CSMA/CD quick post

One of the topics that we need to understand which will happen on our daily tasks maybe even without knowing it is really happening.

we are talking about collisions and how and when they happen on a network.

let's take the two topology below and compare what will happen in each of them.

As we all know, HUB is an old device that used to connect devices (Computers, Printers) together in a shared network.

the HUB is not smart enough to prevent problems like loops or collisions and it is not manageable and still work with one vlan or any vlan that comes from another attached network.

This is why, in our networks today we don't use Hubs no more. and switches replaced these hubs.

ok, so let's talk about collisions.

Simply, they happen when two or more devices talk (send frames) at the same time. this can easily happen on the Hub devices, simply because they don't have any mechanism to prevent this.

on the other hand, stations do have a feature which was created to limit these collisions and it is called CSMA/CD.

which stands for (Carrier Sense Multiple Access/ Collision detection)

The devices connected to Hub ports will sense the traffic on the shared medium and send frames when there is no frames sent by other devices.

If there is a collision, all ports will send jamming signal to stop sending more frames.

this jamming signal will then ends, and each part of the collided frames will wait for a timer before sending more frames again, these timers will be randomized by each device to make sure no collision happens again. Hence the name of MAC came from (Media Access Control)

A better option than CSMA/CD is CSMA/CA which stands for the same but instead of detection, we have Collision avoidance here, in which a computer will send like a test frame to see if there is any senders then continue sending frames to destination so this can lower the collision chance even more.

So this will be done by computers as a feature working in layer2 to solve some of Hub problems.

This however is not the same on switches, where each port is a separated collision domain from the rest of the ports.

Hope this was useful.

Samer R. Saleem