Local area network
First Claim
1. A digital communications system comprising a plurality of stations connected in a ring topology for transmission of information in one direction around said ring, each station in turn initiating transmission of information which is waiting to transmit over the system in response to receipt of a token passed from station to station around the ring, said token being in the form of a unique digital code, and said communications system being adapted to carry at least two classes of information, each of which is assigned a priority ranking and at least one of which is synchronous information for which the transmitting station must be serviced periodically, said system including control means distributed throughout the system, such that each station within the system contributes equally to the control of the system, said distributed control means including system clock generation means, distributed token establishment and control means and distributed cycle control means, said cycle control means comprising a cycle clock in each station of the system to indicate the beginning of each new cycle to its respective station, the cycle clock of each station being kept in synchronization with the remainder of the cycle clocks by a synchronizing signal which is passed around the system at the beginning of each cycle, and each station operating under a protocol whereby a first pass of the token around the ring during a system cycle is used to transmit synchronous information of the highest priority ranking, while subsequent passes of the token around the ring are used to transmit any remaining information classes in order of priority ranking, the time taken for each pass of the token around the ring being dependant upon the number of stations having information of the respective class queued for transmission.
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Accused Products
Abstract
A communications system suitable for use as a local area network (LAN) is provided wherein the network has a ring topology comprising a plurality of nodes 10 each of which is connected to adjacent nodes by links 14 and 15. Each node comprises a Link Interface Unit (LIU) 11, a Network Interforce Unit (NIU) 12 and a Terminal Interface Unit (TIU) 13. By providing forward and reverse links 14, 15 between nodes the system is capable of reconfiguring itself after link or node failure such that the failed equipment can be bypassed to minimize system disruption. Link Interface Units (LIU) 11 are also capable of bypassing their own node if a node failure is detected. System control is decentralized with each active node contributing to system control such that prime-failure sites are avoided. System protocol depends upon a token passing scheme where only the node 10 currently holding the token is entitled to transmit data and once it has finished its transmission the node 10 passes the token to the next node 10 in the ring. The system operates under a strict timing regime wherein a system cycle of substantially fixed period is established and the first rotation of the token around the ring during the system cycle is used to initiate transmission of the highest priority data with subsequent rotations of the token initiating transmission of data of progressively lower priorities. Accordingly, the timeslots available for transmission of each data type have variable-boundaries determined by the system load.
179 Citations
19 Claims
- 1. A digital communications system comprising a plurality of stations connected in a ring topology for transmission of information in one direction around said ring, each station in turn initiating transmission of information which is waiting to transmit over the system in response to receipt of a token passed from station to station around the ring, said token being in the form of a unique digital code, and said communications system being adapted to carry at least two classes of information, each of which is assigned a priority ranking and at least one of which is synchronous information for which the transmitting station must be serviced periodically, said system including control means distributed throughout the system, such that each station within the system contributes equally to the control of the system, said distributed control means including system clock generation means, distributed token establishment and control means and distributed cycle control means, said cycle control means comprising a cycle clock in each station of the system to indicate the beginning of each new cycle to its respective station, the cycle clock of each station being kept in synchronization with the remainder of the cycle clocks by a synchronizing signal which is passed around the system at the beginning of each cycle, and each station operating under a protocol whereby a first pass of the token around the ring during a system cycle is used to transmit synchronous information of the highest priority ranking, while subsequent passes of the token around the ring are used to transmit any remaining information classes in order of priority ranking, the time taken for each pass of the token around the ring being dependant upon the number of stations having information of the respective class queued for transmission.
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16. A method of transmitting information between stations of a digital communications system comprising a plurality of nodes connected in a ring topology for transmission of information in one direction around said ring, wherein each station is in turn given access to initiate transmission of information over the system by receiving a token which is passed from station to station around the ring, said token being in the form of a unique digital code, and said communications system being adapted to carry at least two classes of information, each of which is assigned to priority ranking and at least one of which is synchronous information for which the transmitting station must be serviced periodically, said system including control means distributed throughout the system, such that each station within the system contributes equally to the control of the system, said distributed control means including system clock generation means, distributed token establishment and control means and distributed cycle control means, said cycle control means comprising a cycle clock in each station of the system to indicate the beginning of each new cycle to its respective station, and said method comprising the steps of:
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circulating a cycle clock synchronizing signal, comprising a cycle poll signal containing a time field, at the beginning of each cycle, wherein said cycle poll signal is initiated by the first station to receive a token after the beginning of a respective cycle and the synchronizing signal being used by each station in turn to synchronize its cycle clock; circulating the token around the system for a first time, whereby each station may transmit, upon receipt of the token, any synchronous data which it has queued for transmission; circulating a cycle depoll signal indicating the end of the synchronous data transmission service for the respective cycle, the cycle depoll signal being initiated by the station upon receipt of the token after its first circuit of the system; and recirculating the token around the system whereby each station may transmit, upon receipt of the token, any non-synchronous data which it has queued for transmission, circulation of the token after the cycle depoll continuing until the next cycle poll signal is initiated. - View Dependent Claims (17, 18, 19)
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Specification