Network Certification Exam Tutorial: Ethernet CSMA/CD Explained

By: Chris Bryant

When you're studying for the Network+ exam, you've got a lot of new acronyms to learn!  One of the basics is CSMA/CD, which stands for Carrier Sense Multiple Access with Collision Detection.  That's quite a mouthful, but it's an important term to know for both your exam and real-world success.

On an Ethernet segment, hosts can send data at any time.  As you might guess, that's a bit of a problem in itself, because if multiple hosts send data at the same time, a collision will occur and the data will become unusable.  That's why CSMA/CD requires a host that wants to send data to listen first.  Listen for what?   Data being sent by another host. If the host hears data being transmitted, that host will not begin sending data itself.  That's the "carrier sense" part of CSMA/CD.  If that host hears nothing, the host will begin to transmit.

That's all fine, but what happens if two hosts go through this process at the exact same millisecond, resulting in them sending data onto the segment simultaneously?  Their data will collide, and that's where Collision Detection comes in.  The collision itself generates "noise", and the hosts that just sent data will realize that their data was involved in that collision.  As a result, those hosts will generate a jam signal, which will be heard by all other hosts on the segment.  This jam signal tells the other hosts that there has been a collision and that they should not attempt to send data at this time.

That's an effective technique, but the two hosts still have to send their data.  What if they send the data at the exact same time again?  To avoid that possibility, both hosts that transmitted data will invoke a random timer, and when that timer expires, the hosts will begin the entire CSMA/CD process again - and that process always begins with listening to the segment to see if another host is currently sending data.  In this way, the hosts that were involved in the first collision have a very slim chance of being involved in another one right away.

CSMA/CD isn't the only weapon we have to minimize collisions.  In the next installment of my exclusive Network+ tutorial series, we'll take a look at how routers, switches, hubs, and repeaters can (or can't!) help us keep collisions to a minimum.  Until thenArticle Search, keep studying!

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