Collision Domain Explained

A collision domain is a group of devices that share collision. If two or more devices in a collision domain send data frames at the same time, data frames collide in the wire and prevent all other devices from sending data frames until the collision clears in the wire.

What is a collision domain?

If you want to connect only two devices in a network, you can connect them directly through a cable. Since both devices are connected through a direct cable, they do not face any collision.

But, if you want to connect more than two devices, you need to use a central device to connect them. You can use a repeater, hub, bridge, or switch. Based on the device you use to connect them, they will face collision.

If you use a hub or repeater to connect them, all devices share the same collision. If you use a bridge or switch, only devices connected to a port share collision.

Since the devices that share collision belong to the same collision domain, a hub and a repeater create a single collision domain while a bridge and a switch create a collision domain per port.

Single collision domain

Hubs and repeaters are layer-1 devices. A hub is a multiport repeater. The main difference between a hub and a repeater is a hub has more ports than a repeater. A repeater is mainly used to amplify signals. A hub is mainly used to connect multiple devices. So far functionality is a concern, hubs and repeaters are the same. They work with signals.

When an electrical signal enters one hub port, the hub repeats that electrical signal out of all other ports (except the incoming port). The hub has no concept of Ethernet frames.

If two or more devices transmit signals at the same time, the signals collide and become garbled. The hub repeats all received electrical signals, even if it receives multiple signals at the same time.

Let's take an example.

The following figure shows an example network. In this network, PC-A and PC-B send their frame at the same time. The hub repeats both frames and forwards them toward PC-C. Frames collide on the left side of the hub.

Hub per port collision

Per port collision domain

If we replace the hub with a switch, the switch prevents collision on the left side. The switch operates at Layer 2. It reads the frame's destination address to make a forwarding decision. If it needs to forward both frames from the same port, it forwards them in a queue.

switch per port collision

Key points

  • A collision domain is a group of devices that share collision.
  • In a collision domain traffic generated by one device adversely affects other devices in the same collision domain. It does not affect devices outside the collision domain.
  • A hub or a repeater creates a single collision domain.
  • A bridge or a switch creates per port collision domain.

ComputerNetworkingNotes CCNA Study Guide Collision Domain Explained