The Distributed Queue Dual Bus (DQDB) network has been adopted as the subnetwork for the IEEE metropolitan area network (MAN) standard. Since its. IEEE to protocols are only suited for “small” LANs. They cannot be used for very large but non-wide area networks. IEEE DQDB is designed. Distributed Queue Data Interface (DQDB) and put up as IEEE standard. network. The stations are attached to both the buses in parallel. Each bus.

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A single set is described below. The node concurrently looks for idle slots on the second bus.

If, however, operation indicates that the idle slot count exceeds the DAR register value, then the ntework node is allowed to write to the bus in an operation and the idle slot count is set to zero in an operation The above description assumes a single priority system. The steps beginning with operation 84 and ending with operations 87, 91 or 96 complete the process of controlling the upstream request sequence and check is then performed to see if all bits in the request field have been read.

Given this structure, it dqbd implicit that netdork cell can contain only one access request per priority level but may contain as many as four access requests–one for each of the four possible priority levels.

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The count in the request string counter is compared to the DAR register value in an operation ddqb This pattern is interpreted by nodes 76A, 76B and 76C as meaning that one downstream node is requesting access to Bus A. The RQ counter is decremented by one count for each idle slot passing the node on Bus A since the idle slot can be utilized by one of the nodes downstream on Bus A. The access control field includes three additional bit positions in a segment Bandwidth balancing is established if the BWB modulus has a nonzero, positive value.

The headers on the reverse bus communicated requests to be inserted in the distributed queue so dqb upstream nodes would know that they should allow DQDB cells to pass unused on the forward bus. A check 88 is then made to determine whether the current DAR count exceeds a DAR register value established during a prior operation. To accomplish this, a check 93 is made to see whether the current request string count exceeds the DAR register value.

These bits are not employed in the access control method to be described and networl be ignored. If the payload segment is idle, the node receiving the cell may write data into the segment if certain conditions are satisfied. Thus, the current RQ count indicates the number of currently unsatisfied reservation requests for access to Bus A by nodes downstream on Bus A.

The method described above provides efficient utilization of DQDB busses in the type of system having a four bit request field in the access control field where each of the bits is assigned to a different access priority level. Each access control set also includes a DAR register 58 which is used to store a count of downstream access requests.


The cell is divided into a five byte header 64 and a forty-eight byte data or payload segment Each node can read data being transported on a bus and can modify data as it passes the node on the bus; that is, can read from and write to the bus.

When the CD counter reaches zero, the node writes its data into the passing idle slot for propagation along Bus A. To accommodate operation at fifteen priority levels, it is necessary to include a decoder at each node to decode the bit patterns presented by the request field and fifteen sets of DAR counters, DAR registers, request string counters and idle slot counters; that is one set for each priority level.

If the local node does not require access at the priority level under consideration, the length of the received request string is preserved in the upstream direction. When the added binary 1 is detected at node 76B, that node also adds a binary 1 to the pattern it has been sending so that node 76A then receives a repeated pattern.

The figure does not show the cell delimiters and any data integrity checking characters that might be used as these are well known. This is achieved by incrementing every RQ counter at the node for priority levels for which no access requests are queued and by incrementing all CD counters at the node for those priority levels having queued access requests. The originators of this approach have acknowledged at least two problems. The occurrence of the reset signals can lead to propagation and access delays.

Neither the detailed configuration of the metropolitan area network nor the details of the various components other than the DQDB subnetworks of the metropolitan area network is essential to an understanding the present invention.

A metropolitan area network may include DQDB subnetworks 12 for carrying voice and data signals originating with different users throughout the area. Next Patent Multiple port medium Search Expert Search Quick Search.

Distributed-queue dual-bus – Wikipedia

A node requesting access changes the bit value to a binary 1 as the cell passes. If, however, the next cell arriving at the local node is idle, as indicated by a binary 0 in the busy bit position of the access control field the local cell then checks operation to see whether there are any downstream access requests pending at higher priority levels, as indicated by nonzero values in the local DAR registers assigned to those higher priority vqdb.

If the DAR count is greater than the register value, the count is transferred to the register in an operation Since the operations require processing of multiple cells, netwodk requires a number of iterations of the flow chart to perform even the steps which theoretically happen at what is depicted as a single time in the FIG.

A multiple dqdn system is accommodated with relatively minor changes. The RQ counter is then reset to ddqdb. In a multiple priority system where a node is not requesting access to Bus A, dqeb node’s RQ counter for a particular priority level counts reservation bits received for that priority level and all higher levels.


In accordance with the present invention, each node continuously tracks the number of nodes requiring access to the second bus by counting successive bus request signals received on the first bus. The three remaining bits in the access control field are reserved or perform defined functions that are not relevant to the present invention.

If the node is requesting access to Bus A, the CD counter continues to be decremented for each idle slot detected on Bus A but is also incremented for each reservation bit counted in the RQ counter. While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, details of preferred embodiments of the invention may be more readily ascertained from the following detailed description, when read in conjunction with the accompanying drawings wherein:.

Retrieved from ” https: If, at time T1, node 76D requests access to Bus A, it notifies nodes 76A, 76B and 76C of this request by changing the pattern of request bits being transported on Bus B.

For use in a distributed queue dual bus network having two unidirectional oppositely directed busses along which information is transmitted in successive multi-byte cells, each cell having at least one busy bit and at least one bus request bit, and a plurality of parallel nodes, each of said nodes being connected to both of said busses and being capable of reading data from and writing data to the bus, a method of equitably distributing access to the bus among contending nodes, said method being practiced at each node and comprising: It then allows idle slots to pass to the downstream nodes to service their access requests before it attempts to regain access to the bus.

Network Protocols Handbook by Javvin Technologies, Inc.

A downstream node is one which receives data after the local node. If not, an operation selects the next bit position which equates to a different priority level and the steps beginning with operation 78 and ending at operation are again performed. Control of access to the busses by the node involves multiple sets of access controlling counters and registers. The new pattern delivered to those nodes in four successive cells on Bus B takes the form of alternating 0’s and 1’s or At each zero count, an idle slot is allowed to pass the node so that it can be used by a downstream node.

If a node writes data into an idle slot on one of the busses assume Bus Athe BWB counter at the node is incremented.

If there are no netwkrk downstream access requests at a higher priority level, as indicated by zero values in the local DAR registers for those priority levels, the local node may or may not be able to access Bus A depending on the results of the next operation

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