 We will discuss x dot 25 network, myself Sudhakar Barabade, assistant professor, electronics and telecommissioning engineering, Walchen Institute of Technology. Learning outcome, at the end of this session, students will be able to describe key features of x dot 25. In the following slides, we will discuss features of x dot 25, x dot 25 layers and virtual circuits. If you look at this diagram, this is a x dot 25 network which consists of DCE, data communication equipment and the DTEs that is data terminal equipments are connected to these DCs. So, the network consists of DCs and the communication takes place between the two DTEs. This type of network is a that is x dot 25 is a packet switch network which is developed by ITUT in 1976 and it defines how a packet mode terminal that is DTE can be interfaced to a packet network that is called a DCE used for data communication. So, how this communication takes place in this x dot 25 that we are going to discuss. Here we are going to study the x dot 25 layers. This is a standard OSIM model which consists of seven layers and in x dot 25 only three layers are implemented and the names given to those are physical, frame and packet. So x dot 25 has three layers, physical layer, frame layer and packet layer. Let us see the purpose of each layer. Physical layer deals with the physical interface between station and packet switching node. Station here it is named as a DTE and packet switching node is a DCE. So, how the DTE and DCE of a communication network are connected by a physical interface that is named as physical layer. So, it consists of different electrical characteristics. Then at the next layer, frame layer is responsible for reliable transfer of data across physical layer by transmitting it as a sequence of frames. Data is grouped into different frames and these frames are then transmitted in a physical link in the communication channel. And at last packet layer which facilitates for end to end connection between two DTs. So packet layer performs the circuit establishment between two DTs and once the circuit establishment is done packets are transferred. Let us see what are the functions performed by packet layer. First function is establishing connection as we discussed earlier. Then transferring the data, then terminating a connection. So these are the three stages involved in packet layer. In addition to that packet layer what it does is in X dot 25 packet layer and frame layer both the layers perform error and flow control. In addition to this data transfer, circuit establishment, circuit disconnection packet layer does also the error and flow control. Let us have a question. Connection between two DTs is established by which of the three layers in X dot 25? You pause the video and answer the question. You might have thought over the question. The connection between two DTs is established by packet layer. Now let us go to the virtual circuits. So this is a figure which shows the virtual circuit. So this is the X dot 25 network which consists of DCs, data communication equipments and these DTs are attached to DTE. So here we are having four DTs DTA, B, C and D and see this there is only one physical link between DTA and DTC but here a virtual circuit of three channels is created which can connect this DTA to DTB through this path, then DTA to DTC through this path, then DTA to DTD through this path. So using single physical link, three virtual circuits are created. So virtual circuits are created at packet layer, several virtual circuits are created through the same link using in-band signaling technique. Virtual circuit identifier is known as local channel number which is assigned to the every virtual circuit. Now there are two types of virtual circuits we will see. One is a permanent virtual circuit also called PVC which is somewhat similar to Lease line used in telephone network. That means in permanent virtual circuit the connection is always present between the DTs. So it is same as a private line. Then next is a switched virtual circuit also called SVC wherein links are set up between the local DT, DC and the remote DTDC before actual data transfer. So before transferring the data link must be established between local DTDC and remote DTDC pair. Then virtual circuit is set between the local and remote DTs. So by doing this the DT1 and DT2 circuit is established. Then the data transfer is performed between the DTs and at the end the virtual circuit is released and also the link is disconnected. So this way switched virtual circuit performs set up data transfer and disconnect these different phases. In summary what we have studied here is we have made a table of features of X.25. So different features and at what layer that task is done that is also shown here. So if you look at connection establishment is done at network layer, flow and error control is done both at frame layer and network layer, data rate in this X.25 is fixed one. Then multiplexing is done at a network layer and congestion control which is not necessary because the line is of fixed capacity. So there is no possibility of congestion references. Data communication by professor A. Parl, Department of Computer Science and Engineering, Indian Institute of Technology, Kharagpur and the contents are taken from NPTEL lecture number 24 and the chapter is X.25 and frame relay. The link of the video is also given here. You can refer this for additional details. Thank you very much.