CCNA / Network+ Certification Exam Tutorial: Collision Domains And Switches

In the previous Network+ exam tutorial, we talked about how collisions occur when data sent by two hosts on a shared Ethernet segment transmit data at the exact same time.  Collisions result in the colliding data being unusable, which means the hosts must retransmit the data - and all this extra activity slows the network down!

Carrier Sense Multiple Access with Collision Detection helps limit the number of collisions, but by dividing one large collision domain into smaller collision domains, we can further reduce or actually eliminate the number of collisions.  That's where switches come in!

A collision domain is defined as a group of hosts whose data can be involved in collisions if they transmit at the same time.  If three PCs are all on the same network segment, that's a collision domain.  To many newcomers to networking, it sounds like you’d rather have one large collision domain than multiple smaller ones.  That’s what I thought as well when I started!  What you must keep in mind is that the smaller the collision domain, the smaller the number of hosts in the domain – and the fewer hosts we have in a collision domain, the less chance we have of collisions.

And wouldn’t it be great if we could create a collision domain with only one host? Well, we can, thanks to switches!

Traditional switches run at Layer 2 of the OSI model, with many new switches able to operate at both the data link (L2) and network (L3) layers. Switches help to eliminate the chance of collisions because each port on the switch is actually its own little collision domain!  If we take the three PCs mentioned before and connect them each to their own switch port, they literally cannot be involved in collision with each other - and in networking, we'll take all the guarantees we can get!

While switches do help lower the number of collisions, by default they're not much help with broadcasts.  In the next Network+ tutorial, we'll talk about broadcasts, why they're important, and why it's important to keep the number of unnecessary broadcasts to a minimum.  Until then, keep studying!