 Hello everyone. In my last video, we have studied about various LAN that is Token Bus and Ethernet. In this video, we will see Token Ring. The learning outcomes are at the end of this session, student will be able to describe IEEE 802.5 standard for LAN that is Token Ring and they will also describe the concept of wire center. So, let me start with the history of Token Ring. Token Ring proposed in 1969 and initially referred to as a New Hall Ring. Token Ring Network is a local area network topology where nodes or stations are arranged in a ring topology. Data passes sequentially between nodes on the network and the direction is either clockwise or anticlockwise. It uses the token passing control access mechanism. So what is token and the frame format of the token will see in this lecture only. Data rate is 4 or 16 Mbps. Suppose Token Ring data rate is R Mbps, then Token Ring emits the bit after every 1 by R Mbps. Now let's see Token Ring in detail. Token Ring consists of collection of ring interfaces connected by point to point link which is also known as point to point link. So point to point link or point to point line means the interfaces of one station or the interface of one station is connected to the interfaces of the other two stations which are at the right and left of the station. So here this diagram shows there are 4 stations connected in a ring topology. So these are the stations and these are the ring interface and this is the direction of the token. So token is move around the ring in clockwise or in anticlockwise direction. Token Ring have two interface modes, listen mode and transmit mode. So let me explain the listen mode in detail. Each bit arriving at an interface is copied into a 1 bit buffer and copied out onto the ring again. So in this diagram this is an interface and this is a token which is coming from the station. When this token is coming from the station some modification takes place into this buffer and then that token passes to the same station. Now during this modification 1 bit delay is introduced. So in buffer bit can be inspected and modified and this step introduce the 1 bit delay as I said. Now the another mode is transmit mode. In this mode interface breaks the connection between input and output entering its own data onto the ring. Now let's see the ring or the token ring operation in detail. In token ring a special bit pattern called the token circulate around the ring. When station want to transmit a frame it sees the token and remove it from the ring before transmitting. Only one station can be transmit at a given instance. It can transmit for the length of time called as token hold time that is THT which is 10 milliseconds. When the sender sees or when the sender notice its frame return it drain it from the ring and reinsert a token which is for the next station or when the another station have a frame to transmit it again resend the token. When the last bit of the returning frame has been drain the repeater switch immediately to the listen mode. Now let's see what is token and what is the frame format of token. So token is a 3 byte long frame and function as a place holder and reservation frame. This is the format of token. Here SD stands for start frame delimiter AC is the access control and ED is the end frame delimiter. So here start delimitator and end delimitator mark the beginning and end of the frame. Whereas access control contains priority bit, token bit, monitor bit and reservation bit. So here these 4 bits are explained in detail. So here the first 3 bits are priority bit. When it is 0 0 0 it is the lowest and when it is the 1 1 1 that particular bit has a highest priority. The another bit is or the fourth bit is reserved for token bit. If it is 0 then it is a token frame. If it is 1 that in that is the non-token frame. M bit. So M bit is known as a monitor bit. When it is 0 that means whatever the frame has been arrived that frame is a clean frame and when it is 1 that is the orphan frame that means the erroneous frame. Then the last 3 bits are reserved for reservation bit which is used for reserving the token at the prescribed priority level. So this is the token frame format of the ring topology. Now pause the video and think about this question. Write down the answer. Token ring station operates in which of the following mode? A. Transmit mode. B. Listen mode. C. Bypass and receive mode and D. All of the above. And the answer is all of the above. Let me explain why. D. Answer. Transmit mode. In this mode a station transmit the data. In listen mode station listen from other stations. In bypass mode a station simply bypass the data packet if it is not meant for it and in receive mode a station receive the data packet if it is destined to it. So that's why the answer is D. All of the above modes. Now the another thing is disadvantage of ring topology. So if the cable breaks somewhere the ring collapse and the communication stop. So this is the disadvantage of actually a ring topology. So how to eliminate this disadvantage? So there is one solution and that is wire center. So with the help of wire center we can eliminate this problem. So let me explain wire center in detail. The problem of the ring break in is solved by using wire center how by using the bypass relay. That means what? Bypass relay protect the ring topology from node failure at the hardware layer. So with the help of this diagram I'll explain how bypass relay avoid the failure in the ring topology. Now in this diagram there are total four stations connected logically in the ring topology and all the stations are connected physically with each other with the help of wire center. So just consider here one station connected with this square box which is nothing but the wire center with the help of two with the help of pair of two twisted pair cables. So one twisted pair cable is for data out and another is for data in. So this station connected with this wire center with the help of this connector and this wire center inside this wire center there is a bypass relay. So what is this bypass relay? Let me explain with the help of this diagram. When any frame or any token is coming from the previous host to this host. So this host is nothing but the station. So this is transmitted with the help of this link and once the operation takes place that host transmit that data to the next host. But if this station goes down or there is a breakage in this line then how the communication takes place. So here this bypass relay is on. So how it is on? So in this way in this figure in this next figure it shows this is connected with this line. So whatever the packets or whatever the frames coming from the previous host these are directed to the next host directly with the help of bypass relay. So in this way bypass relay work and with this bypass relay protect the ring topology from the node failure. So here logically or in wire center logically all stations are connected in ring. Physically each station is connected to wire center. Hence it is known as star shape ring. Inside wire center bypass relay is present and the working of bypass relay is depends on what the loss of the power. So if ring breaks or the station goes down loss of the drive current will release the relay and bypass the station. Relay can be operated with the help of software also. It can manually also operate it. Wire center is used to improve the reliability and maintainability. So this is the working of wire center. So in this video we have studied what is token ring. That means what is IEEE standard 802.5. How it works? What is the format of token that means frame format of token and in ring topology if the breakages are introduced how that breakages are avoid with the help of wire center and this wire center uses the bypass relay which because of this bypass relay we can easily bypass the down station here. These are the references. Thank you.