 assistant professor, electronics and telecommission engineering, Walton Institute of Technology, Singapore. Today, we will discuss X.25 Network, and this is in continuation with the previous lecture. Learning outcome, at the end of this session, students will be able to describe key features of X.25. Contents, in the following slides, we will consider frame format, packet format and multiplexing in X.25. Now, let us discuss X.25 frame format first. This is figure one which shows the X.25 frame format, wherein the user data and at packet layer of X.25, a layer three header is attached and in the link layer, LabB header and LabB trailer is attached to this information. This is what the format of the X.25 frame. At the beginning, there is a flag and also at the end there is a flag. In addition to that, address of source and destination is added. Then, some control information is added to perform flow and error control of the frames. Then, this is what the information presents. And in the information, there are different data, that is, i stands for user data, s is empty in case of X.25 and u for control data. The control data is necessary for controlling flow of information, which is used in an in-band signal. The address, as we discussed, there are only two addresses in X.25 because it is a point-to-point communication, address of the source and address of the destination. For example, here 8-bit address 01 is a command issued by the DTE and responds to it. As we know in X.25, DTE wants to communicate with other DTE with the help of X.25 network, which consists of DCs. So, the DC address 03 is a command issued by the DC and responds to it. Since it is a point-to-point communication, the communication is established between two DTEs and the data is transferred. Now, let us go to the packet format of X.25. These packets are broadly divided into two categories, that is, data packets and control packets. Control packets are also broadly divided into two categories. One is RR, RNR, REG packets, which are used for error and flow control. And the other packets are also needed in X.25 for signaling. So, here data packet is used to send user data, whereas control packet is used to perform flow and error control. Here the question, which packet type is used for flow and error control in X.25? You pause the video and answer the question. You might have thought over the question. And the answer is, error and flow control is done with the control packets. Now, let us see in detail X.25 packet format how the error control is done. Here flow and error control can be done with the help of packets like RR, which stands for receive request, RNR stands for receive not request and REG stands for reject packets. Then other packets are also necessary for in-band signaling. Here we can see flow and error control is done by using these control packets, RR, RNR and REG, whereas other packets are used for in-band signaling. Here flow control is done by using sliding windy protocol and error control is done by using go back end protocol. Some additional information specific to that packet may be added to the packet as shown in this figure 4. And there are two formats for sending this information. One is a 3 byte format and another is a 4 byte format. For example, here this is a 3 byte format which consists of 3 bytes. This is a 1st byte, 2nd byte and this is 3rd byte. So, these 3 bytes corresponds to data packet and this 1 byte, 2nd byte and 3rd byte. This corresponds to control packet and these 3 bytes, 1, 2 and 3, corresponds to these control packets named as RR, RNR and REG. Here we have different bits, Q bit, D bit. Then there is a group channel, here group is 4 bit and channel is 8 bit. Then PR, PS, these are used for acknowledgement between DTE and DC. There is also M bit. So, we will see what is the function of these bits in next slide. This is another format that is 4 byte format. Here this is a 1st byte, 2nd byte, 3rd byte and 4th byte. These 4 bytes corresponds to data packet, these 4 bytes corresponds to control packet and these 4 bytes corresponds to RR, RNR and REG packets. So, this is a 4 byte format. Here also the group is 4 bit, channel is 8 bit. So, total 12 bits local channel number we can create. Now we will see the function of these bits. Q bit is not used in context of x dot 25. Here D bit is when D is 0, then it is flow and error control between DCE and DTE. So, it is a local communication or it is to control the error between DC and DTE. And when D equal to 1, then it is used for N to N flow and error control. So, when D is equal to 1, then flow control and error control is done between 2 DTEs. As we seen 12 bit number of group and channel also called LCN. PR, PS are used for acknowledgement and flow control and M bit is corresponds to also the communication between either DTE, DC or DTE, DTE. So, this way we can have different control signals. Now let us go to the another important feature of x dot 25 that is multiplexing. Over the single DTE, DC link as we already seen with the help of 12 bits in the packet. There are 12 bits for forming the local channel number 4 from group and 8 from the channel. So, total 4095 virtual circuits can be created which are used for multiplexing the channel. As this x dot 25 speed limit is only 64 kilobits which is very low in context of today's data transfer. That is why we shall discuss about faster packet switching network in the next lecture. References, data communication by Professor Ajit Pal, Department of Computer Science and Engineering, Indian Institute of Technology, Kharagpur and the contents I have taken from NPTEL lecture number 24 on x dot 25 and frame relay. The link of the same is given here. You can go for further details. Thank you very much.