Wi-Fi DCF and CSMA/CA, what does this mean? We'll soon look into it. Wi-Fi operations are based upon the standards in which DCF distributed coordination function is a technique that is used to enable Wi-Fi communication and in DCF, a core networking technology that is used to access the ISM band channel is called Carrier Sense Multiple Access with Collision Avoidance, for short CSMA/CA. Now once again, we need this because the ISM band is shared with many many devices that are unlicensedly using the ISM band and that is what the ISM band was made for. Now, in that application structure as multiple devices are using it in a unlicensed structure, when they access the channel, when multiple devices access the channel, then we want them to monitor the carrier frequency and if nobody's using it, we want them to decide when and how to use it. So overall that is where we get the word of Carrier Sense that you're sensing the carrier frequency that will be the 2.4 gigahertz or five gigahertz frequency range and you're trying to do very sensitive, accurate, multiple access, where multiple devices are trying to use the channel in a specific way that they don't collide and that is why we add the word Collision Avoidance. The role of DCF and CSMA/CA is because we want to avoid communication failure due to packet collision. So what happens if you have a packet collision for multiple users? Well, those packets will all be wasted, they will not be useful. In addition, if you are in a sequence of communication, then most likely the sequence of the packets may become all wasted, they may all become useless and that would degrade the efficiency, the utilization of the network and therefore we do not want that. Why does this occur? Once again, this occurs because this is a unlicensed ISM band. Operations of CSMA/CA, they first do carrier sensing which means that they're going to first look at the channel that they are going to use and they're going to try to use it when nobody else is using it. In order to know when to use it, in order to know when others are not using it, you need to sense the carrier frequency and that is what the CSMA part CS is standing for Carrier Sensing. The other is Collision Avoidance, you want to avoid collisions. So, if somebody else is using it and you detect that through your carrier sensing, then you do not want to send. So there has to be a period, once you detect somebody else is going to use it, there is a NAV period which you set such that during that period, you do not use the channel because you detected that somebody else is using it and setting this NAV period is very important to avoid collisions. What is that? That is short for Network Allocation Vector, meaning that you've allocated the network and somebody's using it, so I'm not going to use it to avoid collisions and that is what NAV stands for. Here, there are two protocol messages that are exchanged that will set up NAVs in local nodes and that is Request To Send and Clear To Send. This is like the Request To Send is what initiates a communication session, where I will say Request To Send and I will ask somebody to reply back the node that I want to communicate with, the station that I want to communicate with. When that station sends back a Clear To Send as a reply to my Request To Send, then because of these two signals, RTS and CTS, all surrounding devices that have seen as exchange either RTS or CTS, or seen as exchange both the RTS and CTS messages for a NAV period, they will withhold from sending anything and that is how we avoid collision, that is the CA (Collision Avoidance) technology that we are going to use within CSMA/CA that is used within Wi-Fi technology. So the RTS and CTS, these are messages that are exchanged such that local devices will see this and withhold from sending any packets during the NAV period. This is a solution to the hidden node problem. In the ISM band, we do not know who is overlooking the channel and who is wanting to send something. So therefore, these nodes that we don't know who wants to send and who is planning to send, we can prevent them from colliding with our packets using the RTS and CTS and the NAV duration and that is why we say that it's a solution to the hidden node problem, because these hidden nodes that my node is not aware of, will be able to see our RTS and CTS exchanged and avoid collision. Then you will see that the communication is requested using an RTS and the CTS frame is used to accept the communication request made by the RTS. In addition, when you're communicating, there are Inter-Frame Spaces and these are used to set a priority. Think of it this way, if two people are communicating and they are fast talkers, they're sending in, they're talking and replying, talking and replaying at a very high pace, very fast, then it is very difficult to get yourself to interrupt them in the middle without having your words overlap with somebody else's words. This is because they are talking and replying very quickly, then you cannot squeeze in into the middle to hijack or intercept the communication session. However, if they are talking slowly, where they say something, they wait and they reply, then they wait and then they reply again, back and forth because they're communicating slow, there is a big gap between their words and their replies, then because of this gap is wider you can easily get into it and inject your message, your sentence and in other terms you can hijack the conversation. So, based upon this Inter-Frame Space which is space. It's a little bit of a short duration of time in-between messages and messages and messages, packets and packets and packets. With this small interval time, is it large? Is it a wide period of time or a small period of time will determine the priority? One thing that you will see is that if the session is very short, which is what is accomplished by the SIFS; the short interframe space. Then a sequence that is so fast in sending and replying, sending and replying that the small Inter-Frame spaces that are the SIFS, the short Inter-Frame spaces they are so fast that once a communication session is in mode using the SIFS Inter-Frame durations, you cannot bother them. You have to leave them alone because they are sending and replying very quickly. After a communication session is over, then there is a longer period before the next communication session is conducted and that is our DIFS duration which is longer than the SIFS. This is a period in which after a quick session of communication using frames, packets, and SIFS durations, then at the end you will use a DIFS before another one will start communicating. That is why as you see right here there is a DIFS and then there is a SIFS. In addition, there is a contention window. This is also called a backoff window. The contention window is a contention window because if there are multiple devices that are trying to communicate and this is detected, who will send first is decided through the contention window. This is first set for a given duration where each device that is wanting to send something will select a random number within the contention window period, and they will wait for that duration and after that duration, they will check to see if the channel is empty, nobody's using it and then they will use the