 Sudhakar Barabadek, Assistant Professor, Electronics and Telecommunication Engineering, Valkan Institute of Technology, Singapore. In continuation with the previous lecture, we will discuss switching techniques and in that we will see message switching and packet switching. Learning outcome, at the end of this session, students will be able to describe what is message and packet switching, contents. In the following slides, we will consider these points like message switching, what are the advantages and disadvantages of message switching, then packet switching, virtual circuit, datagram and the comparison between datagram and virtual circuit. Let us go to the first one. As we discussed in the previous lecture, one of the disadvantage of circuit switching is constant rate of data transfer. Disadvantage, because nowadays the nature of data to be transmitted is burst U1 and this type of data transfer is not possible with the circuit switching method. Then we go for this message switching, wherein if you look at these are the, this is what message switching network, which consists of nodes. These are different nodes, which has a facility of store and forward. That means, whatever a message is coming, which is stored and then forwarded to the next node, until it reaches to the destination. In earlier circuit switching, node where only dumb nodes, those were not having storing facility. Here storing facility is added, so that we can remove the drawback of circuit switching. This is a source DTE, this is a destination DTE and these are connected by nodes, which comes under this communication network. There are four nodes here and each node is having capability of store and forward. If you look at the advantages of message switching, the line efficiency is greater as the line, which is not used by a particular DTE can be used by other DTE, so the line efficiency increases. Because of this store and forward facility available in the nodes, we can have a variable data rate and with that a burst in nature of data can be transmitted. Also we can assign message priorities to the messages, so that the higher priority message can be forwarded first from a node. Then the disadvantage of message switching are message of large size dominates the link and storage. If the size of the message is large, all the links between nodes are used by this message only, no other messages are allowed to take the links. So the large message dominates the link and storage and that is why this message switching concept has been extended to packet switching. So this disadvantage is removed in the packet switching. Now here a longer message is broken up into small series of packets. Now earlier in message is broken now into small packets and these packets are then transmitted from a source to destination through the nodes in a network which is having store and forward facility. So here data are transmitted in a small packets and if you look at this is a large message, this large message is broken into n number of packets, packet number 1 and packet number n and each packet is having some header information which is used for routing purpose. That means to find the path from source to destination. So every packet contains the control information in the form of header used for routing. Now let us have a question in which of the following node is buffered, one circuit switching, message switching, packet switching both A and B both B and C. You pause the video and answer the question. You might have thought over the previous question. The buffering is available in message switching as well as packet switching. So the correct answer is B and C. Now let us go to the virtual circuit which is created in a given physical link. So how the virtual circuits are created and what are the different types of virtual circuits that we will discuss in this slide. There are two approaches used to handle packet in a packet switching. One is a virtual circuit and another is a datagram. So let us see what is virtual circuit. In virtual circuit route between stations is set up prior to data transfer means the circuit is established before the data transfer in virtual circuit which is similar to circuit switching and a packet is buffered at each node and queued for output over a line. So buffering is available that means storage is available at each node and it is like message switching. In addition to data the packets carry a virtual circuit identifier required for routing. So in addition to this data VCI virtual circuit identifier is required for routing and that is why the intermediate nodes do not take routing decisions. Who takes the connection between the two DTEs established prior to data transfer? So the connection is established before data transfer at that time only the routing information is available. So there is no need of routing done by individual nodes in a communication network whereas in datagram approach treats each packet independently. So there is no need of prior circuit connection in datagram because each packet is treated as independently and even the connection is not established prior to the data transfer in datagram. So every packet contains source and destination addresses and here you can see here every intermediate node maintains routing table so that the packet can be transferred to the appropriate destination. If you compare these two approaches what we have seen is in virtual circuit all packets are sent through the predefined route path is established before data transfer that is why predefined route is there packets are transmitted in sequence. There is no need of what we can say or we cannot transmit the packets in random way. So it should be transmitted in a sequential manner packet number one then two then three like this. No routing decision is taken by the node routing decisions are taken at the in the beginning only so nodes in the communication network does not play any role in deciding the path for the packets. Link is prone to congestion in virtual circuit since the packets are transmitted in a predefined route there is a possibility of congestion whereas in datagram call setup phase is avoided see here the circuit establishment is not done prior to the data transfer it is done dynamically. So each packet is transmitted independently and this approach is more flexible and reliable in a communication networks references data communications by professor Ajit Pal department of computer science and engineering Indian Institute of Technology Kharkur and I have taken this from NPTEL lecture number 20 switching techniques in that the packet switching. So the link is also provided with you thank you very much.