Chapter 1 - Internetworking - 02

Figure 1.2  A switch can replace a hub, breaking up collison domain

COLLISION DOMAIN

Before start to talk about this picture, what we called "collision domain" is a network segment connected by a shared medium or through repeaters where simultaneous data transmissions collide with one another[1]. The collision domain is a scenario in which when a device send out a message to the network, all other devices which are included in its collision domain have to pay attention to it, no matter if it was destined for them or not. This causes a problem, because, in a situation where two devices send out their message simultaneously, a collision will occur leading them to wait and re-transmit their respective messages, one at a time.

 

SWITCH

Actually every port on a switch is in a different collision domain, it means switch is a collision domain seperator. So messages that comes from devices connected to different ports never experienced a collision. Let's back to the Figure 1.2, This network is still using one broadcast domain, and it extended the one collision domain from the switch port. Of course that it replaced main hub with a switch. Basically, hubs don't segment  a network and they just connect network segments together. So it's inexpensive way to connect couple of PCs together. However, there is still a problem with a switches. They never break broadcast domains, means it is not a broadcast domain seperator. All the ports on the switch are in still in a single broadcast domain.If a device sends a broadcast message, it will still cause congestion.

 

[1] https://en.wikipedia.org/wiki/Collision_domain