One topic that is introduced early on in the CompTIA Network+ study guide is about collision and broadcast domains. They are not explained until Chapter 5 though.
Devices listen for and generate frames on the network. With wired networks, a device transmits a frame over the wires while also listening to the wires to see if there’s been any corruption. This is Carrier Sense Multiple Access Collision Detection (CSMA/CD) media contention access.
We’ve all been in conversations where Person A is talking and then Person B starts talking over them. If both people continue talking, I usually hear garbage and give up on both simultaneous conversations. Thankfully, social customs usually work out that one person stops talking and lets the other person continue. A device will give up sending its message as soon as it “hears” not what it sent out. That means that if two frames are sending exactly the same thing…well, you actually have a bigger problem on the network (the MACs should differ and those are transmitted early on in the frame).
By contract, wireless devices use Carrier Sense Multiple Access Collision Avoidance (CSMA/CA) because they have no way of looking for collisions in the air. CSMA/CA is two people who are very polite. They listen to see if anyone else is talking, then they talk and wait for an acknowledgement. Pretty neat.
Collision domain
A collision domain consists of all the devices that could talk over one another. If you’re in at a dinner party in the ballroom at a hotel, the room is one giant collision domain. Good luck hearing the people across the table from you.
Network devices called hubs create one collision domain. The hub is the ballroom in which your dinner party is hosted. Every port is a table. The hub radiates every bit from every port to every other port. Talk about a noisy room!
Network devices called switches break up collision domains. Switches separate the ballroom into individual (smaller) conference rooms which brings down the noise level in each room. Each conference room is a port, and only devices within the conference room (attached to the same port) will collide with one another . You can hear Aunt Martha at your table now.
Network devices called routers also break up collision domains. Routers look similar to switches for this conversation with one collision domain per port.
One thing to note about hubs and counting collision domains. If you have a hub connected to another hub, there is still only 1 collision domain and it’s big.
Broadcast domain
A broadcast domain consists of all the devices that could send and receive a broadcast frame. A broadcast frame is a special frame that is meant to be delivered to all devices. It has a MAC address of FF:FF:FF:FF:FF:FF. Broadcast frames are useful when devices first come on to the network and need to find services or other computers. More on this later.
Hubs radiate broadcast frames because they radiate everything. Switches also radiate broadcast frames. Routers also break up broadcast domains – one broadcast domain for each port. Unlike switches, routers do not pass on broadcast frames from one port to the other. Routers are kind of like the hotel staff making sure that a person only gets into the dinner party they were assigned to; no crashing parties.
| Network Device | Number of Collision Domains | Number of Broadcast Domains |
| Hub | 1 | 1 |
| Switch | 1 per port | 1 |
| Router | 1 per port | 1 per port |
One interesting topic to look into is broadcast storms. A broadcast storm is when a broadcast frame is continuously transmitted on the network. It just won’t die!
This can happen on your network when there is a loop. This is more technically called a spanning tree issue. There’s a great animated video here by Kevin Wallace Training, LLC.
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