DQDB NETWORK PDF

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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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Therefore, it is intended that the appended claims shall be construed to include both the preferred embodiments and all such variations and modifications as fall within the true spirit and scope of the invention.

Distributed-queue dual-bus – Wikipedia

A proposed alternative method would require that a node send a single reservation request when it first finds it needs access to the bus, rather than for each unit of data it wishes to send. Method of providing reserved bandwidth in a dual bus system.

Second, a node that has an access request pending uses the first available idle slot which it detects. 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 a node writes data into an idle slot on one of the busses assume Bus Athe BWB counter at the node is incremented. A metropolitan area network may include DQDB subnetworks 12 for carrying voice and data signals originating with different users throughout the area. A private DQDB ne work, such as network 24, may support a number of directly-attached components. If a check 80 shows a bit value of 1, the current count in the Downstream Access Request or DAR counter for 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.

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. If the node is requesting access to Bus A, the CD counter continues to be decremented for each idle slot detected on Bus A but is also incremented for each reservation bit counted in the RQ counter. Those conditions are discussed below. Each cell is currently defined as being a fixed length or fifty-three byte cell.

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Distributed-queue dual-bus

For one thing, the method does not adequately consider the priority levels assigned to access requests and can result in inappropriate handling of high priority access requests. 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 netwkrk.

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:.

If the local node does not require access at the priority level under consideration, the length of the received eqdb string is preserved in the upstream direction. There is no wrapping of data from one of the busses to the other at any node, including the head and tail nodes.

If nnetwork idle slot count is found, in operationto be equal to or less dqb 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 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. Dqfb the DQDB algorithm, a distributed queue was implemented by communicating queue state information via the header. Reliable local area network including active stations. The above description assumes a single priority system.

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First, each node counts the number of downstream nodes that are requesting access to Bus A simply by counting the number of successive binary 1’s in the appropriate bit positions in the request fields of cells received on Bus B. If, however, the local node is itself requesting access to Bus A, a series of steps must be performed to determine whether the local node can claim the next available idle slot it receives on Bus A.

Access is not a function of the number of data segments that nework nodes wish to write onto the bus. The three remaining bits in the access control field are reserved or perform defined functions that are not relevant to the present invention.

In a multiple priority system where a node is not requesting access to Bus A, the node’s RQ counter for a particular priority level counts reservation bits received for that priority level and all higher levels. The node concurrently looks for idle slots on the networl bus. Each bit position in the reservation field is assigned to one of four priority levels.

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The initial situation is illustrated at time TO.

If operation 93 indicates the request string count is less than or equal to the DAR register value, a binary 1 is forced into the current bit position in an operation 95 and the request string counter is incremented in an operation Click for automatic bibliography generation. If it does, operation 89 forces a binary 0 into the current bit position and the request string counter is reset in operation An upstream node is one which receives data before the local node.

The use of a single reservation request reduces the number of requests which must be processed and, potentially, any queueing delays associated with that processing. Thus, the current RQ count indicates the number of currently unsatisfied reservation requests for access to Bus A by nodes downstream on Bus A. The node is arbitrarily labelled as node 38 although the description which follows applies to any generalized node in a DQDB system. These bits are not employed in the access control method to be described and will be ignored.

First, the access control field structure of the presently defined DQDB system does not allow four levels of request and idle signals. More details about the operation of counter 60 are provided later. Metropolitan area networks Local area networks Link protocols.

The remainder of the access control field comprises a request field 74, shown as having 4 bit positions. When the first of the two idle slots reaches node 76C at time T5, that node uses the idle slot, allows the second idle slot to pass and changes the request bit pattern being transported on Bus B to a repeated 01 pattern.

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. For example, node 36 may send data to node 42 on Dqdbb A at the same time node 42 is sending data to node 36 on Bus B. The count in the request string counter is compared to the DAR register value in an operation