CSMA/CD and CSMA/CA – CompTIA Network+ N10-006 – 5.2

Device access to a wired or wireless network must be carefully controlled to allow everyone on the network to communicate successfully. In this video, you’ll learn about the access controls provided by CSMA/CD and CSMA/CA.
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We have so many different devices that we put onto our wired and wireless networks. But how are they able to communicate on the network without disrupting communication from anyone else? In this video, we’ll look at the c CSMA/CD and CSMA/CA access control methods. Let’s start with CSMA/CD, and let’s break down what this acronym really means. The CS means “carrier sets.” That means that the devices that are on the network are going to listen for a carrier before they begin transmitting over the network.

The MA stands for “multiple access.” This means that there will be many devices on the same network that will need to communicate. CD stands for “collision detect.” We use the term “collision” in ethernet networks to mean that two devices are communicating at the same time over that same media. The term “collision” has such a bad connotation, but in the use of ethernet this only means that two devices happen to be talking at exactly the same time, and it’s a normal part of the ethernet topology.

When we talk about CSMA/CD, we’re usually referring to half duplex ethernet networks where only one device can communicate at one time over the network. But our networks these days are using switches, and we’re connecting to the switches at full duplex and with full duplex we’re able to transmit and receive at exactly the same time.

With CSMA/CA, we still have the carrier sense multiple access, but in this case we have the CA at the end, which stands for “collision avoidance.” This is a very common way to communicate over wireless networks, because we’re not able to detect when a collision might occur. Whenever we’re transmitting out of our wireless connection, we’re effectively overloading our local receiver and we can’t hear if anybody else might be transmitting at the same time.

In this way, we use something called RTS and CTS, where we are ready to send and clear to send, so that we’re communicating to everybody else on the network that we’re going to send information so everyone should be quiet while we’re performing that process.

This is really useful for wireless networks, because you have the problem of a hidden node. That means that you have a central access point and station A can hear all of the communication to the access point. Station B can hear all of the communication to and from that access point. But A and B are so far apart that they can’t hear each other. If you have the access point in the middle and it’s handling the collision avoidance process, all of those devices can communicate easily on the network, even though they can’t hear each other.