.
BNC port (Figure 3): It is a standard thin cable interface. It can connect to 50Ω coaxial cables in the 10BASE-2 network standard. Since most of the common Ethernet networks we nowadays are 10BASE-T or 100BASE-T. Therefore, our BNC headers have been basically eliminated. But if you want to use it as a cascade, its speed also has certain advantages.
AUI port (Figure 3): It is a transceiver interface. It is used to connect signal transceivers connected to thick cables. That is a 10BASE-5 network standard. Like 10BASE-2, it is already a past tense.
Power supply interface (Figure 4): If I don’t mention this interface, you will know it. Yes, how does the hub work without it? Next to the power interface of our hub, the voltage, frequency and maximum carrying current parameters are marked for us, which is very careful.
Power switch (Figure 4): It is a switch. There is really nothing to explain. If you really don’t know what it is for, just ask the kindergarten children, and they will definitely tell you.
RJ45 interface (Figure 5): It is the most commonly used thing we use now. The twisted pair cable connected to the computer network card in our network will be plugged into this interface without exception. RJ45 is an interface form in the 10BASE-T network standard and is now widely used. There are 8 wire slots inside, and the meaning of the wire slot follows the EIA/TIA568 international standard. In the 10BASE-T network, lines 1 and 2 are sending lines, and lines 3 and 6 are receiving lines. The "UTP" marked in the figure refers to an unshielded twisted pair wire.
Cascade port (Figure 6): The role of cascade port is a bit special, so let’s say a few more words. The cascade port is specially used for cascade use between HUBs. In one network, we may have dozens of machines. Take our 16-port HUB as an example. If there are 30 machines in our 10BASE-T network, our 16-port will not be enough.
What to do?
By the way! Wouldn't it be enough to get a few more HUBs!?
After we had 3 16-port HUBs, our RJ45 ports had 4X16 in total, but how could we connect these machines to a network?
Use cascading ports!
There is a cascade port switch next to the cascade port in the figure, and its function is to allocate 1 port as needed. In other words, in a network that does not require cascading, we keep the cascading button in the off state, so that port 1 is like other 15 ports, and can be directly connected to standard twisted pair cables. When we need to cascading the HUB, we put it on. At this time, the transceiver and receiver of port 1 is switched so that we can use a standard twisted pair cable to connect the two HUBs to work together.
Some friends will ask: "What if my HUB does not have a cascading interface?"
It's easy to deal with, then we can artificially change the transceiver and receiver terminal positions of the twisted pair cable.
OK, everything is done!
Our 30 machines are really connected to a network.
(Note: In the 10BASE-T network structure, the HUB cascade cannot exceed level 4)
Indicator light part (Figure 7): In order to better manage and supervise the network, HUB manufacturers have designed some very practical working status indicators for us. Let’s explain the meaning of several commonly used indicator lights:
BNC/AUI: Displays errors caused by BNC/AUI connection. On a general 10BASE-T hub, the Metropolis retains a BNC and AUI interface for connecting coaxial cables. But the hubs at 100BASE-T do not provide them. This light is not illuminated under normal circumstances. If this light is on, it means that there is a problem with the connected coaxial cable, such as the BNC connector is broken, the terminal resistor is loose, short circuit, etc.
Power: Power light. When the HUB has a power supply and the switch is on, the light is on, which means that the HUB has powered on.
COL: Many friends have asked me about the function of this lamp, but it cannot be seen from the literal meaning. In fact, it is the header of Collision, which shows the situation of transmission collisions on the network. Since Ethernet uses the CSMA/CD method to transmit data, the more likely it is to cause signal collisions when there are more workstations. Under normal circumstances, this light does not light up. If a collision occurs in the network at this time, it will flash. The higher the flicker frequency, the more serious the collision is.
Link/Rx: refers to the connection and working status light, which corresponds to the RJ45 interface of HUB, and is the same as its number. When the correct signal is connected to the RJ45 port, the light is on, and it flashes when it has a signal read and write. At this time, it means that information exchange has occurred.
The internal structure of HUB
After we looked at the external structure of HUB, we seem to have a certain understanding of it. Let's open it to see what it looks like inside.
The HUB motherboard shown in Figure 8 is the HUB motherboard. There are many chips that we have never seen before. Fortunately, I am from the power engineering major and have learned so well in electronic technology... several books. Let's take a look at a few main parts.
Power supply part (Figure 9): The HUB's power supply is a process from AC to DC. It is a typical rectifier circuit, and we can see the fuse and main transformer marked by the arrow in the figure. The fuse is used to protect the primary side of the power supply. It will automatically fuse when it is attacked by a large current (exceeding its blowing value) to protect the devices on its inner side. The main transformer is a transformer device, and its principles cannot be explained here. The circles in the figure are some electrolytic capacitors, and the size of their voltage and capacitance can be seen from their head size. There are also some three-terminal voltage regulator and rectifier diodes. Haha, everyone must be annoyed? Let’s not talk about it!
Crystal Oscillator (Figure 10): As we all know, our network is composed of digital circuits. In digital electrons, all signals are composed of 0 and 1, 0 and 1, which are actually high potentials and low potentials in logic circuits.
How do 0 and 1 come about?
It's a pulse!
How does the pulse signal come about?
By the way, that's it! The crystal oscillator we see in the figure generates pulse signals of a certain frequency to realize the transmission of our network information. The figure shows a 20MHz oscillator.
The square-shaped large chip on the left side of the clock chip in the above picture is the main control chip of HUB, which is used to manage and implement HUB work.
How does it work?
Haha, I don't know much more than you.
How HUB works
We have been talking about so much, but I still don’t know how HUB works on the network. Let's talk about its working principle.
As its name suggests, HUB is the center of the network. In network terms, it is a connection point based on the star network topology. The basic function of a hub is information distribution, which distributes signals received on one port to all ports. Some hubs regenerate weak signals before distributing them, while others organize the timing of the signals to provide synchronous data communication between all ports.
In our common LANs, hubs are roughly divided into shared hubs and switched hubs.
Shared hubs cannot improve network performance and cannot detect signal errors. They simply receive data from one port and distribute it through all ports, which is the easiest thing a hub can do. It is an entry-level device for star topology Ethernet, and the hubs in this article fall into this category. The total bandwidth of this hub is 10M/s. If we connect to 4 workstations in total, when these 4 workstations are online at the same time, the flat bandwidth of each workstation will be only 10/4, that is, 2.5M/s.
Switch HUB is different from shared hubs, and it has the function of signal filtering. It only transmits the signal to ports of a known address and does not transmit the signal to all ports on the network like a shared hub.
In addition, each port on a switched hub has dedicated bandwidth, which allows multiple ports to talk simultaneously without affecting each other. The switched hub can transmit data in pass-through transmission, storage-forwarding and improved pass-through transmission, which is much more efficient than the shared hub.
Above, we use a shared entry-level hub as an example to analyze and illustrate the structure and simple working principle of the hub. Since I have always been learning online knowledge, the content in the article is inevitably thin. I hope that experts who see this article will criticize and correct it more.
Article entry: csh Editor in charge: csh