 Hello everyone, welcome to the session on Conjection Control, Learning Outcome. At the end of this session, student will be able to describe the concept of Conjection and general principles of Conjection Control. So, before starting the Conjection Control, let me explain you what is Conjection. So, the definition of Conjection, when too many packets are present in the subnet or in the part of a subnet performance degrade, then this situation is called Conjection. Look at this diagram. In this diagram, three LANs are connected to three routers that is R1, R2 and R3. These R1, R2 and R3, three routers are connected with each other with the communication line. So, whenever there is a communication or whenever there are the routers and the routers are communicated with the communication line, then it forms the subnet. Now consider, if there is a traffic from router 1 to router 2 and some heavy traffic is there, some heavy traffic is there and some packets are saturated here at the R2 side, then R2 is not able to retrieve that packet. So, this condition or this situation is known as Conjection. Now look at this graph shows the three different conditions, when too much traffic is offered Conjection set in and the performance degrade sharply. Now look at here in this graph on x-axis the packet sent from the source is given and on y-axis the packet delivered to the destination. This dotted line shows the maximum carrying capacity of the subnet. Now whenever the packet sent or the packet deliver is proportional, directly proportional to the packet sent, then that situation is the perfect situation, right means there is no any loss of packet. Now consider the next situation is Desirable situation. So, when the traffic increases too far, the routers are no longer able to cope and they begin losing the packet and this is known as Desire condition or the desirable condition. Now look at the Conjected condition, at very high traffic the performance collapse completely and almost no packets are delivered, then that condition is known as Congested condition and look at this third graph here it is shown it completely collapse means not a single packet is delivered to the destination. Conjection occur by several factors. The first thing is what inefficient memory to hold the packet. If there is this low processor then also congestion is occur, when there is a low bandwidth line then also the congestion is occur. Now look at the general principle of Conjection control. The first thing is what the Conjection control refer to the technique and the mechanism that can either prevent the Conjection before it happen or remove the Conjection after it has happen. So, on that basis or on that principle the Conjection is divided into two category, the first one is open loop category and the another one is closed loop category. Now let's see the category open loop Conjection control. In open loop Conjection control policies are applied to prevent the Conjection before it happen. In this mechanism Conjection control is handled by either the source or the destination and in closed loop Conjection the mechanism try to elevate the Conjection after it happen. Several mechanism have been used by the different protocols or by using different protocols. We will see the different protocols in this session only. Now let's see the open loop policies and closed loop protocol. So here there are total five open loop Conjection control policies, the retransmission policy, window policy, acknowledgement policy, discard policy and admission policy and for closed loop Conjection control the back pressure protocol is used, choke packet is used, implicit signaling and explicit signaling is used. So let's see all these one by one. So here retransmission is sometimes unavoidable. If the sender feels that a sent packet is lost or corrupted, the packet need to be retransmitted. Retransmission increase the concession in the network, however a good retransmission policy can prevent the concession also. The retransmission policy and the retransmission timer must be designed to optimize the efficiency and at the same time it prevent the concession. Now let's see the another policy that is window policy. Two methods are used in this window policy that is selective repeat and go back in. The selective repeat window is better than the go back in window for concession control. Why? Because in go back in window when the timer for a packet expire then the several packets may be recent although some packets may have arrived safe and sound at the receiver. So this creates the duplication of the packet and because of this duplication the concession is occur. But in selective repeat window it tries to send the specific packet that have been lost or the corrupted. So that's why the selective repeat window policy is better than the go back in window policy. Pause the video and write down the answer. Which of the following policies are known as open loop concession control policies? Back pressure, acknowledgement, choke packet or discarding policy? Here is the answer the acknowledgement policy and the discarding policy are the open loop concession control policies. Now let's continue with the open loop concession control policy. The third policy is acknowledgement policy. If the receiver does not acknowledge every packet it receive it may slow down the sender and help to prevent the concession. Several approaches are used in this case. A receiver may send an acknowledgement only if it has a packet to be sent or a special time expire. A receiver may decide to acknowledge only n packet at a time. Acknowledgement are also part of the load in the network. And sending the fewer acknowledgement means imposing the fewer load on the network. The another policy is discarding policy. A good discarding policy by the router may prevent the concession and at the same time may not harm the integrity of the transmission. For example in audio transmission if the policy is to discard the less sensitive packet when the concession is likely to happen then the quality of the sound is still preserved and the concession is prevented or elevated. Now the last policy of open loop concession control is admission policy. An admission policy which is a quality of service mechanism and can also prevent the concession in a virtual circuit network. A router can deny establishing a virtual circuit connection if there is a concession in the network or if there is a possibility of a future concession. Now let us see the closed loop concession protocols. So as I said there are various protocol like back pressure, choke packet, implicit and explicit. So let us start with the back pressure. Back pressure is a node to node concession control and start with the node and propagate in the opposite direction of the data flow or to the source. So the back pressure technique can be applied only to virtual circuit network in which each node know the upstream node from which the flow of the data is coming. Now look at this diagram here source transmit the data to destination so data flow is from left to right. Now suppose concession is happen at the node third then it send the packet to the router 2 then from router 2 to 1 and 1 to source like this. So this is the back pressure policy or we can say that back pressure protocol. So here in this protocol there is no direct transmission from conjected node to source. So the middle routers take the part in this back pressure. Now look at the choke packet. A choke packet is a packet sent by the node to the source to inform it of concession. In the choke packet method the warning is from the router which has encounter concession to the source station directly. So the intermediate node through which the packet have travel are not worn. Now when a router in the internet is overwhelm with the IP datagram it may discard some of them and but it inform the source host using the ICMP message ICMP that means internet control message protocol. Now the warning message goes directly to the source station. The intermediate router does not take any action. Look at this diagram. In this diagram whenever the concession is occur here at the third router then this router directly send the choke packet to the source and these middle or this intermediate node does not know about the concession. So this is choke packet policy. Now look at the another closed loop concession control protocol that is implicit signaling. In implicit signaling there is no communication between the conjected node or the source. The source guesses that there is a conjection somewhere in the network from symptom and means like for example when the source sends the several packet and there is no acknowledgement for a while. One assumption is that the network is conjected and the delay in the receiving and acknowledgement is interpreted as a conjection in the network and the source should, source itself slow down the transmission. Then the next one is explicit signaling. Now what happen in this? The node that experiences the concession can explicitly send a signal to the source or the destination. In the explicit signaling method the signal is included in the packet that carry the data and the explicit signaling can occur in either the forward or the backward direction. So direction is not fixed here. They can transmit the data from source to destination or from destination to source. So all these are the policies and the protocol provide by the conjection control. These are the references. Thank you.