What is a collision domain?

Alongside various other network technologies you will often hear the term “collision domain”. What is behind this phenomenon and how does a collision domain compare to a broadcast domain? We will give you the answers below.

What is a collision domain?

A collision domain is a closed network, which among other things is composed of lines and stations in layer 1 of the OSI model.

The main feature of a collision domain is that only one device can send data via a physical transfer medium. As soon as two or more devices try to transfer data a collision is caused. This means that the data being transferred doesn’t arrive at its destination. The reason for this is that when accessed by multiple devices the voltage in the medium increases and the signal overlaps. The more devices accessing the transfer medium at the same time, the more likely it is that a collision will occur. A long distance between individual stations can also cause collisions.

How the CSMA/CD process works to recognize and avoid collisions

A tried and tested way of easily recognizing and avoiding collisions is carrier sense multiple access/collision detection – or CSMA/CD. With regard to ethernet, this flow control must meet the IEEE 802.3 standard.

During the process, the stations connected in the domain monitor the voltage which is in the medium. As soon as two stations send data at the same time, the overlapping signals exceed the previously defined values. This means a collision is detected and the process of sending the data will be reattempted after a short time. However, if the medium is free then the data can be sent without any problems.

What are the conditions for a collision domain’s size?

So that the CSMA/CD works as desired, a maximum of 1,023 stations can be connected in the affected ethernet. In addition, no more than five cable segments and four repeaters can be used and only three segments can be connected to end stations. If these maximum values are exceeded the collision domain will be too large and collisions will no longer be recognized by the process set out above.

How a collision domain is defined

How you define a collision domain depends on which network segments belong to them. Bridges and switches operate essentially as separating stations. Both components are connection elements which belong to layer 2 of the OSI model and are used to forward data packages. Switches usually access more than two ports. Each of these ports is connected to a station with a cable and, therefore, creates its own collision domains.

The same doesn’t apply if a hub belongs to a network. Unlike bridges and switches, hubs work in layer 1 of the OSI model. They connect multiple network stations with one another and have almost the same function as a distributor. A hub and the stations connected to it create a separate collision domain. This set up means that only one station can share data signals with the hub at any time. If another device wants to send data to the hub, it will not be able to.

Disadvantages of a collision domain

Simple collisions are part and parcel of using different media and are generally no great big problem. However, this only applies when they don’t happen often. High collision rates can disrupt data traffic and mean data sending processes need to be stopped. For this reason collision domains aren’t or are seldom scalable since collisions occur too often causing the networks to be overloaded. This then results in sensitive output losses.

This is a problem because scalability is essential for a company’s long-term success. The key word being “economies of scale”. The CSMA/CD process is essentially a useful solution to recognize collisions but managing and maintaining the protocol is sometimes rather complex.

What are the differences between collision and broadcast domains?

Essentially both collision domains and broadcast domains are phenomena, which describe logical partitions of a network. However, there is one difference between the levels and restrictions.

A broadcast domain is a cluster, containing one of more collision domains. In a broadcast domain, broadcast data traffic is sent and received. A restricting component may be either a VLAN or a router. If two devices are connected via routers, bridges or switches then this will not cause a collision. However, a collision may occur between the end device and the switch. The reason for this is due to collision domains and broadcast domains operating on different layers of the OSI model.