 Today we will discuss switching techniques which are usually in a communication networks. Learning outcome, at the end of this session students will be able to describe what is circuit switching contents. In the following slides we will discuss the contents, all these are related with the communication network and the switching. Let us have a look at figure one which is of a communication network. We have these nodes, node A, B total 5 nodes are there, C, D and E. A wants to communicate with remaining 4, also B may want to communicate with remaining 4, so this way any terminal wants to communicate with any other terminal, so that is also called as an end station, so these are end station and these wants to communicate with each other. But to facilitate this what we require is a communication network shown in this circle, 6 different nodes are there which are interconnected and by which the message from say A end station can be transmitted to E through different nodes in the communication network. So this is the basic model of the switched communication network and various switching techniques are used. For example if A wants to communicate with C as we can see here, A wants to communicate with C using this communication network then which nodes are involved in that we can find out, then there are multiple paths available. So the data can be transferred in different ways or different paths, so one of the path just I will let you know from station A to node 1 then node 2 then node 6 then node 5 and then finally to C, so this is one of the path. Another path is node 1 to 3 to 5 and finally to C, so there are different paths available so with which path is chosen that depends upon the switching technique used. Let us again the purpose of this network is to provide a switching facility that will move data from node to node until it reaches the destination that is our aim is again node to node links are of higher bandwidth that means the inside the communication network each node is connected to every other node that is why these links are of higher bandwidth and to make maximal use of the bandwidth we use FDM that is frequency division multiplexing or TDM time division multiplexing techniques to improve the performance of a channel. Now let us see what are the different switching techniques. There are main three switching techniques that is circuit switching, message switching and packet switching. So in this lecture we will discuss more on this circuit switching and in the next lecture we will see the other techniques. Circuit switching consists of a dedicated path between two stations that means link is there between source station and destination station in circuit switch network before actual transfer of data. These network nodes are connected in such a way that it forms a path between source and destination and each physical line can have many logical channels and dedicated logical channel is associated with a particular communication. Now let us see the how circuit switching works. Circuit switching involves three phases first that is circuit establishment, second data transfer and third circuit disconnect are the three steps required for circuit switching to transfer data from source to destination. There are multiple paths from source station to destination and this allows us to improve the reliability of the network. So circuit establishment is done before actual data transfer is done and after circuit is established then data transfer occurs. What type of network it is whether it is handling analog signals or digital signals depends upon that but the circuit switching concept remains same for analog data as well as digital data. The connection between the two DTs which are communicating with each other through this communication network is generally a full duplex one. Then the last phase of circuit switching is circuit disconnect. So this is also necessary for the purpose that the link which was used for transferring data should be released for the new DTs. So in this terminate connection at the end of the data transfer and also the signals must be propagated to deallocate all the resources those are dedicated for this connection. Now let us have a question. The data transfer from source to destination is full duplex or half duplex. You pause the video and answer the question. You might have answered the question and the answer is in circuit switch network the data transfer takes place between source and destination in full duplex form. Now to connect each DTE for example here we have taken 8 terminals 8 terminals. So we require a matrix of 8 by 8 8 rows are there and 8 columns are there. So the connection is established between every input and every other output physically separate paths are provided for each of the each of these links by a crossbar matrix this is called crossbar matrix in space division technique. So in space division technique this is called a crossbar matrix. What happens here is a connection can be established between a vertical line and a horizontal line as we saw here. For example if row 3 for example this row 3 and column 8 this switch must be closed so that there can be a connection between 3 and 8. As the number of crosspoints grows with the square of the number of attached stations the complexity of the switching network increases and the solution to the problem is multi stage space division total 40 crosspoints are there in this network. But if you implement in a single stage then we would require 64 crosspoints advantages of circuit switching in end to end link bandwidth is fixed and it does not decrease. So with the fixed bandwidth we get data communication. In dedicated link delay is constant and so there is no congestion occurs. So congestion occurs only when there is a huge traffic but here since delay does not vary the bandwidth remains same and that is why the congestion does not occur. These advantages circuit establishment and circuit disconnect adds extra overhead and delay. So we know there are three steps so the first step is circuit establishment so that adds extra overhead and it causes a delay. Also it has got constant data rate which does not allow bursty data and third is channel capacity is dedicated for the duration of the connection even though no data is being communicated. So dedicated channel is goes west if you do not transfer the data. So these are my references data communication by professor A. Pal NPTEL lecture number 19 and the chapter is switching techniques thank you very much.