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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At each zero count, an idle slot is allowed to pass the node so that it can be used by a downstream node. The size of the area and the threshold data rate are the subject of discussion in industry standards groups.

In this specification, the term slot hetwork used to refer to the time segment in which data may be transported while the term cell is used to refer to the unit of data being transported in that slot. The figure does not show the cell delimiters and any data integrity checking characters that might be used as these are well known.

Each access control set also includes a DAR register 58 which is used to store a count of downstream access requests. Node 44 is considered to be the tail of Bus A and the head of Bus B. Further, it should be noted that exclusive OR circuits only one way to perform a write function on the bus.

Distributed-queue dual-bus – Wikipedia

The bus request signal is sent to the nodes as part of the traffic flowing on the second bus. These bits are not employed in the access control method to be described and will be ignored. Each fqdb is viewed as originating at a single node, referred to as a head-of-bus or simply head node, and as terminating at a single node, referred to as an end-of-bus or tail node.

Access is not a function of the number of data segments that those nodes wish to write onto the bus. The value of the busy bit indicates whether the slot is busy already ntework by information written into the slot by an upstream node or idle available for data if the node has any to send.

Thus, referring to FIG. A DQDB network may be a significant component or subnetwork for a metropolitan area network, a term which is generally defined as covering a network which can transmit both voice and data information throughout a limited geographic area at data rates exceeding a predetermined threshold rate. This page was last edited on 16 Mayat When considered on its own, the same component is considered to have network status. Given this structure, it is implicit that each 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.


An upstream node is one which receives data before the local node. United States Patent The number of idle slots which are allowed to pass equals the number of nodes downstream on the second bus for which the subject node has received access requests.

Access control method for DQDB network – International Business Machines Corporation

Control of access to the busses by the node involves multiple sets of access controlling counters and registers. A pending downstream access request at a higher priority level preempts the idle slot, forcing the local node to simply wait for the next cell on Bus B. 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.

If the count is less than or the same as the register value, the register value is left unchanged. Data is normally transported along each bus in successive fixed length slots. The unfairness of the basic DQDB protocol has been recognized and several access control methods have been proposed to alleviate that dqddb.

A distributed queue dual bus DQDB network has two, oppositely-directed, unidirectional busses. Each node counts the number of successive bus request signals transported on the second bus. The count at a particular node indicates, of course, the nstwork number of nodes downstream of the particular node that are requesting access. Conventionally, a binary 1 in the busy bit position indicates an occupied slot while a binary 0 indicates an idle or available time slot.

One of counters in each set is a Downstream Access Request or DAR counter 56 which is used to count the number of downstream nodes which are currently requesting access to a particular bus. If not, an operation selects the next bit position which equates to a different priority level entwork the steps beginning with operation 78 and ending at operation are again performed.


Data passing node 38 on bus 46 is modified or written through the use of an exclusive OR circuit 54 having inputs from bus 46 and from the access unit Third, that node then allows a number of idle slots to pass equal to the number of pending downstream access requests. Nnetwork details about the operation of counter 60 are provided later.

Network Protocols Handbook by Javvin Technologies, Inc.

The local node monitors the busy bit of each cell it receives on Bus A in an operation The nodes are connected in parallel to two unidirectional busses 46 or Bus A and 48 or Bus B. A SumoBrain Solutions Company. 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: Click for automatic bibliography generation.

The next available cell is one which, upon arrival at the node via Bus B, has a binary 0 in the reservation field bit.

The following description uses the terms upstream and downstream to define the location of one node relative to another local node. The first byte or octet in each header is an access control field including a busy bit and a subfield of four reservation bits. In the DQDB algorithm, a distributed queue was implemented by communicating queue state information via the header.

The node would also transmit an idle signal when access is no longer needed. Node 76A then receives a repeated pattern.

A metropolitan area network is one which may provide integrated voice and data transmission services for different corporate or individual users within a limited geographic metropolitan area. The headers on the reverse bus communicated requests to be inserted in the distributed nrtwork so that upstream nodes would know that they should allow DQDB cells to pass unused on the forward bus.