Carrier Sense Multiple Access (CSMA)

CSMA stands for carrier sense multiple access.

Let's talk about the MA part, or the multiple access. That's really simple. That just means this network is being accessed by multiple devices. There's multiple access from many different devices.

Then let's talk about the carrier sense. Carrier sense means that before this device decides to speak, it's going to listen, or it's going to sense. It's going to sense to see if any other device is talking on the line at that time. So it's going to sense the line and only communicate if it senses that no other communication is trying to happen during that time. That's carrier sense.

Collision detection

Collision detection is still carrier sense multiple access. Devices on this network still listen to see if anyone else has communicated. If they're not communicating, then they will try to communicate. And if two devices happen to be listening and try to communicate at the same time, and there's a hub here in between — since it's half duplex, we will cause a collision.

And so we have a collision detect. A collision detect just speeds up this whole process by, as it is trying to communicate, it will sense to see if there is a collision, and if there is a collision they will stop communicating right away rather than trying to continue to communicate. So collision detect helps speed up this process by: hey, I am in the process of communicating and I sense that there's a collision. I'm going to stop speaking right away. And now I'm going to go into my backoff algorithm. I'm going to wait a random amount of time. This device waits a random amount of time, this device waits a random amount of time; the chances are those are going to be two different time frames. And then this device will try to communicate, and then this device will wait until that device is done communicating before it will try to communicate.

So this is collision detect. Collision detect detects to see if there's a collision, stops communicating right away, and goes through the backoff algorithm to wait a random amount of time.

Where collision detection matters

We see this in networks like Ethernet networks, where they're all cabled together, and specifically where there's a hub in between. If we have a switch in between, this is a non-issue because a switch is full duplex.

What happens with a switch is those collision domains — everything that we have within here, if it's a hub, everything is part of this collision domain. This is capable of having a collision. But if we have a switch that's involved here, they can be full duplex. Communication can happen all at the same time. And so it separates out our collision domains to every single port on this switch as a separate collision domain, and we don't get collisions.

So when we talk about CSMA/CD, it's a protocol that exists more when we have collisions. It's needed more for when we have collisions within here. But when we replace this equipment with things like switches, it becomes a lot less necessary, because we have separated out our collision domains and we don't have near the same amount of collisions that are happening on our network.

Collision avoidance

Collision avoidance operates in much the same way. It's still carrier sense multiple access, so a device that wants to speak on the network is still listening to see if any other device is talking before they speak. There's a small difference with collision avoidance, and that is that before they speak they are going to send a little message out saying I intend to speak.

So the difference here between CD and CA is that CD, collision detection, will try to send all of its information out that it wants to get out there — the whole frame, it wants to send the whole frame out and just see if there's some sort of collision. Versus collision avoidance will just say I intend to speak, make sure that that doesn't collide with anybody else's intent to speak. And once it determines yes, then it sends its information out. There could be some sort of moderator even in between, saying okay, this is your opportunity to speak. So collision avoidance tries to avoid the collision to begin with.

We still see this widely implemented. In fact we see it implemented in a lot of networks, because it's what wireless uses as its communication method. Essentially your wireless access point is just a hub. It is only half duplex. Only one device can speak at one time, because of the limitations of the access point. And so this is the method that is used to avoid those collisions, and if there is a collision, then it has to go through that backoff algorithm and once again wait.