What does it mean that all PCs under the shared Ethernet hub belong to the same collision domain?

First of all, you should understand the meaning of conflict domain. When two nodes on the network try to send data at the same time, conflicts will occur, so the two nodes are in the same collision domain, that is, the traditional Ethernet, that is, * * * shared Ethernet. Now switched Ethernet will not conflict, because each port of the switch is an independent collision domain, and different ports belong to different collision domains. Pay attention to the working level of network equipment here. Generally speaking, switches are at the second layer of OSI, while hubs work at the first layer of OSI. The function of the hub is only to regenerate the received signal and then send it out from all interfaces. It can only work in half-duplex mode. In other words, it runs CSMA/CD, while the switch does not need to run CSMA/CD, and it works in full-duplex mode.