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.
For this reason, although the hub technology is constantly improving, it has essentially added some switch technologies, and developed into today's stacked hub technology, and some hubs also have the function of intelligent switches. It can be said that hub products have been technically transformed to switch technology, and have certain intelligence and data exchange capabilities. However, with the continuous decline in the price of switches, the only price advantage is no longer obvious, and the market of hubs is getting smaller and smaller, and it is on the verge of elimination. Nevertheless, hubs are economically attractive to families or small businesses, especially suitable for networks with several computers at home or small and medium-sized companies as branch networks.
The hub is a multi-port repeater. When the HUB is the central equipment, one line in the network fails, which does not affect the work of other lines. Therefore, hubs have been widely used in local area networks. Most of the time, it is used in star and tree network topology, and is connected to the host through RJ45 interface (BNC interface is also available). According to different statements, there are many kinds of hubs.
According to the way of processing input signals, hubs can be divided into passive hubs, active hubs and intelligent hubs.
Passive hub: it is the lowest kind (the word is a bit _), which does not process the signal, does not extend the transmission distance of the medium, and has certain influence on the signal. Each computer connected to the hub can receive signals from all other computers on the same hub;
Active hub: the difference between active hub and passive hub is that it can amplify or regenerate signals, thus extending the effective transmission distance between two hosts;
Smart HUB: As soon as I heard this word, I knew this guy was definitely better than those two! In addition to all the functions of active hubs, smart hubs also have network management and routing functions. In the intelligent hub network, not every machine can receive the signal, and only the computer with the same destination address can receive the signal. Some smart hubs can choose the best path by themselves and manage the network well.
According to other methods, there are many kinds, such as 10M, 100M, 10/ 100M adaptive hub, and so on, which will not be introduced here. In short, the market price is not so expensive now, try to buy a better one.
If we often contact the network, we should be familiar with the hub, the basic equipment for building a local area network, but how much do we know about the knowledge behind the hub?
I. Definition of Hub
Hub belongs to the basic equipment in data communication system. Like twisted-pair transmission media, it is a hardware device that needs no software support or little management software. Widely used in various occasions. Hub works in the local area network (LAN) environment and is applied to the first layer of the OSI reference model like the network card, so it is also called the physical layer device. The hubs are electrically interconnected. When the environment for maintaining LAN is logical bus or ring structure, a physical star or tree network structure can be established with the hub. In this regard, the hub acts as a multi-port repeater. In fact, the hub is actually a kind of repeater. The only difference is that the hub can provide more port services, so the hub is also called a multi-port repeater.
The external board structure of common hub is very simple. For example, the simplest 10BASE~T EthernetHub of D-Link is a cuboid, with an AC power socket and switch on the back, an AUI interface and a BNC interface, and a row of 17 RJ-45 interfaces distributed in most positions on the front. On the right side of the front, there are LED interface indicators and LED status indicators corresponding to each RJ-45 interface. From the appearance, high-end hubs are not much different from modern routers or switched routers. Especially the modern two-speed adaptive Ethernet hub, because of the built-in switching module that can realize the communication between internal 100 MB/s and 100 MB/s network segments, this hub can completely realize the communication exchange between nodes in the network segment with the hub as the node. Sometimes, this kind of switching hub is simply called a switch, which makes it difficult for users who use the hub for the first time to distinguish it correctly. However, it is a simple method to distinguish hubs according to the type of backplane interface.
Second, the working characteristics of the wheel hub
According to the IEEE 802.3 protocol, the role of hub is to randomly select the devices at a certain port, so that it can monopolize all the bandwidth and communicate with the upstream devices (switches, routers or servers, etc.). ) hub. It can be seen that the hub has the following two characteristics when working.
First of all, the hub is just a multi-port signal amplification device. When a port receives a data signal in operation, because the signal has been attenuated in the transmission process from the source port to the hub, the hub shapes and amplifies the signal, so that the attenuated signal can be regenerated (restored) to the state when it was sent, and then forwarded to all other ports in operation. It can be seen from the working mode of Hub that it only plays the role of signal amplification and forwarding in the network, and its purpose is to expand the transmission range of the network, but it does not have the directional transmission ability of signals. This is a standard * * * shared device. Therefore, some people call the hub "stupid hub" or "dumb hub".
Secondly, the hub only communicates with its upstream devices (such as superior hubs, switches or servers). There is no direct communication between ports at the same level, but information is broadcast to all ports through the upstream devices. It can be seen that even the communication between two different ports of the same Hub must go through two steps: the first step is to upload the information to the uplink device; The second step is to connect devices and broadcast information to all ports.
However, with the development of technology and the change of demand, many hubs have broadened their functions and are no longer affected by this working mechanism. The network composed of Hub is a * * * shared network, and Hub can only work at half-duplex.
Hub is mainly used to set up a * * * shared network, which is the most economical scheme to directly solve the problem from server to desktop. In a switched network, the hub is directly connected to the switch and sends the data of the switch port to the desktop. Hub networking is flexible. It is located on a star node of the network, and centrally manages the workstations connected to the node, so that the faulty workstations will not affect the normal operation of the whole network, and users can freely join and quit.