Layer 2/Layer 3/Layer 1-7 devices

Layer 2 devices (bridge switches)

 A hub (HUB) unconditionally copies and forwards received traffic to all ports other than the receiving port.

 In other words, traffic will flow even to a port that does not have a communication partner. If unnecessary traffic flows like this, traffic jams are likely to occur.

This area where congestion is likely to occur, formally called a segment, is a collision domain.

 As the number of hosts in a collision domain increases, the probability of collisions inevitably increases. If you send it at the wrong time, it will collide with a packet sent by a host in the same collision domain.

 Network performance suffers when there are many collisions. And in the worst case, the network goes down.