Token bus network
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'Token bus' is a network implementing the token ring protocol over a "virtual ring" on a coaxial cable.Note that a physical ring may not be present.A token is passed around the network nodes and only the node possessing the token may transmit. If a node doesn't have anything to send, the token is passed on to the next node on the virtual ring. Each node must know the address of its neighbour in the ring, so a special protocol is needed to notify the other nodes of connections to, and disconnections from, the ring.
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Token frame When no station is transmitting a data frame, special token frame circles the loop. This special token frame is repeated from station to station until arriving at a station that needs to transmit data. When a station needs to transmit data, it converts the token frame into a data frame for transmission. Once the sending station receives its own data frame, it converts the frame back into a token. If a transmission error occurs and no token frame, or more than one, is present, a special station referred to as the Active Monitor detects the problem and removes and/or reinserts tokens as necessary (see Active and standby monitors). The special token frame consists of three bytes as follows (J and K are special non-data characters, referred to as code violations):
Token priority
Token ring specifies an optional medium access scheme allowing a station with a high-priority transmission to request priority access to the token. 8 priority levels, 0-7, are used. When the station wishing to transmit receives a token or data frame with a priority less than or equal to the station's requested priority, it sets the priority bits to its desired priority. The station does not immediately transmit; the token circulates around the medium until it returns to the station. Upon sending and receiving its own data frame, the station downgrades the token priority back to the original priority. Token ring frame format A data token ring frame is an expanded version of the token frame that is used by stations to transmit medium access control (MAC) management frames or data frames from upper layer protocols and applications. Token Ring and IEEE 802.5 support two basic frame types: tokens and data/command frames. Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. Data/command frames vary in size, depending on the size of the Information field. Data frames carry information for upper-layer protocols, while command frames contain control information and have no data for upper-layer protocols.
Data/Command Frame
SD AC FC DA SA PDU from LLC (IEEE 802.2) CRC ED FS 8 bits 8 bits 8 bits 48 bits 48 bits up to 18200x8 bits 4 bits 8 bits 8 bits
Token Frame
SD AC ED 8 bits 8 bits 8 bits Abort Frame SD ED 8 bits 8 bits Starting Delimiter consists of a special bit pattern denoting the beginning of the frame. The bits from most significant to least significant are J,K,0,J,K,0,0,0. J and K are code violations. Since Manchester encoding is self clocking, and has a transition for every encoded bit 0 or 1, the J and K codings violate this, and will be detected by the hardware. J K 0 J K 0 0 0 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit Access Control this byte field consists of the following bits from most significant to least significant bit order: P,P,P,T,M,R,R,R. The P bits are priority bits, T is the token bit which when set specifies that this is a token frame, M is the monitor bit which is set by the Active Monitor (AM) station when it sees this frame, and R bits are reserved bits. + Bits 0–2 3 4 5-7 0 Priority Token Monitor Reservation Frame Control a one byte field that contains bits describing the data portion of the frame contents.Indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information. + Bits 0–2 3 0 Frame type Control Bits Frame type - 01 indicates LLC frame IEEE 802.2 (data) and ignore control bits 00 indicates MAC frame and control bits indicate the type of MAC control frame
Destination address a six byte field used to specify the destination(s) physical address . Source address Contains physical addressa of sending station . It is six byte field that is either the local assigned address (LAA) or universally assigned address (UAA) of the sending station adapter. Data a variable length field of 0 or more bytes, the maximum allowable size depending on ring speed containing MAC management data or upper layer information.Maximum length of 4500 bytes Frame Check Sequence a four byte field used to store the calculation of a CRC for frame integrity verification by the receiver. Ending Delimiter The counterpart to the starting delimiter, this field marks the end of the frame and consists of the following bits from most significant to least significant: J,K,1,J,K,1,I,E. I is the intermediate frame bit and E is the error bit. J K 1 J K 1 1 E 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit Frame Status a one byte field used as a primitive acknowledgement scheme on whether the frame was recognized and copied by its intended receiver. A C 0 0 A C 0 0 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit A = 1 , Address recognized C = 1 , Frame copied Abort Frame Used to abort transmission by the sending station Active and standby monitors Every station in a token ring network is either an active monitor (AM) or standby monitor (SM) station. However, there can be only one active monitor on a ring at a time. The active monitor is chosen through an election or monitor contention process. The monitor contention process is initiated when · a loss of signal on the ring is detected, · an active monitor station is not detected by other stations on the ring, or · when a particular timer on an end station expires such as the case when a station hasn't seen a token frame in the past 7 seconds. When any of the above conditions take place and a station decides that a new monitor is needed, it will transmit a "claim token" frame, announcing that it wants to become the new monitor. If that token returns back to the sender, it is OK for it to become the monitor. If some other station tries to become the monitor at the same time then the station with the highest MAC address will win the election process. Every other station becomes a standby monitor. All stations must be capable of becoming an active monitor station if necessary. The active monitor performs a number of ring administration functions. The first function is to operate as the master clock for the ring in order to provide synchronization of the signal for stations on the wire. Another function of the AM is to insert a 24-bit delay into the ring, to ensure that there is always sufficient buffering in the ring for the token to circulate. A third function for the AM is to ensure that exactly one token circulates whenever there is no frame being transmitted, and to detect a broken ring. Lastly, the AM is responsible for removing circulating frames from the ring.
Token bus was standardized by the IEEE 802.4 Working Group. It is mainly used for industrial applications. Token bus was used by GM (General Motors) for their Manufacturing Automation Protocol (MAP) standardization effort.
