Collision Domain Explained

An in-depth understanding of collision domains is integral to efficient network design. The term “collision domain” refers to a network segment where packet collisions can occur. In other words, if one device’s digital data transmissions, or frames, can collide with the frames of another device, these devices are part of the same collision domain.

Frame collisions reduce a network’s efficiency because the network devices must retransmit the packets, which takes up bandwidth.

A Hub Creates a Single Collision Domain

A hub is a Layer 1 network device that repeats a frame to every interface that connects with it. Consequently, all wireless or ethernet hub interfaces are in the same collision domain.

For example, suppose you have a network that consists of four PCs that connect to each other via a hub. When one of these computers transmits a frame, the hub will repeat this transmission to the other three computers. If one or more of the other three computers source a frame at the same time, it causes a frame collision.

Adding another hub to this network will extend the collision domain. For example, if this second hub also connects to four computers, you will now have eight computers that form part of one collision domain.

Collision Domain Examples

Here is a network diagram of a collision domain.

collision domain example

PC1, PC2, and PC3 are all connected to a hub so they are part of the same collision domain.

If PC1 and PC2 send a frame to PC3 at the same time there will be a packet collision.

packet collision

How to Separate Collision Domain Networks

Separating collision domains minimizes frame collisions and prevents your network from reaching its usability threshold.

You can separate a collision domain by adding a bridge to the network. A bridge is a network device that operates at Layer 2 of the OSI model.

Instead of repeating a frame to every connected interface, a bridge learns the address of each interface. Then, the bridge directs the frames to their correct destinations. Every interface that connects to a bridge is a separate collision domain.

switch collision domain example

Alternatively, you can use a switch to separate a collision domain in your network. A switch is also a Layer 2 network device but provides more functionality than a bridge.

Even though bridges and switches prevent frame collisions, they still allow broadcasts between interfaces. In other words, these Layer 2 devices separate collision domains, but they don’t separate broadcast domains.

A router, which is a Layer 3 network device, also separates collision domains. One of the similarities between collision domains and broadcast domains is that a router separates both types of network segments.

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

CSMA/CD is a Media Access Control (MAC) protocol to prevent frame collisions in a local area network.

This protocol monitors a shared transmission medium to prevent devices from simultaneously transmitting data. The protocol allows only one device to send a data packet when the medium is free. You can implement this protocol to eliminate wasteful transmissions and optimize your network’s efficiency.

CSMA/CD is ideal for local area networks that use ethernet technology. However, networks with switches as collision domain separators don’t require this protocol.

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