 Hello and welcome to this video session on medium access control protocol aloha. So as Mac is one sub layer of the data link layer and one of the function of data link layer is the access control. So in this video we will try to understand how we can have access control for a shared channel. These are the learning outcomes of this video. At the end student will be able to identify the problem with the shared channel that is channel allocation problem and then solution for that may be static or dynamic channel allocation. So student will be able to differentiate between what are the advantages of static or dynamic channel allocation schemes and at the end we will see aloha protocol for channel allocation and we will compare the performance of two aloha protocols pure aloha and slotted aloha. So these are the contents of this presentation. So let us try to understand what is channel allocation problem. How we can determine that who will get access to a shared channel when there is a competition for it? In this network diagram you find that there are different nodes named A, B, C, D, E but you find that they are sharing a common channel. Now when A wants to have access of this shared channel, A wants to communicate with D let us say at the same time suppose B wants to communicate with E what happens in that case will be if you are directly simultaneously sending the data on to the shared channel A is sending data to D as well B is sending data to E at a time on to the same shared channel two different nodes are transmitting the data. This will result into invalid frames. The reason for generation of these invalid frames is called as collision. The reason for the collision is the invalid frames are called because other stations transmitted at the same time when one station was doing the transmission of the data. So collision happens and when collision happens you have to discard that particular frame. So this is the problem where a shared channel if is used by more than one nodes and simultaneously they are trying to send the data on to the same channel then collision will occur and then you need to discard those frames and then maybe receiver will ask for retransmission of the frame to the sender. Now how this problem can be solved? So medium access control protocols tries to solve this shared access shared channel problem by having some access control protocols. There are two basic ways of solving this problem is shared channel can be allocated statically to a particular sender or that channel allocation can be done dynamically. The same channel allocation can be done in three different ways. You can have time division multiple access protocol where different time slots are given for the different nodes on to the network. Though the channel is shared the nodes will be transmitting in their own time slot only hence you will get that access for the channel available and then it will never happen that two stations are transmitting on to the channel same time. But the problem with this is if you are allotting time for a particular station but that station does not have any data to be sent in that case your channel will be idle. Others have data but others do not have their time slot and that is why other nodes are not able to transmit. So this is the problem with this TDMA static channel allocation. Another approach may be you may go for frequency division multiplexing where different nodes will be transmitting their data in different frequency bands. So if B is the bandwidth available made available by the channel then every suppose n different nodes are there in the network then B divided by n will be the bandwidth available to each node and in different bands the data will be transmitted by a particular node. Again we know that if the bandwidth is decided by number of nodes which are present as number of nodes increases in the network with the shared channel the bandwidth available to each node will go on reducing and we know that the data rate which you can get for your transmission is a function of bandwidth hence the as number of nodes increases the bandwidth will reduce and that may result into reduction in the data rate available. Another approach is CDMA code division multiple access where here some modulation is done by making the use of orthogonal codes though you are doing that modulation and that modulated data is transmitted on the shared channel simultaneously the collision will not occur. So these are the static channel allocation schemes but instead of going for the static channel allocation another approach may be you can have allocation of the channel dynamically as per your requirement you may go for allocation of the channel for a particular node. So this dynamic channel allocation can be done in two different ways either you can go for scheduling approach which may involve a protocols like polling for the channel after fixed interval of time or reserving the channel itself for a particular node these are the approaches possible and another way of having that dynamic channel allocation is you can go for some random access protocols. So the today's video session is based on this random access protocol one example of that random access protocol is aloha when it is aloha it is considered as a pure aloha and another counterpart is slotted aloha. Another approach for random access may be CSMA protocol or its improvement is CSMA CD now let us try to understand how this aloha protocol works so aloha is used for solving this channel allocation problem so there are two types of aloha one is called as pure aloha another is called as slotted aloha. So let us try to understand what is the difference between this pure aloha and slotted aloha. Basic difference is in terms of timing when you talk about the pure aloha it does not require as such global time synchronization in the next slides we will come to know what this time synchronization requirement is but here pure aloha does not require this time synchronization while slotted aloha does require this global time synchronization. At the end we will compare the performance of this pure aloha and slotted aloha also. Now let us try to understand what this pure aloha is it is a dynamic channel allocation where the whenever a node has data to be sent directly that node will send that data on to the channel frames are transferred completely at random times. You find here A wants to send the data A has sent the data B wants to send the data B has sent the data again A wants to send the data at this time so data is available. So you can imagine this axis is the time axis but with this you can find that let us talk about this particular interval what has happened here is C wants to send the data he C has started sending the data at this interval E wants to send the data E has started sending the data so those frames are sent but if you look at this time interval what has happened is C as well as E both nodes are sending the data on to the same shared channel so this will result into the collision but if I know that load of my network is very low so then this can be a very suitable approach where whenever you want to send just go on sending the data if you find that some collision occurs then you may go for retransmission of that particular frame so this pure aloha works as you want to send the data you send that data on to the channel maybe collision will occur so check for the collision and if you find that collision has occurred then just neglect that frame destroy and then use some acknowledgement or feedback so that you can have you check the status and then retransmission of the same frame will be done by the sender so this is how pure aloha works now if you look at the vulnerable period for the frame in case of pure aloha is you'll find that if at this instant some frame was sent and another frame this is the frame which collides with the start of the shaded frame so here is the collision which is taking place and again here you find collision it is collides with the end of the shaded frame so this frame has got collision this frame also has got collision so what is the vulnerable time for this pure aloha is from t0 up to t0 plus to t if t is the time duration for that particular frame which is resulting into the collision so this vulnerable time will be almost to t now the performance of this pure aloha can be improved by going for the slotted aloha in slotted aloha what we have is we have these time slots and any station can transmit its data in up at the start of the time slot only randomly you can send the data but it is not just when you want to send the data you'll be sending the data on to the channel but you'll be sending the data on to the channel at the start of the time slot so this is how slotted aloha will work so time slot is divided into slots and then this discrete slot so stations are synchronized to the frame so this is where this synchronization is required and then you transmit the data at the start of the frame interval at the first slot after the frame interval so arrival you will be transmitting your frame so this slotted aloha doubles the performance of the pure aloha almost vulnerable time period is getting half so this is how you can compare the performance of the pure aloha and slotted aloha so if you compare yes is the throughput what you get from the network and x is indicating the frame transmission time and g is indicating the load which is average of transmission attempts per x seconds so you find that if you go for the pure aloha the maximum throughput you can get is 0.184 but the same thing if you compare for the slotted aloha the throughput is almost doubled so this is the advantage of advantage in performance for the slotted aloha these are the references used for this presentation thank you