This is a series of posts designed to help readers understand how the Internet works.  This specific post looks directly at collision domains.

Ethernet uses a process called “Carrier Sense Multiple Access with Collision Detection” or CSMA/CD for short.  This is a very long way of explaining the process of how network adapters can share the same media to communicate.  Think about it if you have 10 machines on a network that are all sharing the same wires or devices, how can any of the devices understand anything with all those frames?

Devices on the same collision domains share media but need to make sure that there is no traffic on the lines before transmitting data.  The devices will listen on the line for traffic and when it believes it’s safe to transmit, will send the data frames out on the line.  If another machine transmits at the same time, the electrical signals will “collide” and transmissions won’t go through.  If this happens, they will each wait a random period of time and try to re-transmit.  This process is standardized in IEEE 802.3 http://www.ieee802.org/3/.

Now that you know how collisions work, it’s easy to see why adding more and more devices to a collision domain can cause problems.  Below we’ll explain a bit how we can overcome these issues.

Hubs

A network hub is a form of a repeater in the sense that any incoming transmissions get repeated out all of the rest of the ports.  Hubs operate at Layer 1 of the OSI model so they are essentially like a multi-port network interface card (NIC).

Here is what a collision domain would look like if you had four machines connected to a network hub.

Switches

A network switch is similar to a hub because it has multiple ports, but this device works at layer 2 of the OSI model (data link layer) so these devices understand MAC addresses.  Since these devices know MAC Addresses, they can determine what devices are connected to each port.  So, when they receive a frame destined for another machine, they only need to send out the frame on that single port.  This dramatically increases the number of collision domains and decreases the number of collisions.

After looking at the two diagrams, it should be easy to see why switches can increase network performance even if the port speeds are the same as a switch.