 Hello friends, in this video we are going to see about Error Control Mechanisms used in Sliding Window Flow Control Methods. At the end of this course we are going to understand the working mechanisms of various Error Control Methods and we are going to compare the performance of these methods. The goals we are trying to find the effect of errors and how to control these errors in Sliding Window Flow Control Mechanism and there are two methods are defined. The first is go back in ARGU, it is standing for Atomatic Repeat Request and second is Slate Reject Atomatic Repeat Request. At the end we are going to compare the performance of these two methods. Before we start the actual mechanism, let us try to understand some important terms used in Sliding Window Flow Control Method. In this mechanism we use the frame-based communication, each frame is given with the sickness number, then at the center side we manage one window, also we manage one window at receiver side. Before we start communication, we define the window size and finally during communication they use a feedback signal, it is named as receiver ready for nth sickness number. Let us try to understand the basic working mechanism of Sliding Window Flow Control. There are two sides are shown in the figure, one is transmitter, other is a receiver. At the transmitter side we manage one window, it is called sender's window and also we manage one window at receiver, it is called receiver's window. The size of windows are very much same, at the center side the sender stores all the frames which he want to transmit on the channel. The sender sends some frames on the channel. In the figure it is shown that it is sending frame F1, F2, F3 and F4 and after some time the frames are coming at receiver side. The receiver is applying some check against error and he is going to plan the feedback. Now in the figure it is shown that the receiver is applying check on frame F1, F2, F3 and he is planning for giving the feedback here. So he gives the feedback as RR4, it means that he acknowledges all the three frames that is F1, F2, F3 and he is waiting for the fourth frame onwards here. Now in this method, in this case even the frame number 4 is issued correctly and suppose the feedback is coming as RR4, the sender has to start its next phase of transmission from the frame number 4. That is frame number 4 is discarded by the receiver. This is the policy we use in the studying window flow control mechanism. Now we will consider errors on the channel during communication. Now in the practical channels we normally are finding some errors on the channel. Now these errors are because of noises and various parameters occurring on the channels. Now because of these errors the data inside the data frame is get changed here. That is the 0 may change to 1 or 1 may change to 0 here. And this single bit error is going to garble entire frame because a single bit error is going to mismatch the checks that we insert in the frame and the receiver side is going to give the feedback as the negative acknowledgement. Now there are two types of error control methods we define. The first is called go back and ARQ. The second is related to reject ARQ. Let us try to understand the working mechanism of go back and now in this diagram again the sender is going to plan to send the frame F1, F2, F3, F4. After sometime these frames are coming at the receiver side and again this is going to apply the check against error and first he is going to keep all the received frames in the window that is F1, F2, F3 and F4 and when he check the frame F2 against error he is finding some error here and so that he is going to plan the feedback as RR2. The number 2 is going to acknowledge the frame F1 and he also denotes that the receiver is going to expect the frames to onwards here. Now getting this feedback on sender side the sender again plan for the next phase of communication by re-aligning its window that is he will start its next phase of communication from F2 and onwards here that is F2, F3, F4 and onwards here. In my in this case even this frame F3 and F4 are received by receiver successfully the sender is going to start its communication from frame number 2 onwards here that is frame number 3 and frame number 4 are discarded by the receiver. This kind of mechanism is called as the go back in ARQ method. Let us try to understand one more method here it is called slate to reject ARQ mechanism. Now again in this diagram the sender is planning to send the frame F1, F2, F3 and F4. Again the same thing is happening here the F2 is having error inside the frame. So the receiver is going to give the feedback as the feedback signal now like in this case the feedback signal is taken as Rej and after that we are going to write the frame number in which we are finding the error. In this case we are having error inside the frame number 2. Now when this feedback comes back to the sender side the sender only repeats the frame F2 and he will continue to send the previous frames onwards here that is he will plan to send the frame F5, F6 and onwards here that is in this mechanism we only repeat that frame which is under error and this kind of method is called as slate to repeat or also it can be called as slate to reject method. Now how to compare the performance of these two different methods here. Now to understand the effect of its performance we take two things in talk on the first is total delay of transmission and second is the bandwidth required for entire file transmission. Now just try to put a question here out of these two methods which is the best method and when to use which method during communication. Now in the go back end method the transmitter is going to repeat some of the frames he already sent on the channel hence in this method we require more frames to transmit to send this entire file and because we are sending more frames in this method we also require more bandwidth on the network now whereas in the slate to reject ARQ in this method the transmitter only transmit those frames which are under error and he is not repeating any additional frame except the error frame on the channel hence we require less time period to transmit the entire file on the channel and also we require less bandwidth because we are having less frames to transmit now going beyond now when to use which method actually go back and method is recommended for those networks in which errors are occurring frequently on networks because the error pattern normally we encounter it is seen that when there is coming in a single frame it remains for number of frames on the given channel it is normally happening with the networks having low performance issues there so go back in methods is recommended for those networks which are having some lower performances during communication whereas the slate to reject ARQ method is recommended for those networks which are very good and they're having very less errors during communication that is the networks which are having better performance during communication so it is found that in this networks errors are coming quite rarely and even this errors are coming on network they don't remain more than one frame here so in this case we are recommending a method which is called the slate to reject ARQ so as a underline we can say go back in method is recommended for those networks which are having poor performances or errors are coming quite frequently and the slate to reject method is used in that network in which the error is coming quite rarely now these are my references I use two books here one is for us and other is William Stylings