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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Data being transported on bus 48 can be modified at an exclusive OR circuit 52 connected both to that bus and the access unit Second, a node that has an access request pending uses the first available idle slot which it detects.

The following description uses the terms upstream and downstream to define the location of one node relative to another local node. The present invention relates to data communications networks and more particularly to a method of controlling access by individual nodes to busses in a Distributed Queue Dual Bus DQDB network.

If the idle slot count is found, in operationto be equal to or less than the DAR register value for the same priority level, the idle slot count is incremented in an operation and the local node simply waits for the next cell on Bus B.

A binary 0 signals the end of string of successive bus request signals.

A downstream node is one which receives data after the local node. To determine whether a sufficient number of idle slots have passed the local node, an idle slot count is maintained for metwork number of idle slots that have passed the local node since the last time it loaded data onto the bus. The node is arbitrarily labelled as node 38 although the description which follows applies to any generalized node in a DQDB system.

Distributed-queue dual-bus

These bits are not employed in the access control method to be described and will be ignored. The same node serves as the head of one of the busses and as the tail of the other. Those conditions are discussed below. If, at time T2, node 76B requests access to Betwork A.

The node includes an access unit 50 which can read data from each of the busses 46 and 48 and can modify that data as is passes the access unit on the bus. In the DQDB algorithm, a distributed queue was implemented by communicating queue state information via the header. If the node is not currently requesting access to Bus A itself, a Request RQ counter is incremented for each reservation bit received in a cell on Bus B.


While there dqeb been described what are considered to be preferred embodiments of the invention, variations and modifications in those embodiments will occur to those skilled in the art once they are made aware of the basic dwdb of the invention.

Under these conditions each node sees four successive 0 request bits in four successive cells being transmitted along Bus B. As will be described in more detail later, each DAR counter actually counts the number of successive bus request bits at each priority level in cells being transported on bus 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 If the counting node also desires access to the second bus, it adds dqfb bus request signal to the stream, thereby increasing the number of successive signals seen by the next node on the first bus.

As noted earlier, a cell which has written data to a bus must then allow a sufficient number of idle slots to pass to service pending downstream access requests at the same priority level. Node 76B and node 76C continue to receive the repeated 01 pattern since only one node downstream of those nodes is requesting access to Bus A. In this specification, the term slot is 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.

In a multiple priority system, multiple dqsb are maintained, one for each priority level.

Access control method for DQDB network – International Business Machines Corporation

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. The count in the request string counter is compared to the DAR register value in an operation If the local node does require access at the priority level under consideration, the length of the received request string is extended by one in the upstream direction.

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 the steps of: In telecommunicationa distributed-queue dual-bus network DQDB is a distributed multi-access network that a supports integrated communications using a dual bus and distributed queuing, b provides access to local or metropolitan area networksand c supports connectionless data transferconnection -oriented data transfer, and isochronous communications, such as voice communications.


The unfairness of the basic DQDB protocol has been recognized and several access control methods have been proposed to alleviate that unfairness. If, however, operation indicates that the idle slot count exceeds the DAR register value, then the local node is allowed to write to the bus in an operation and the idle slot count is set to zero in an operation Finally, each set includes a request string or RS counter, the basic function of which is to pass on the appropriate number of successive request bits to the next upstream node.

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. For example, nodes downstream of node 38 on bus 46 can request access to that bus by including request bits in cells transported to node 38 on bus Conventionally, a binary 1 in bit position 70 indicates a busy or occupied cell while a binary 0 indicates an idle cell.

If there are no pending 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 At time T3, it is assumed that node 76C requests access to Bus A by inserting a binary 1 into the 01 pattern it has been receiving.

The initial situation is illustrated at time TO. Data being transported on the busses is discarded or lost at the tail node after being read there. The figure does not show the cell delimiters and any data integrity checking characters that might be used as these are well known.

There are three salient points about the method.