 Hello everyone, today we are going to study IEEE 802.5 token ring frame format. In my last video, I have explained what is token ring and how the breakages are find out with the help of wire center and how the break station is eliminated by using the bypass relay. So, learning outcomes are at the end of this session student will be able to describe IEEE 802.5 token ring frame format, they will also explain how ring maintenance is carried out in token ring. Let us start with the token ring frame format. Token ring frame consists of various field. First is start delimitator and the end delimitator. Start delimitator and end delimitator reserved 1-1 byte for itself. Start delimitator and end delimitator specifies the start and the end of the frame. It contains invalid differential Manchester pattern to distinguish them from data byte. So, what is Manchester and differential Manchester, I have explained it in my last video. Differential Manchester and Manchester, it represent the transition in the middle of bit. That means, when the one signal is there, it represent the signal high to low and when 0 is there, it represent low to high, but ring in this IEEE 802.5 token ring, it shows the pattern high high and low low. So, this is invalid pattern, invalid pattern to distinguish from the data bytes. Then the next field is access control, which reserved 1 byte for itself. So, it contains the priority bit, token bit, monitor bit and reservation bit. Priority bit that means, from this 1 byte, first 3 bits are reserved for priority bit. When all these 3 bits are 0 0 0, that means that frame has lowest priority and if these bits are 1 1 1, that means has a highest priority. So, similarly token bit, monitor bit and reservation bit has its specification specified to work. So, this priority bit, token bit, monitor bit and reservation bit, I have explained in my previous video. The next bit is frame control, that is FC. The frame control byte indicate whether the frame contain data or control information in control frame, this byte specify the type of control information. We will see what is frame control and how these bytes are reserved for the various frame in this video only. So, the next field is destination address and source address. 2 6 byte field that identify the destination and source station MAC address. The next field is data field which is the no limit field, but the maximum length is limited by the ring token holding time which define the maximum time a station can hold the token. So, if we compare this token ring with the previous LAN that is Ethernet and token bus, see data capacity is higher than the previous one. Then the next field is checksum, it reserved 4 byte for itself. This checksum is filled by the source station with the calculated value depends on the frame content. The destination station recalculate the value to determine whether the frame was damaged or not and if it found the frame is damaged it immediately discard that frame. Then the last byte is FS that is frame status. Frame status is a byte or is a field which is not present in Ethernet and token bus. So, this is something a special frame status field present in token ring. So, here it contains 2 bits A bit and C bit. When the frame is arrived at the interface of the station, interface check whether that particular frame has destination address or not. If the destination address is present then the A bit is turn on or interface turn on the A bit. If that frame contain data then that frame is copied into the interface of that station then interface set C bit, C bit as a 1 or it turn on the C bit. So, similarly there are 3 possible combinations. So, if A equal to 0 that means the destination address is not present, if C equal to 0 that means the frame is not arrived and not copied to interface. Now, the next combination is A equal to 1 and C equal to 0 that means the destination address is present but frame not accepted. The third combination is A equal to 1 and C equal to 1 that means the destination address is present and frame is copied to the interface. So, this is about the all fields of token ring. Now, pause the video think about this question and write down the answer. The question is in token ring which field is present only in data or command frame but not in the token frame. These are A, SD that is start frame delimiter, B, AC access control, C, ED that is end frame delimiter and D, FC that is frame status and the answer is frame status. This field is present only in the data or command frame because a token frame consists only three fields that is SD, AC and ED, therefore the option D is the correct answer. Now, let us move toward the token ring maintenance. In token ring LAN ring maintenance is fully decentralized way. Each token ring has a monitor station that oversees the ring. If monitor goes down, contention protocol immediately elected other station as a monitor station. There are various responsibility carried out by monitor station and these are seeing that the token is not lost, taking the action when the ring breaks, cleaning the ring when the garbled frame appears and watching out for the orphan frames. So, there are various type of erroneous frame present in a ring during the transmission. So, these frames are orphan frames, lost token and garbled frame. So, what do you mean by orphan frames? So, here the monitor detect orphan frames by setting the monitor beat in the access control byte, ok. If an incoming frame has the beat set something is wrong, so the same frame has passed the monitor twice and evidently it was not recovered by the source. So, the monitor drain that token when that frame is a orphan frame. Then the next is lost token. The monitor has a timer that set to the longest possible tokenless interval. When each tokenless interval that means what? When each node transmit for the full token holding time. If this timer goes off, the monitor drain the ring and issue a fresh token to that particular ring. Then the third is garbled frame. The monitor can detect such kind of frames by their invalid format or checksum. That means when the frame is arrived at the interface, the station check the checksum field first and then it find out whether that frame is a garbled frame or a non garbled frame. So, such type of frames are drain from the ring and issue a fresh frame or a fresh token. Now let us see ring maintenance or ring maintenance with the help of frame control. Now frame control reserved 1 byte for itself. So how many beats are present? Yes, total 8 beats are present. When all these 8 beats are 0, 0, 0, 0 then the duplicate address test is provided. That means what? That means what? When this session found, there are two stations which having the same addresses. So, at that time station sends duplicate address test, ok. Now then the next frame is beacon frame. When the frame control field set to 0, 0, 0, 0, 0, 1, 0 then the beacon frame is sent which is used to locate the break in the ring. Then the next frame control is claim token, ok. It is attempt to become a monitor. When the ring has been initialized and any one station want to become a monitor station then it sends the claim token frame to the ring and it become a monitor. Then the next frame is purge frame. So, why purge is used to re-initialize the ring, ok. When in a previous ring when all station has transmitted its data and it does not have anything to transmit then it sends the purge to re-initialize the ring. Then the next frame control is active monitor present. Now suppose in a ring a monitor is selected or any one station that becomes a monitor. So that monitor issue periodically its present into a ring by sending active monitor present frame. And the another frame is a standby monitor present. So suppose that active monitor is dead for some time, ok. Then the other station sends the claim token to become a active monitor, ok. So at that time that second station sends the standby monitor present frame to announce that the presence of the potential monitor and it become a monitor till the first one get activate. So this is the ring maintenance of token ring. So in this video we have studied the frame format of token ring and the how ring maintenance takes place during the ring. These are the references, thank you.