Describe the basic working principle of the hub through an instance

We know that there is only one physical signal transmission channel in the ring network, which is transmitted through one transmission medium. In this way, there is a conflict between nodes competing for channels and the transmission efficiency is low. After a hub is introduced, each station connects to the hub using its own transmission medium. Each node no longer has one transmission channel, the signals sent from each node are concentrated through the hub, and the hub then shapers and zooms them up and sends them to all nodes, so that there is no collision at least on the uplink channel. However, the hub-based network is still a LAN with shared media. The "sharing" here is actually the internal bus of the hub, therefore, when both the Upstream Channel and the downstream channel send data at the same time, there will still be a signal collision. When a hub detects a collision from its internal port, it generates a collision enhancement signal Jam) and transmits it to the target port connected to the hub. At this time, all data cannot be sent successfully, forming a network "congestion car ".

In this network phenomenon, we can use an image of the actual situation, that is, there are two directions on the bicycle road at the same time, as shown in 1.

Figure 1

We know that only one vehicle is allowed to pass through a bicycle lane, but in small towns, there are usually no such restrictions, and the bicycle lane is likely to allow two vehicles in the same direction, but it must go through at different times. The same is true in the hub. Although the connections between nodes and the hub have their own independent channels, there is only one common channel inside the hub, upstream and downstream data must be sent and received through this shared channel, which may lead to congestion when data is sent at the same time as in a single lane. That's easy to understand, right?

Because of this deficiency of the hub, it cannot be used separately in a large network to share a small part of the network communication load with devices such as switches ), just as in the center of a big city, there is no single lane, because the more the network comes, the more opportunities for network collision. Because of this, the data transmission efficiency of the hub is relatively low, because it can only transmit data in one direction at a time, that is, the so-called "ticket" method. If you want to use a hub as a single set line device in the network, the network size should be less than 10, and the bandwidth of the hub should be more than 10/100 Mbps.

In addition to bandwidth sharing, a hub must also be considered when selecting a hub, that is, its broadcast method. Because the hub is a hardware-only underlying device, it basically does not have the "smart memory" capability, let alone the "Learning" capability. It does not have the MAC address table of the vswitch, so it does not send data in a targeted manner, but uses broadcast to send data. That is to say, when it wants to send data to a node, it does not directly send data to the target node, but sends data packets to all nodes connected to the hub, as shown in figure 2.

Figure 2

This type of broadcast data transmission method has two disadvantages: 1) user data packets are sent to all nodes, which may lead to insecure data communication, some people with ulterior motives can easily illegally intercept others' data packets. 2) because all data packets are sent to all nodes at the same time, in addition to the shared bandwidth method described above, network Congestion is more likely to occur and network execution efficiency is reduced.

1. Differences and connections between switches, routers, hubs, network cards, and other network devices