Ethernet : CSMA/CD


CSMA/CD is made up of following three components (process). So if you have clear understanding of each of the component processes, you would understand what CSMA/CD is.

  • CS : Carrier-Sensing
  • MA : Multiple Access
  • CD : Collision Detaction


Now let's think of each of these component process. The centrol component among this is 'MA', so I will start with MA.



What is MA ?  


MA stands for Multiple Access. What does it mean ?

Simply put, Multiple Access is 'sharing (accessing) a single physical resource among 'Multiple' entities (e.g, multiple device, multiple persons etc).


For example in our daily life, I can say 'any road (or street) with a lot of cars running on it is a kind of Multiple Access. The road in front of your house is a physical resource and that physical resouce is shared (accessed) by many cars.


In similar logic, Ethernet (e.g, LAN system) can be Multiple Access system. In LAN (Wired LAN) system, many network cards are connected to a single LAN cable backbone. Even Wireless LAN can also be a Multiple Access system since multiple Wireless device is sharing same physical resource which is same RF frequency.


What is CS ?


To make this kind of Multiple Access system work properly, all the device sharing the resource should have a certain techniques to avoid conflict among the multiple devices competing for the same resource. This 'conflict avoidance technology' (called 'collision avoidance system') used in ethernet is called 'CS' (Carrier Sensing).


CS (Carrier Sensing) may sound intimidating but the concept is simple. it is what you do every time you drive your car from your house into a big road in front of your house. When you drive into a road, you don't just drive into the road. If you do that, you may easily get hit by other cars driving along the road.  Usually you take a look at the traffic on the road first. and if there is no cars passing by the road, then you drive into the road. and if there is a car passing in front, you just wait until it passes by. In other words, you are 'Sensing' the traffic and then drive into (or wait).


It is same logic in Ethernet System. Before a device send any data, first it checks if there is any other traffice sent by other device. In other word, it 'Sense' if there is any traffic sent by other device. If there is no traffic, it sends its own data. If there is traffic, it wait until the traffic passes by.


What is CD ?


Even though a device is trying very hard for CS (Carrier Sensing) and send the data when there is no other traffic on the line, there is still possibility of collision. For example, two device were trying CS at the same time and they found a time slot of no traffic at the same time, then it would be possible for the two devices to transmit the data at the same time and in that case it would lead to collision.

Once the collision happens, the device that transmitted the data which collide with other data should stop transmitting the data right away. If it keep transmitting data without recognizing the collision, it would cause the collision continuously. To prevent this kind of continuous collision, a device should have a mechanism to check whether the data it transmitted collide with other data or not. This mechanism is called 'CD' (Collision Detection).