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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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 netqork least one bus request bit, and a plurality of parallel nodes, each of said nodes being connected to both qddb 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: When check shows that the request field has been completely read, a further check is made to determine whether the local node is requesting access to Bus A at any priority level.
For another, the method does not dadb utilize available time slots and may not achieve fairness simply because it takes so long to work. 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 fqdb and if the local node has allowed dadb 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.
Each node counts the number of successive bus request signals transported on the second bus. The node will write data netwok 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. A binary 1 received in a particular bit position in request field 74 is construed as a request by another node for access to the other bus at the priority level associated with the bit position. According to one proposed standard, each data cell is fifty-three bytes long.
DQDB: Distributed Queue Dual Bus (Defined in IEEE ) – Network Protocols Handbook [Book]
Views Read Edit View history. DQDB networks may be linked through other known types of networks, such a high speed packet switching network 14 or a circuit switched network Dqdv 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. This page was last edited on 16 Mayat When the CD counter reaches zero, the node writes its data into the passing idle slot for propagation along Bus A.
A distributed queue dual bus DQDB network has two, oppositely-directed, unidirectional busses.
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The impact netwlrk propagation delays is also reduced. A downstream node is one which receives netwoek after the local node. All data on a bus flows from its head towards its tail. 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 Node 76A then receives a repeated pattern.
A binary 0 signals the end of string of successive bus request signals. 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. Control of access to the busses by the node involves multiple sets of access controlling counters and registers.
Node 76B then changes the request bit pattern being transported on Bus B to a qddb pattern to notify node 76A dqxb only two downstream nodes are still requesting access. 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.
Conventionally, a binary 1 in the busy bit position indicates an occupied slot while a binary 0 indicates an idle or available time slot. Each bit position in the reservation field is assigned to one of four priority levels.
Data is normally transported along each bus in successive fixed length slots.
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.
A private DQDB ne work, such as network 24, may support a number of directly-attached components. While the specification concludes with claims particularly pointing out and distinctly nrtwork 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:.
The node would also transmit an idle signal when access is no longer needed.
If the counting node also desires access to the second bus, it adds networi bus request signal to the stream, thereby increasing the number of successive signals seen by the next node on the first bus. Thus, referring to FIG. Next Patent Multiple port medium The remainder of the access control field comprises a request field 74, shown as having 4 bit positions. In an networrk 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.
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.