In Tanenbaum's Computer Networks:

What happens if more than one of the stations or ports wants to send a frame at the same time? Again, switches differ from hubs. In a hub, all stations are in the same collision domain. They must use the CSMA/CD algorithm to schedule their transmissions. In a switch, each port is its own independent collision domain. In the common case that the cable is full duplex, both the station and the port can send a frame on the cable at the same time, without worrying about other ports and stations. Collisions are now impossible and CSMA/CD is not needed. However, if the cable is half duplex, the station and the port must contend for transmission with CSMA/CD in the usual way.

I can't find the definition of collision domain in the book, or by searching.

Is it a definition that is not just for the context in the quote, but also for a more general case?

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    Aug 6, 2017 at 23:55

3 Answers 3


This will be a bit of a layman's answer, rather than quoting spec, but it should be accurate.

The collision domain is the sum total of the network segment (hosts, hubs) affected by the collision, and forced to deal with it by a random backoff delay.

Anytime a collision is detected on an Ethernet line, any and all interfaces on that line are required to stop transmitting. They each then come up with a random amount of time to wait before transmitting again, in an attempt to avoid another collision. In a crowded network, this delay will start to add up very quickly, and the collision domain is essentially the sum total of equipment that delay affects.

As in your quote, it comes up a lot less these days. The original Ethernet gear that filled the role of the switch was a hub, which simply repeated any frame sent to it out all of the other ports. Once a network got even lightly congested, this made collisions a common symptom. Modern switch technology virtually eliminates that problem, since any frame received by the switch is only sent to the host it's intended for. The only possible collision is between a host and the switch, making it a very small collision domain, affecting only that host.

  • Might be a question that better fits a different stack, honestly, but there you go.
    – Radhil
    Oct 4, 2015 at 4:37
  • One correction. If you are transmitting you continue transmitting a jam signal for the slot time. This ensures that all devices in the time domain hear the collision.
    – dbasnett
    Oct 6, 2015 at 11:38
  • "The only possible collision is between a host and the switch, making it a very small collision domain, affecting only that host. " and even that can't happen if the switchport is running in full duplex mode. Oct 12, 2022 at 18:54

A collision domain is all devices connected by shared medium. Originally a collision domain was a physical cable, then repeaters and hubs were added. The shared medium ended at a router port and was temporally bounded (512 bit times).

With the advent of switches the concept of collision domain shrank to the port on the switch, and as was noted collisions do not occur on full duplex ports.

Collision domains are still pertinent in WIFI networks. WIFI uses CSMA/CA.


Well, I see your question as "What is a collision domain". In simple terms, a collision domain, like the name suggests, is an area where collision can occur. Think of it as a narrow tunnel where space for only one car to pass through exists. If two cars pass through, they collide. Hence, the tunnel is known as a collision domain.

Now, let's take this to networks. Like the other answers here, before, these devices called hubs were used to connect various hosts. The hub was also a dumb device, in the sense that whatever data it received from one port, it would simply copy to all the other ports. Anyway, the problem with hubs was that it had only 1 collision domain. What do I mean by that? It meant that if I connected 5 hosts to the hub, it would only allow space (like our tunnel example) for just one system to transfer data at a time. If one host is transmitting data, and another host tried to transmit data, a collision would occur, or the two cars would collide and go boom! So when you talk about a collision domain, we think of a shared medium wherein collisions can occur. Of course, lot of prevention mechanisms came out like switches and CSMA/CD, etc. etc, you can read about those in the other replies.

Collision Domains - Hub

The image above shows that two hosts on either side of the hub tries to send data at the same time and since there is only one collision domain, only one is allowed to send. Two = boom (as shown in the image).

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