channel. Now naturally, every device is going to select a random number within the contention window period. That means that they will all select different numbers, and that will help them avoid because whoever luckily chooses a smaller random number will try to attempt to communication quicker than a device, a station that chooses a larger number in the contention window period. This duration of not sending until the random number of what you selected within the contention window period is weighted upon, that is what we call our backoff window. We are backing off from sending. We're withholding from sending, until we reach that time duration that we once again detect if somebody else is sending and then we go into transmission mode if nobody else is sending. Of course, there is a possibility that you have some stations that randomly choose numbers in the contention window and they choose maybe similar numbers or the same number, and then their packets may collide. There is always that possibility. So, to avoid that, then if you detect something is going on or you have a collision in packets, then the next time the contention window is set it is enlarge to double size, and everybody will choose, a random number in this contention window that is doubled in size, they will choose numbers in this double size contention window period. That means that the likelihood of them selecting again similar numbers such that they will collide again will be extremely reduced in probability. If this happens again then the contention window size will be doubled again and then random numbers will be selected inside. This will be repeated 10 times to avoid collision. After the window size, the contention window size is doubled and doubled for the tenth time, then it's going to be tremendously large. That maximum size is maintained six times without doubling and random numbers are selected for their backoff and then it is attempted again. So, once it's enlarged ten times and you failed and failed and failed again ten times, then that large contention window size will be kept and random numbers for all stations will be selected again up to six times will be repeated without doubling the contention window because it reached its maximum size. So, you have ten times of doubling it if you continue to fail and then after you doubled it 10 times, you keep that huge size and you repeat the random number selection again six more times and if you still fail, what do you do? You give up communication and then you go back and then start from the beginning again. This is what Wi-Fi is using, and this is all in the CSMA/CA operation in which Wi-Fi is using, in which your smartphone has been using, in which you have been using everyday you have been using Wi-Fi. Now you know what you've been using. Continuing on with this, there is also a slot time. A slot time is a basic amount of time. The smallest amount of time. It is like a time unit, in which, this is used, the slot time is used to create multiples of slot times to form a contention window. Multiple of these times are used to form a DIFS or SIFS. Then once again, the SIFS is the short Inter-Frame space and this is for immediate action between RTS-CTS data where you're in a data session. You're sending back and forth. Then you will reply within the SIFS duration. Then there's the DIFS which is the DCF IFS. This is a minimum idle time for contention based services. Looking at the numbers that you see right here, the slot times are very small in units of microseconds and based upon what IEEE 802.11 Wi-Fi mode, you see that these numbers may be slightly different, and then the SIFS here is in microseconds as well. Let's look at the relation. Stations use SIFS to maintain highest priority of channel usage during its transmission opportunity, and because it is the shortest duration of a IFS Inter-Frame space, it maintains the sequence of communication without other people intercepting into that communication session. The DIFS is larger than the SIFS based upon having its two slot times added. So, you see the SIFS here, the slot time is the smallest, the SIFS is larger, and the DIFS is even larger than the SIFS, and you'll see that the number difference is based upon two slot times are added coming from here going to hear, and that's what these are. Now, lets look at this operation inaction in this illustrated diagram. At the beginning, DIFS is weighted such that after a session is done, this is a basic waiting period. Then, a device sends out its RTS and naturally the devices that are near around this device will see the RTS signal. That will make them all set their NAV such that they don't collide with the packets that are going to be exchanged between this device and another device, and of course, the RTS time is added. Then after a SIFS duration amount of time, immediately the receiving device of the other communication side will send back a clear to send. This is a reply back to the RTS request to send that it received. Now because of this the devices surrounding this device in this blue dotted range, these devices will naturally set their NAV. They will withhold from sending anything to avoid collision with these two devices that exchange the RTS/CTS in communicating. Naturally, so the time for the CTS is added in how much time we consumed and then after a short SIFS duration, then data will be sent, and this data packet will be protected because the devices that are surrounding are not sending because they have said the NAV duration. Then, after the data packet, there is a SIFS duration very short one, and then an acknowledgment packet that it's confirming that I received your data packet well without errors is sent back to the overall station that sent the data packet, and that is added to the time. After the ACK packet, the data was exchanged. So, after the DIFS period is done, then the next session can begin. Looking at this in a time flow diagram in which time is going down and we have one node representing a neighbor node over near the receiver, and we have one node representing a neighbor node way over there as a representative node of a neighbor near the node that will send the RTS, and let's see how time-wise this operation works. First, after waiting a DIFS from the former session is over, we wait DIFS and an RTS signal is sent out. This will reach the other node that it wants to communicate with. Based upon this moment, the surrounding device will have a NAV duration set where it will not send anything. Then after an SIFS duration, a clear to send CTS reply will be sent back, and this will trigger the surrounding nodes to also set an NAV such that collisions are not occurred and this communication session is protected. That's why this device will withhold from sending anything during this NAV period. Now, we secure the channel. No collisions will occur. So, we can go ahead and send our data packet after an SIFS period and then after that data packet is received with a SIFS period, an acknowledgment will be sent back. After that, then the session is done. I send my data packets safely and I know that it was received safely because an acknowledgment was returned. Then after DIFS period, somebody else can now use this channel. These are the references that I used and I recommend them to you. Thank you.
