 Hello dear learner friends, welcome to this video session on data link layer protocol called HDLC part 2. So, as we know HDLC is high level data link layer control protocol and was one of the popular protocols at data link layer. Myself Mr. Vipul Kondekar from Walchand Institute of Technology, Sulapur. So, let us go for this HDLC protocol, these are the learning outcomes of this video session. So, in this session we will try to understand the frame format for HDLC, how exactly a frame is formed, what are the different parts or portions of that HDLC frame, what is the significance of each and every portion, so that we will try to understand. As well you will be able to understand how some frame based protocol or frame based communication takes place. Now, in this presentation we will have the contents small introduction, then we will go for different HDLC frame types and then what are the different control field formats in HDLC frame. So, in the HDLC when you are talking about HDLC it is basically a bit oriented protocol which can be used for point to point communication as well as point to multi point communication that is what we have seen. All the specifications are for HDLC are developed by ISO and rather this HDLC is improved version or superset of IBM's SNA, SDLC protocol. As we know that OSI layer model suggests that there will be layer 2 called as data link layer. So, HDLC is one of the most widely used data link layer protocol when initially they started thinking of having computer networks. So, this HDLC was quite popular because it was supporting half duplex communication as well full duplex communication it was supporting synchronous or synchronous point to point as well as point to multi point communication. Now, as we want to concentrate in this session on frame format for HDLC can you enlist what are the different framing methods recall right. So, framing is nothing but dividing the data into the manageable data units, framing is done by the data link layer of the sender, data link layer receives the data from the upper layer called as network layer. So, it receives the packet and those packets has to be merged into frames so that those frames I can send on to the physical layer and there are different framing techniques how you will form the frames. So, framing techniques starts with the character count technique, second technique may be you can have byte stuffing then there can be a technique called as bit stuffing framing with bit stuffing and you may have a technique called as physical layer coding violation. So, these are the different approaches for framing. So, it will be interesting to know what type of framing is used in HDLC also. So, with this there are three types of frames in case of HDLC either it will be a information frame, supervisory frame or unnumbered frame. So, they are named as iframe as frame or u frame now this iframe is the information frame means it is used to carry the data which you want to communicate also it is used for flow and error control information when you want to transfer. Supervisory frame is used for providing additional functions for flow control and error control and unnumbered frames are there those unnumbered frames are used for management of the network, management of the link, carrier information management. So, those are done by the unnumbered frames. So, these three types of frames are possible in case of HDLC now this is the general format for the frame and how exactly it varies for the three kinds of frames for HDLC. So, in general the HDLC frame start with a flag bit it will be of 8 bits and it will end up with again a flag bit. Then you have a address field, control field, information and fcs fcs stands for frame check sequence. Now, few of these fields are available as it is and few are not there. So, just if you compare this general format with the i frame you will find that almost all fields are there in information frame, but if you compare this format with the s frame in s frame the information field is not present. In u frame the information field is there, but that has got some different significance that is the management information you are having in the s frame. So, this is something you are getting from the upper layer may be from the user information may be the information you are getting from the upper layer called as network layer, but when how it is converted into i frame is these headers these fields are appended and this is the format for the HDLC frame and this is how the frames are sent. So, this is how you are sending flag, then address, then control information, then user information fcs and flag again. Now, let us try to understand what is the significance of these different fields. Now, if you look at the flag bit pattern the flag bit pattern used here is this 0, then 6 consecutive ones and 0s and the type of framing you can recall the framing technique used for HDLC is bit stuffing. So, in bit stuffing you have to take care of this format will be there or this will be the flag format for indicating the start of the frame as well end of the frame, but in the information field you have to make sure that this format will never occur. So, bit stuffing works like in the data if you find 5 consecutive ones I will stop 1 0 bit. So, that I will make sure that 6 consecutive ones will appear only for the start of the frame or end of the frame. So, bit stuffing technique is used for framing and this is what is the explanation already we have seen in the data link layer functionality how bit stuffing technique works. So, it checks for 5 consecutive ones. If 5 consecutive ones are detected then it will what we can say is tough 1 0 in the data. Now, address field, now the use of address field is to identify the station to which you want to communicate may be secondary station in most of the times and usually it is of 8 bit long, but it is extendable means if there are some requirements here where you may extend that address field and it will be extended in multiples of 7 bits. And there also you will look for the LSB least significant bit of each octet of extended address and if that bit is 1 then that will be considered as last octet of the address. And if it is 0 then still more octets are there representing the address that is one special address if all ones are there in the address field means that is a broadcast frame. So, that frame is to be received to the all the hosts within the network. So, address field consist of address of the receiver if the frame is sent by the primary station. So, it contains the address of the secondary station if it is sent by the secondary station if secondary station is sending the data who will be receiver primary will be the receiver. So, the address field will have the address of the primary station and as I said it is extendable. So, as the address field may be 1 byte or several bytes long. Then you can have a control field and then the control field can be either 8 or 16 bit and this control field is used for flow management flow control is done. And different means flow control field will decide what type of frame it is. So, if the control field starts with 0 then it is information frame. If it starts with 1 0 it will be a supervisory frame. If it is 1 1 then it will be a unnumbered frame and you will find this with this slide. So, if this is a control field format so, if it is 0 then this is a information frame if it is 1 supervisory 1 1 then it is unnumbered frame. And then based on these bit values other bits have got the significance means if it is a information frame. So, this bit will be representing that it is a information frame and then next 3 bits will be representing N of S. N of S is what? N of S is representing here the sequence number of the sender. N of R is the received sequence number. As sliding window protocol is used by this HDLC for flow control. So, there is a requirement of giving the sequence number for the frame and this is what is done. This is how the sequencing is done, sequence numbering is given done. So, these are the different fields where S stands for supervisory function bit, N of R is receiver sequence number, received sequence number, N of S is transmit sequence number, M stands for unnumbered bit, P oblique affix stands for poor oblique final bit. So, it is used for interaction purpose. So, this is something how we use that P oblique affix bit. So, if it is a command frame, so P bit is used for pole for primary, may say P bit is one. So, what you are doing is you are soliciting response from the peer. In the response frame that F is its interpretation taken as F bit and if it is one it indicates that response to soliciting command. So, this is about P oblique F bit. So, these are the different bits used in the bit formats used in the HDLC frame and this is how the HDLC frame is formed and as additional functions as we have discussed HDLC does error control. So, it uses CRC for error control, cyclic redundancy check for error control, it does flow control uses sliding window protocol for the flow control. So, this is in brief the frame format how framing is done for the HDLC protocol and significance of different fields in the frame format. With this we will stop here. These are the references used for this video presentation. Thank you.