Hey everyone, thanks for coming back to another chapter of Wi Fi fundamentals with location and analytics. This course will help you to master the air. In our previous part, we have seen the basic Wi Fi process throughout the exchange of management frames. How a station connects to the access point. We've also discussed the role of the MAC layer and the physical layer, each with huge responsibility for the operation of your network. But hey, our Wi Fi networks are crowded.
Everyone wants to talk who takes care of their if one station starts to transmit slowly and the other consumes a lot of bandwidth, there is a mystery for the most of us what actually happens in the air, Whose turn is to transmit data and who just wait for the others. This is our topic today, how air is shared between our station Whose turn is to talk and how fair is Wi Fi to legacy clients. So, what are we up to today, we will learn of csma ca Wi Fi is mechanism to sense and avoid collisions in the air. And we will also understand and see how time is coordinated between stations through the usage of DCF scary words, it will all be clear soon. Ethernet has no issue with collusions. It used to be an issue in the old days when hubs transfer their data.
But as switches replaced hubs collusions are dealt in a very effective way. Wireless is another story clients operate under a medium that is available to everyone the air, two major things must happen. There has to be a mechanism that will avoid collisions and second, share the airtime to every stations around. How do we do it? This CF a distributed coordination function. This year grants access control to the physical medium.
He is actually your doorman is doing so by setting a period of time but the station is to wait in different scenarios. So this half is actually a time period. There are several types of time periods but for now, we will look at three Which are the most dominate DFS DCF interframe space is the most used yet it's the lowest priority time period. s Fs is a shorter period of time used mostly by our access point as it transmits back and acknowledgement free Fs which is used before quality of service frames with a high priority such as in Voice over IP applications and video csma ca which stands for carrier sense multiple access with collision avoidance happens in two steps. The first part physical carrier sense, our station detects energy in the air, it can be Wi Fi or even an energy that comes from other sources such as our microwave.
We will assume that it thanks Another Wi Fi station that transmits and there comes the second part the virtual carrier sense. With each Wi Fi frame, there is a duration value that tells stations how much they have to wait for the end of the transmission. So if a station is in the middle of a transmission, station B will read its duration value and set up what is called enough a network allocation vector. That only after it it can start the steps needed to send x frame. nerve is actually a period of time that is equal to the duration value of station a. But that's not all.
As enough ends, our station will wait another time at diff and interframe space As defense it will start another short time called the back off timer. The station actually generates a random number, number of slots time periods that the station needs to wait after this has ended. It is also called the contention window. The number of slots is derived from the modulation type for DSS modulation a topic we haven't touched yet, contention windows can be up to 31 slots and for OFDM a more advanced modulation technique it is up to 15 now station B can transmit. So if we try to summarize the first thing is to sense the air. Read the question Then you've set up enough, wait some time usually diffs generated backup timer and transmit.
But that's only the beginning. Next step csma. ca part two. See you there.