 Hello friends, myself, Darshan Pandit, Assistant Professor, Department of Computer Science and Engineering from Walchand Institute of Technology, Solapur. So today we are going to discuss about medium access control. So the learning outcomes are students will be able to describe hidden and exposed terminal problem in a wireless network. So in this we are going to discuss about introduction of medium access control, need of medium access control, motivation and what are hidden terminal, exposed terminal, near and far terminal. So what is medium access control? So medium access control is a sub layer of data linklayer of open system interconnection that is OSI reference model. So in the figure you can see position of MAC layer. So MAC layer is also responsible for flow control and multiplexing for transmission medium. So it also controls the transmission of data packet via remotely shared channels. So why medium access control? So in the figure you can see suppose there are 10 sender. So all these 10 sender want to access the medium at a same time. Then there might be the collision. So to avoid this collision we require medium access control. So here medium access control provides chance for the sender to access this medium one after the another so that collision can be avoided. So we are having several medium access schemes. So through these schemes, so these senders are selected one after the another to access the medium. So the motivation from fixed network, so in fixed network CSMA CD that is carrier sense multiple access with collision detection was used. So when we apply this technique in wireless network then there are some problems with the wireless network. So as we know in wireless network we are using air medium for transmission of data. So here signal strength decreases proportionally to the square of the distance between sender and receiver. Also when sender applies carrier sense and collision detection but if collision happens at receiver then sender might not hear this collision. So that is what collision detection do not work in wireless network when we apply CSMA CD. Next problem is of hidden terminal in which carrier sense might not work. So let us see this problem of hidden terminal. So in this let us consider the scenario with 3 mobile phones A, B and C the transmission range of A reaches B but not C and transmission range of C reaches B but not A. So this scenario to consider that is transmission range of A reaches B and transmission range of C reaches B but not A and finally transmission range of B reaches both A and C. So in this scenario A cannot detect C or C cannot detect A. So here what happens when sender starts sending data to B that is sender A starts sending data to B. C does not receive this transmission as C cannot hear A. So now when C wants to send data to B and C senses the medium as free as C is unaware of the transmission going on between A and B as C cannot hear A. So when C senses the medium as free their carrier sense fails that is when we apply CSMA CD carrier sense multiple access with collision detection. So in this case carrier sense fails when C senses the medium as free and when collision happens at B A cannot receive this collision. So this is what collision detection fails. So in this way we can say that C is hidden from A or A is hidden from C. So in this way hidden terminal may cause the collision. So next is exposed terminal so let us see what is exposed terminal problem. So now consider the situation that B sends data to A and C wants to transmit data to some other mobile phone. So you can say it D which is outside the range of A and B. So C senses the carrier and detects the carrier is busy as transmission is going on between B and A. So what C does C corresponds it transmission until it detects medium as idle but A is outside the range of C so waiting is not necessary. So causing a collision at B does not matter because the collision is too weak to propagate to A. So in this situation C is exposed to B. So here C has to wait unnecessarily when C has to send data to another terminal that is D. So that is what we have said C is exposed to B. Let us see the analogy to near and far terminal. So in the classroom scenario we can see so when teacher is teaching and if student A talks in between then student B will hear voice of student A rather than voice of teacher. So why this is so? Because student A is near to student B then the teacher. So this is what the problem of near and far terminal that is student A draws out the voice of teacher that is B will hear voice of student A then voice of teacher. So let us see near and far terminal. So in this scenario consider A and B both are sending with the same transmission power and C is receiving. So as we know that signal strength decreases proportionally to the square of the distance between sender and receiver. So B's signal draws out A's signal draws out is nothing but B signal fade out A's signal. As a result C cannot receive A's transmission that is A's signal won't reach to C because of B. So this is the problem of near and far terminal. So here you can pause the video and answer this question. So what is the main physical reason for the failure of many MAC schemes known from wire network? So what is done in wired network to avoid this effect? So stations in wired network here each other wired LAN uses CSMA CD as a access scheme and in wireless network. So communicate with central station but not with each other and medium is air. So if carrier sense applied in wireless network then collision of data packet at receiver is not detected as carrier sensing take place at sender. So in wireless network carrier sense and collision detection fails as sender are not able to hear other station signal or collision at receiver. So these are the references which I have created for this video. Thank you.