Cisco CCNA Exam Training: A Guide To CSMA/CD

By Chris Bryant, CCIE #12933

When you're studying for your CCNA exams, you're going to study the theory of technologies we basically take for granted in networking. CSMA/CD is one of those technologies. It works beautifully and you don't even have to configure it. But to be an effective network troubleshooter (and to pass the 640-811, 640-801, and 640-821 exams), you have to know Ethernet inside and out, and that means knowing CSMA/CD.

The first Ethernet standards were 10Base5 and 10Base2. Network devices such as hubs, repeaters, and switches weren't in the picture yet. The sole physical components were the Ethernet cards in the computers and coaxial cable. The cable made up a bus that all the connected devices would use. (This type of bus is referred to as a shared bus.)

If only one of the hosts on this network wanted to send data, there would be no problem. When two hosts would send at the same time, though, a collision would occur. When the signals would collide, both would be rendered unusable. A standard had to be created that would have the hosts follow rules relating to when they could send data and when they could not. This standard is Carrier Sense Multiple Access with Collision Detection, more commonly referred to as CSMA/CD.

If two of the three computers on this segment send data at the same time, a collision occurs.

To avoid this, CSMA/CD forces computers to “listen” to the Ethernet before sending in order to make sure that no other host on the wire is transmitting. When the Ethernet segment is not busy, the device that wants to send data can do so. The sender will then continue to listen, to make sure that sending the data didn’t cause a collision.

If a collision is heard, both of the senders will send a jam signal over the Ethernet. This jam signal indicates to all other devices on the Ethernet segment that there has been a collision, and they should not send data onto the wire. (A second indication of a collision is the noise created by the collision itself.)

After sending the jam signal, each of the senders will wait a random amount of time before beginning the entire process over. This random timer helps to ensure that the two devices don't transmit simultaneously again.

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com