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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The remainder of the access control field comprises a request field 74, shown as having 4 bit positions. Except for these changes, the same basic method steps would be performed. Each network component in a metropolitan area network may be viewed either as a subnetwork or as a network, depending on the context in which the component is being considered. The two busses, considered together, provide effective full duplex communications between any two nodes. When the CD counter reaches zero, the node writes its data into the passing idle slot for propagation along Bus A.

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 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. 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 The following description uses the terms upstream and downstream to define the location of one node relative to another local node.

The idle slot on Bus A is allowed to pass for use by one of the downstream nodes on Bus A. Conventionally, a binary 1 in bit position 70 indicates a busy or occupied cell while a binary 0 indicates an idle cell. Thus, referring to FIG.

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In an ideal system, the basic DQDB protocol described above would permit the first node with an access request to claim nefwork first available slot and every slot would be perfectly utilized. 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.

Node 76B then, even if another access request occurs immediately, allows the next two idle slots to pass on Bus A since it dqdg that two downstream nodes have access requests pending. A binary 0 signals the end of string of successive bus request signals. 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.

Access control method for DQDB network.


The BWB method does not provide a complete solution to the unfairness problem. Bandwidth balancing is established if the BWB modulus has a nonzero, positive value.

Distributed-queue dual-bus – Wikipedia

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. The next available cell is one which, upon arrival at the node via Dqdbb B, has a binary 0 in the reservation field bit. When a node initiates a request for access to Bus A, the current RQ count is transferred to a countdown CD counter in the node.

All data on a bus flows ddqdb its head towards its tail. 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 The access control field includes three additional bit positions in a segment The fifty-three bytes are divided into a five byte header and a forty-eight byte data segment, sometimes referred to as the payload segment of the cell.

If the local node does not require access at the priority level under consideration, the length of the received dqdh string is preserved in the upstream direction. Node 76A then receives a repeated pattern. DQDB networks can also be linked through a multiport bridge The three remaining bits in the access control field are reserved or perform defined functions that are not relevant to the present jetwork.

This pattern is interpreted by nodes 76A, 76B and 76C as meaning that one downstream node is requesting access to Bus A. Each bus 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. United States Patent Each neetwork can read data being transported on a bus and netdork modify data as it passes the node on the bus; that is, can read from and write to the bus.

A multiple priority system is accommodated with entwork minor changes. Retrieved from ” https: In a multiple priority system, multiple counts are maintained, one for each priority level. Node 76B then changes the request bit pattern being transported on Bus B to a repeated pattern to notify node 76A that only two downstream nodes are still requesting access.

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 netwkrk bus among contending nodes, said method being practiced at each node and comprising the steps of: When a given or local node detects an idle slot on the first node, it may write data into that slot if there are no pending downstream access requests of higher priority and if the local node has allowed enough idle slots to pass since its last write operation to service access requests existing at the time of that operation at the same or higher priority levels.


If the cell contains data written into the payload segment by an upstream node on Bus A, it is not available to the local node. 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.

There is no wrapping of data from one of the busses to the other at any node, including the head and tail nodes. A metropolitan area network may include DQDB subnetworks 12 for carrying voice and data signals originating with different users throughout the area.

Each cell is currently defined as being a fixed length or fifty-three byte cell. If there are no access requests pending, the system simply waits operation for the next cell to arrive on Bus B. While there have 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 concepts of the invention.

Each set further includes an idle slot counter 60, the basic function of which is to make sure the node allows an adequate number of idle slots to be delivered to downstream nodes to satisfy access requests from those nodes. 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.

A single set is described below. A DQDB network is defined as a network having dual, unidirectional busses which carry data in opposite directions. The count in the request string counter is compared to the DAR register value in an operation If the idle slot count is found, in operation netqork, to 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 neteork local node simply waits for the next cell on Bus B.

Requests for access to one bus are forwarded to the local node in the form of request bits in cells arriving on the other netowrk.

If netork 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.

As shown in FIG.