 Hello everyone, in my last video we have learned about IEEE 802.3 that is Ethernet. In this video we will learn IEEE 802.4 token bus. In 802.3 that is Ethernet, frame does not provide the priority or the priorities are not provided to the frame and that's why it's time to send the frame is very long and because of this it is not used in the real time system but these lacunas are eliminated in token bus system. Token bus provide the frame priorities to each and every frame and that's why the waiting time is very small. So let's see the token bus in this video. Learning outcomes at the end of this session student will be able to describe IEEE 802.4 standard for LAN that is token bus. They will also explain each and every field of token bus frame format. Let's start with the introduction of token bus. Physically the token bus is a linear or a tree-shaped cable to which the stations are attached but logically the stations are organized into a ring. Let's see this diagram. Here the number of stations are connected with the help of coaxial cable and this is the broadband coaxial cable but all these stations are logically connected with each other like this. So here station A, B, C, D, E and F are logically connected with each other. Now the token bus is a physical bus that operates as a logical ring using token. Token bus is a special frame format. Token bus combines the feature of Ethernet and token ring. A token is passed among the station if the station want to send the data. It must wait and capture the token first and then it can able to transmit the data. Token bus is limited to factory automation, process control and has no commercial application in data communication. Now let's see the token bus in detail. The token is passed from one user to another in a sequence and that sequence is either clockwise or anticlockwise. Each station knows the address of the station to its left and right as per the sequence in the logical ring. A station can only transmit the data when it has the token. The logical ring is formed based on the MAC addresses of the station in descending order. So suppose there are so many number of stations then the highest order station or a station who has highest MAC address will get the chance to transmit the data first. End station suppose in a network there are n number of station and it takes the t seconds to transmit the data then the remaining station has to wait n t seconds. So this is the n t seconds is nothing but the waiting time for all the stations present into a network. Let's see with the help of this diagram. So here number of stations are present and all stations are physically connected in a linear topology but station who want to transmit the data they entered into a logical ring. So here station number 20, 17, 13, 11 and 7 are entered into a logical ring whereas station number 14 and 19 which are not a part of a logical ring. So here it means what 14 and 19 does not have any data to transmit. So the priority is provided to 20th station because its address is higher than the other station who are in the logical ring. Now this is a token which is circulate around this ring okay. Actually this takes place with the help of linear topology. And the direction of token is fixed. Now let's see the logical ring organization of token bus. When the ring is initialized stations are inserted into it in order of station address from highest to lowest. So in this diagram first station that is 20th station entered into a ring first because it has highest address as compared to other station. Station passing is done from high to low address. Again the station when the token is coming or when the token is in include in this ring it goes first to the highest priority station that is 20th station. When the station 20 transmit its frame into that token and it finish its transmission that token is moved to the 17 because 17 has a high second highest priority after the 20th second station. Whenever the station acquire the token it can transmit frame for the specific amount of time because the timer is set for each and every station to transmit the frame. If a station has no data it passes the token immediately upon receiving it. Only one station holds the token at a time and that's why the collision does not occur. Now let's see the ring organization in detail. The token bus define the four priority classes that is 0, 2, 4 and 6 for the traffic control. When the token comes into the station over the cable it is passes internally to the priority 6 substation which can be transmitted its frame. When it is done or when its time expire the token is passed to the priority 4 substation which can then transmit the frame until its timer expire. After that the token is then pass internally to the priority 2 substation and after that this process continue until either the priority 0 substation has send all its frame or the time expire. So this is all about the ring organization of token bus. Pause the video and write down the answer. Can token be pass from one user to other sequentially? Here IEEE standard 802.3 and 802.4. The answer is true. Yes, token can be pass from one user to another sequentially either clockwise or anticlockwise direction. Now let's see the comparison of IEEE standard 802.3 and 802.4. 802.3 as well as 802.4 uses the linear topology that is burst topology. Whereas the logical topology is not present in 802.3 and in 802.4 it is the ring topology. If we want to add a new station into 802.3 system then it can be added almost anywhere on the cable at any time. But in 802.4 distributed algorithms are needed to add new stations. If we see performance wise the 802.3 for light load the stations often transmit immediately but for heavy traffic it reduce the effective data transmission. But in 802.4 for light load station must wait for the token even if no other station is transmitting. But for heavy traffic token passing provides the fair access to all the stations. Now in 802.3 twisted pair coaxial cable and fiber optics are used whereas in 802.4 coaxial cables are used. Now let's see the another important topic that is token bus frame format. So here the first field is preamble. It reserve the one byte with beat pattern 10101010 and it is used to synchronize the receiver's clock. The another field is start delimiter and end delimiter. It reserves one byte with beat pattern 10101011 and used to mark the frame boundaries. Let's see the another field of token bus that is frame control. Frame control is used to differentiate the control frame from the data frame. Now let's see data frame. It carries the frame priority and it carries the indicator for destination station. That means destination station send the acknowledgement to the store station so that store station comes to know that whether the frame is transmitted properly or not. Frame control it specify the frame type and provide the information about the token during the ring initialization. And this is the information which is given in the table like claim token, solicit successor one, two, who follows, resolve contention, token and set successor. It gives the information of passing the token or the information of station which can be included into the ring or which can be removed from the ring. Now the another field is destination and source address. So assign individual and group addressing also local and group addressing. So here the source address and the destination address are same as that of IEEE 802.3. Then the next field is data. Maximum data length is 8182 byte which is 5 times more than the 802.3 that is Ethernet. The next field is checksum used to detect the transmission error. This field contains a 32-bit hash code of data. If the checksum computed by destination is not the same as sent checksum value then the data received is corrupted and that frame is discarded. So this is all about the 802.4 that is token bus. These are the references. Thank you.