 Hello learner friends welcome to this video session on data link layer protocol called as HDLC high level data link layer control protocol part one of this video myself Mr. Vipul Kondekar from Walton Institute of Technology, Sholapur these are the learning outcomes of this video session. So, you will be able to explore and explain that HDLC protocol and there are some concepts like station modes of operation and how you can have transition from one mode to another mode. So, you will be very easily able to understand that mode transition also. So, these are the contents of this video. So, let us start with this introduction part. So, HDLC is basically a bit oriented protocol used for communicating point to point and multipoint links P to P and point to multipoint. If you want to have the communication. So, at the data link layer you can go for using a protocol called as HDLC. The specifications for this HDLC protocol are developed by ISO. It is improved version of IBM's SNA HDLC protocol. It is a superset of HDLC and it is a protocol following all the specifications in the OSI layer too. So, that is data link layer. HDLC supports the communication half duplex as well as full duplex over point to point and multipoint connection as well it supports asynchronous synchronous communication. One important aspect of HDLC is there is full transparency in HDLC data. Transparency in the sense looking at the HDLC frame I will be able to easily interpret all the fields of that particular HDLC protocol. But the point is when you are saying that this HDLC is OSI layer 2 protocol OSI layer 2 is nothing but a data link layer. So, can you recall what are the functions perform at the data link layer? Yes. So, the various function perform at the data link layer most important function is data link layer does framing, data link layer does error control, data link layer does flow control and access control. Now it will be interesting to understand how all those functions are accomplished in this HDLC protocol. When you want to understand HDLC protocol, so there is one concept used called as station. So, data link layer at each node is called as a station. There can be three kinds of station possible in HDLC. A station can be either a primary station. If it is a primary station it is a controller of communication. So, it is responsible for issuing the commands. Primary station will be controlling the communication, it will be sending the commands and it is responsible for maintaining the link to the receiver that may be a secondary station. In the network supporting HDLC protocol, a station may be a secondary station also. What station secondary station will do? Secondary station will operate under the control of primary station and secondary station will issue responses. What secondary station will send will be responses to the commands sent by the primary station. Third kind is possible which is called as combined station which is the combination of primary and secondary station. Where if it is a combined station then combined station may issue the commands or may give the responses also. It combines the feature of primary as well as secondary station. As well in HDLC there can be two kinds of line configuration. One is called as unbalanced line configuration another is called as balanced. So, when in the network one is a primary and other one or more secondary stations are there. One is primary and more than one or one or more than one secondary stations are there. So, that line configuration is called as unbalanced configuration. But if you have two combined stations and combined station you know they can work as primary as well as secondary. So, both combined stations are trying to communicate with each other then that will be called as balanced configuration. So, two configurations possible with HDLC unbalanced and balanced. Now, whenever you have a station that station may be operating in one of the modes. So, there are three important modes we will try to understand for HDLC. The first mode is normal response mode. So, what happens if the no if the host in the network is there in normal response mode. Normal response mode corresponds to the unbalanced configuration. Where there will be a primary station and one or more secondary stations will be there and primary station can only initiate the transmission. And when primary has initiated the transmission secondary may only transmit the responses to the frame sent by the or the command sent by the primary. This NRM is used for multi drop lines. Where the host computer is considered as primary and other terminals are termed as secondary stations. So, this is normal response mode. Now, you will come to know here is one example this is a primary station this is a secondary station primary station is sending the command and secondary station is sending the frame which is the response to the command. This is a point to point configuration you may have point to multi point configuration where a command is sent to more than one secondary stations and both the secondary stations are responding to the primary station who has sent the command. So, this is about the normal response mode. Second important mode of HDLC is asynchronous balanced mode. Now, balanced mode means you know that the stations are combined. Now, what is the property of the asynchronous balanced mode is it is a balanced configuration either of the station can send the command or response means any station can initiate the transmission without receiving the permission which was not possible in the earlier mode. Primary only has to initiate the communication there, but in case of ABM. So, a station can send command or it can send response to the command sent by other station. So, here command and responses are possible here also command and responses are possible in earlier case this was not possible command was only sent by the primary station secondary is sending only responses in normal response mode, but in asynchronous balanced mode both command as well as responses are sent by each station because the station is of kind combined station and hence it is the most widely used mode of operation. So, there is no polling overhead invoke in this particular mode also. Now, let us go for third mode possible which is rarely used and that mode is asynchronous response mode. So, again it is a unbalanced configuration, but here secondary may initiate transmission it is unbalanced and secondary may initiate the transmission and primary will be responsible for doing the maintaining the link means connect disconnect error recovery initialization all those tasks will be given to the primary station. So, this mode is rarely used, but still it is one of the modes for the HDLC protocol. Now, along with these modes there are few non operational modes also. So, those modes are normal disconnect mode, asynchronous disconnect mode and initialize mode. So, these are the two disconnect mode whenever you power on you will go in one of these modes. So, if you want to go to the normal response mode you will enter into the normal disconnect mode first. This you will understand with the next block diagram which we will discuss about the state transition diagram. So, these are the non operational modes two disconnect modes are there and one is initialization mode which is quite important where you want to need regeneration of what we can say parameters or exchange of parameters you will go to the initialization mode. So, these are three non operational modes in HDLC. Now, let us try to understand how we can have transition between these modes in case of HDLC transition. So, this transition is taking place with the help of mode setting commands. Now, these are the different mode setting commands SNRM stands for set to normal response mode, set to asynchronous response mode, set to asynchronous balance mode, set to initialization mode, DIS stands for disconnect mode. Now, if you look at this transition diagram see whenever you power on a station either you will be in the AC mode synchronous disconnect mode or normal disconnect mode. Assume that you are in the normal disconnect mode. So, from normal disconnect mode what you can do is you can switch to normal response mode and if you are already in the normal response mode you can be in normal response mode with the help of mode setting command called as SNRM. If you are in normal response mode and if you want to get disconnected your communication is over. So, you can come back to the disconnect mode by using mode transition command called as DISC. Like that at the power up you may be in the asynchronous disconnect mode and from asynchronous disconnect mode you can go to one of these asynchronous mode. Either you can go to the asynchronous response mode by using command SARM or asynchronous balance mode by using the mode setting command SABM. Again similar to the normal response mode if you are there in asynchronous balance mode or asynchronous response mode and your communication is over you want to come back you want to switch off. So, again you will use the DISC command. So, that you will come back to the disconnect mode and if you want to be in the balance mode or response mode asynchronous response mode there can be a command called as set to ARM. If you are in ARM and you want to be in ARM. So, more transition command will be SARM itself. Now interestingly sometimes if you want to switch from asynchronous mode to normal mode or normal mode to asynchronous mode is it possible? Yes it is possible. What you can do is if you are in the normal response normal disconnect mode and you want to go to the asynchronous balance mode or asynchronous response mode what can be done is first you go to the initialization mode. Go to the initialization mode. Here also if you want to change the parameters go to the initialization mode and from initialization mode after initialization mode you can go to any of the modes you can go to an ARM you can go to ABM you can go to SARM. So, this is how the mode transitions are possible with the help of these different mode setting commands in case of HDLC protocol. So, this is all about the part one of HDLC protocol. These are the references used for this video presentation. Thank you.