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TechKnowSurge Catalog Videos What’s a Collision?
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What’s a Collision?

Network collisions occur when two devices transmit data simultaneously, and Ethernet handles this through CSMA/CD, a protocol that detects conflicts and manages retransmission. Understanding this process is foundational to how wired networks operate.

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About this video

A network collision happens when two devices attempt to transmit data at the same time, causing their signals to interfere and rendering both transmissions invalid. This is a predictable characteristic of hub-based Ethernet networks, where incoming data on any port is broadcast to all other ports, creating the conditions for simultaneous transmission. Rather than treating collisions as failures, Ethernet is designed with the expectation that they will occur and includes a built-in mechanism for handling them. That mechanism is Carrier Sense Multiple Access with Collision Detection, or CSMA/CD. The carrier sense component means a device checks whether another device is already transmitting before sending its own data. Multiple access reflects the reality that many devices share the same network at any given time. If a collision occurs despite this precaution, the collision detection component triggers both devices to immediately stop transmitting. Once both devices halt, a backoff algorithm assigns each one a random countdown value. Because the values are chosen independently, the two countdowns will almost always differ, allowing one device to begin retransmitting before the other. The second device, still counting down, detects that the line is now in use and waits until it is clear before attempting its own retransmission. This process restores orderly communication after a collision with minimal disruption to overall network traffic.

What you'll learn

What's covered

Ethernet Collisions & CSMA/CD

Aligned to

Cisco CCNA
1.13 Describe switching concepts
CompTIA Network+
1.6 Compare and contrast network topologies, architectures, and types.
1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.
Cisco CCST Networking
4.5 Explain basic switching concepts

Key terms

Carrier Sense Multiple Access with Collision Detection
CSMA/CD
The media access control method used in traditional Ethernet where devices listen before transmitting and detect collisions if two devices transmit simultaneously, then retransmit after a random backoff period. CSMA/CD is defined for half-duplex Ethernet and is not used in full-duplex switched networks.
Collision Domain
A network segment where data packets can collide with each other when transmitted simultaneously.
Half Duplex
A communication mode that allows data to be transmitted in both directions but only one direction at a time.
Media Access Control
MAC
A sublayer of the Data Link layer that controls how devices on a network gain access to a medium and transmit data.
Random Backoff Algorithm
A collision recovery mechanism in which each device waits a randomly selected period of time before attempting to retransmit, reducing the chance of repeated collisions.

Transcript

We've all been in that place where we've started to talk and at the same time somebody else has started to talk, and usually what happens is we just say, "Oh yeah, you go ahead first." One of you will let the other person talk. The same thing happens with computers. We call it a collision when two computers try to talk at the same time.

Here's our network, and in the middle we still have our hub. What happens with a hub is that every time somebody talks and it comes in on a port, it gets broadcasted out all other ports. So it's quite conceivable that some other device could talk at the same time, causing a collision. Ethernet is just built on the fact that there are going to be collisions, and we expect it to happen all the time.

CSMA/CD

Ethernet uses something called carrier sense multiple access collision detect, or CSMA/CD. I don't want to get too far in depth into this, but essentially multiple access means that you've got a lot of devices that are accessing this network at any given time. And carrier sense just means that before one of these devices talks, it's going to sense to see if somebody else is talking on this network. If somebody else is talking, then they're going to be receiving a message, and this computer is not going to try to talk at that same time. So that's the carrier sense. And collision detect just means that once this computer starts communicating, if it detects a collision then it will stop speaking, it will stop communicating at that point.

What happens during a collision

Let's say two devices did talk at the same time and caused a collision. For number one, the communication from both of these streams is no longer good. They've stepped on each other's toes, and the communication that happened has to start all over. No matter which device you're at, whether you started talking milliseconds before the other one, it doesn't matter. The communication is bad.

So what happens is both devices stop altogether, and then they have a backoff algorithm where they choose a random number and start counting down that random number. They're going to choose different numbers here, and they start counting down that number, and then one will start communicating before the other one has a chance to communicate. Then this other device will sense that the line is now in use, that somebody's communicating on that line, and will wait until the line's free before it starts communicating again. So that's what happens when there's a collision.

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