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.