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 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 first step in the process begins when a cell is received on one of the busses, arbitrarily designated as Bus B.
Views Read Edit View history. A single set is described below. The initial situation is illustrated at time TO.
If a check 80 shows a bit value of 1, the current count in the Downstream Access Request or DAR counter dsdb the priority level associated with the bit is incremented in an operation 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. For example, node 36 may send data to node 42 on Bus A at the same time node 42 is sending data to node 36 on Bus B.
The RQ counter is then reset to zero. A check 84 is made by the node netork see if it requires access to Bus A at the priority level defined by the current bit position.
The size of the area and the threshold data rate are the subject of discussion in industry standards groups. 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. A set is assigned for each priority level on each bus and includes three different counters and a register.
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. The figure does not show the cell delimiters and any data integrity checking characters that might be used as these are well known.
From Wikipedia, the free encyclopedia. Metropolitan area networks Local area networks Link protocols. 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.
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. 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. 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.
This pattern is interpreted by nodes 76A, 76B and 76C as meaning that one downstream node is requesting access to Bus A. The value of the busy bit indicates whether the slot is busy already occupied by information written into the slot by an upstream node or idle available for data if the node has any to send. If the payload segment is idle, the node receiving the cell may write data into the segment if certain conditions are satisfied.
If check 85 shows that the request string count equals the DAR register value, a binary 0 is forced into the passing bit position in operation 89 and the request string counter is reset to zero in an operation If the two dqrb not equal the request string count can only be less than or equal to the DAR register valuea binary 1 is forced into the bit position in the passing cell in operation 86 and the request string count is incremented in an operation In a multiple priority system, multiple counts are maintained, one for each priority level.
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. Each access control set also includes a DAR register 58 which is used to store a count of downstream access requests. There are three salient points about the method. Communications network having bit-mapped refresh cycle. The node concurrently looks for idle slots on the second bus.
The use of a single reservation request reduces the number of requests which must be processed and, potentially, any queueing delays associated with that nehwork.
Click for automatic bibliography generation. In an ideal system, the basic DQDB protocol described above would permit the first node with an access request to claim the first available slot and every slot would be perfectly utilized. The idle slot networm allowed to pass even if the subject node has another access request pending.
A check 88 is then made to determine whether the current DAR count exceeds a DAR netwoork value established during a prior operation.
More details about the operation of counter 60 are provided later.
The nodes are connected in parallel to two unidirectional busses 46 or Bus A and 48 or Bus B. Control of access to the busses by the node involves multiple sets of access controlling counters and registers. 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 new or all zero pattern notifies nodes 76A, 76B and 76C that there are no outstanding access requests for Bus A. A binary 0 signals the end of string of successive bus request signals.
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:.
Data being transported on bus 48 can be modified at an exclusive OR circuit 52 connected both to that bus and the access unit At time T4, it is assumed that a free or idle slot is made available to node 76B by node 76A. The node will write data into the first available idle slot and then allow a number of idle slots to pass before it attempts to regain access to the second bus. If the count is less than or the same as the register value, the register value is left unchanged.
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.