Collision Domain

A collision domain is a section of a network where devices share the same transmission medium.

If two devices try to send data at the same time, their signals interfere, causing a collision.
As the name suggests, it is a zone where collisions can happen.

Figure 1 – All devices connected to the hub share the same collision domain

When this happens, the data gets corrupted and must be resent. This causes delays and negatively impacts network performance. The more devices that share the same collision domain, the higher the risk of collisions.

Let’s now see how this works in practice.

A hub operates at Layer 1 (Physical Layer) of the OSI model.
It is a dumb device that forwards data to every port, regardless of the destination. It doesn’t inspect traffic or make any forwarding decisions.

In other words, all ports on a hub belong to a single collision domain.

Case 1 — No Collision

In a hub-based network, imagine PC1 sends data to PC2.

Figure 2 – hub broadcasts it to all ports without causing a collision

Since the hub broadcasts the signal to all ports, PC3 also receives the frame, but it simply ignores it.
As only one device is transmitting, there’s no collision.

Case 2 — Collision Occurs

Now, suppose PC2 and PC3 both try to send data to PC1 at the same time.

Figure 3 – PC2 and PC3 send data simultaneously, causing a collision

Since all ports on a hub share the same collision domain, their signals collide on the link going to PC1 (Gi0/1).

This collision corrupts the data, forcing both PCs to wait and retransmit, which introduces delays.

As networks grew, hubs became a major performance bottleneck due to frequent collisions. To address this, bridges were introduced.

To reduce collisions, bridges were introduced as an improvement over hubs.