 Hello friends, myself, Darshan Pandit, Assistant Professor, Department of Computer Science and Engineering from Walton Institute of Technology, Solapur. So today we are going to discuss about access methods in medium access control. So the learning outcome is student will be able to describe multiple access scheme used in wireless communication. So in this we are going to see what is space division multiplexing, time division multiplexing, frequency division multiplexing and code division multiplexing. So to regulate user access to a medium we need to have SDM, TDM, FDM and CDM schemes. So as we know there are presence of hidden and exposed terminal or near and far terminal. Because of this terminal there is a need of specialized MAC in wireless domain. So here SDM and FDM are typically used in a fixed manner that is certain space or frequency assigned for longer period of time. And in this main focus is on TDM mechanism which allows time allocation on demand and in a distributed fashion. So before going to the schemes let us see one analogy to wireless communication. That is a highway problem and the goal of this highway problem it should be like from highway many vehicles should pass that is maximum number of vehicle should use highway without any collision. So the collision should be less and utilization should be maximum. So for this problem we can divide common highway into number of lanes that is we can apply space division. So every vehicle can use single lane so but problem is that number of vehicle which can pass without collision is limited to number of lanes. So subsequent solution is that we allow more than one vehicle to pass the same lane provided they move with different speed that is nothing but frequency division. So hence avoiding the collision. So here the problem is that vehicle moving with same speed in a same lane would collide. So also to solve this problem vehicles moving with same speed in same lane should arrive at different time that is nothing but time division. And lastly allowing special vehicle on same lane that is nothing but code division that is there are some vehicle which are having special code like ambulance. So when someone sees an ambulance then you would leave space for the ambulance that is it has some special code. So let us apply this analogy for these schemes. So space division multiplexing access so in this figure you can see channel K1 to K6. So introduced with three dimensional coordinate system. So in this system dimensions are like code, time and frequency that is C, T and F. And for this first type of multiplexing a space division multiplexing SDM with three dimensional space that is S1, S2 and S3 are shown and these spaces are mapped to channel K1 to K3. And to avoid the overlapping we are having guard space in between these spaces. So the spaces between interference range is sometimes called as guard space. So in every multiplexing scheme we require guard space. So space division multiplexing is used for allocating separate space to the user in wireless network. So an application involves assigning optimal base station to a mobile phone user. So a MAC algorithm could now decide the base station by taking into account which frequency, which time slot, which code are still available for the mobile station depending upon the technology. So next we are having code division multiplexing access. So the figure shows all channels Ki uses same frequency at a same time for data transmission. So now separation is achieved by assigning each channel with its own code. That is guard spaces are realized using code with necessary distance in the code space. So this enables use of code to separate different user in code space and provide access to the shared medium without any interference. So the main problem in this CDMA is to find good codes and how to separate signal from noise generated by other signals and the environment. So next we are having frequency division multiplexing access. So FDMA comprises of all algorithm allocating frequency to the transmission channel according to the frequency division multiplexing scheme. So allocation can be either fixed or dynamic that is as per the radio station or it may be demand driven. So in this scheme two partner typically establish a duplex channel that is both partner can send and receive data at a same time. So it is also known as frequency division duplex which allows simultaneous transmission in both direction. So the two direction are mobile station to base station that is known as uplink and from base station to mobile station that is known as downlink. So in the figure you can see you are having 124 channels for uplink and 124 channels for downlink and each channel has bandwidth of 200 kilohertz. So this is an example of GSM next that is time division multiple access. So in time division multiple access shared medium network is allowed several user to share same frequency channel by dividing the signal into different time slots. So time division multiple access is more flexible scheme which comprises various technology that allocates certain time slot for communication. So tuning in a certain frequency is not necessary here in time division multiple access. So the receiver can stay at same frequency for whole time. So in figure you can see there are 12 slots for downlink and 12 slots for uplink. So each slot has duration of 417 microsecond. So here connection is allotted with the pair of uplink and downlink. So for mobile station slot 1 is used for uplink and slot 1 is used for downlink. So in this pair is allotted for the mobile station. So the drawback is what there is a wastage of bandwidth. So if no input signal is present in one of the channel since fixed time slot is assigned to each channel. So that time slot for that channel does not carry any information and there is a wastage of bandwidth. So using same frequency the receiver can stay at the same frequency whole time. So in this fixed TDMA pattern helps in implementing multiple access and duplex channel between base station and mobile station. So it assigns different time slots for uplink and downlink using same frequency. So it is known as time division duplex. So here you can pause the video and answer this question. So explain the term interference in space, time, frequency and code. So what are the countermeasures in SDMA, TDMA, FDMA and CDMA system? So answer is so interference is like combination of two or more signals. So interference and countermeasures are like SDMA. So in SDMA interference happens if sender are too close to each other. So the terminal or base station have to keep a minimum distance between them. In TDMA interference happens if sender transmit data at a same time. So countermeasures are tight synchronization and guard space that is time gap between the transmission. FDMA so interference happens if sender transmit data at same frequency thus different frequencies have to be assigned to sender next that is CDMA. So interference happens if sender transmit data using orthogonal codes that is correlation is not 0 thus sender have to use orthogonal codes for transmission. So these are the references which I have used to create this video. Thank you.