 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 MACA and polling technique in medium access control. So learning outcome is at the end of the session student will be able to describe the solution for hidden and exposed terminal problem. So TDMA access methods in this we are going to see that is multiple access with collision avoidance, polling and inhibit sense multiple access. So first MACA technique that is multiple access with collision avoidance, it is a random access Aloha scheme with dynamic reservation. So this scheme solves hidden and exposed terminal problem as with the help of RTS and CTS packet and this scheme does not require any base station. So this RTS and CTS packets are short signaling packet which are used to avoid collision. So RTS is sent by sender so which request write to send before send any data packet and CTS so the receiver grants write to send as soon as it is ready to receive. So this signaling packet consists of sender address, receiver address and the packet size. Now let us see this scenario in hidden terminal. So MACA avoids problem of hidden terminal by using RTS and CTS packet. So in this A and C want to send to B that is A and C terminal want to send data to terminal B. So first A sends RTS packet so this RTS packet consist from sender address that is A, receiver address B and the packet size. After that C waits after receiving CTS from B as CTS packet consist of again sender address and receiver address so that C will get it like there is a transmission going on between A and B. So C will not send any data packet till this transmission gets over. So in this way we can avoid collision by using RTS and CTS packet. So let us see next that is how MACA avoids collision in exposed terminal. So in this scenario B want to send data to A and C want to send data to another terminal so let us say D. So now C does not have to wait for it and cannot receive CTS from A. So this was the exposed terminal problem. So now what B will do? B will broadcast the RTS packet so that A and C both will receive this RTS packet and C will get it like there is a transmission going on between A and B. So now C does not have to wait unnecessarily and directly C can transmit its data to that is terminal B. So in this way we can avoid exposed terminal problem by using MACA technique. So in the figure you can see so sender is idle until a user request the transmission of data packet. After that sender then issues an RTS and wait for write to send. If receiver gets an RTS and is an idle state it send back a CTS and wait for the data. After that sender issues CTS and sends data otherwise the sender would send RTS again after time off. So after transmission of data the sender waits for positive acknowledgement to return to an idle state. After that the receiver sends back a positive acknowledgement if received data was correct. If not or if waiting time of the data is too long the receiver returns to again idle state. If the receiver does not receive any acknowledgement or negative acknowledgement it sends an RTS and again waits for write to send. Alternatively a receiver could indicate that it is currently busy via separate Rx busy. So real implementation have to have more states and transition. So in this example is to limit number of retries. So here you can pause the video and answer this question how MACA still fail in case of hidden exposed terminal think of mobile station and changing transmission characteristics. So the answer is like when stations move new channel can arise all the time and old one can break down. Furthermore due to increase or weakening of signal based on propagation characteristics the same connection between two station in both direction can have completely different signal strength. So it is possible that one station do not receive the CTS of another one but still can disturb other station with sendings so with its sendings. On such reason the mechanism is only suitable for more or less deterministic propagation characteristics and static device. So ADOCK network cannot make use of it. So next scheme is polling. So polling is strictly centralized scheme with one master station and slave stations. So the master can poll slaves according to many scheme that is round robin or randomly according to reservation etc. So here base station will poll every mobile station like whether he has to send data then it will ask for next mobile station. So in this way this goes in round robin fashion or randomly according to their reservation. So as an example you can see randomly address polling where base station signals readiness to all mobile terminal. So this terminal ready to send transmit random number without collision using CDMA or FDMA. So here base station chooses one address from polling from the list of random numbers. So here collision will take place if two terminal choose same address. So this is the problem of polling. So next type of scheme is ISMA, inhibit sends multiple access. So in this base station signal on downlink if medium is free or not, current state of medium is signaled via busy tone and terminal must not send if medium is busy. So terminal can access medium as soon as busy tone stops. So here also we are using centralized base station. So this mobile node have to send their data through the base station. So there is a centralized access. So the base station signals collision and successful transmission via busy tone and acknowledgments respectively. So media access is not coordinated within this approach. So this approach is used by your mobile phones. So whenever the medium is busy so you will hear a busy tone like the number is busy. So once the medium is idle you can access the medium and you can connect to the call. So this is the thing but inhibit sends multiple access. So these are the references which are used to create this video. Thank you.