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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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. 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. The operation of the RS counter newtork described later.
DQDB: Distributed Queue Dual Bus (Defined in IEEE ) – Network Protocols Handbook [Book]
To simplify the following explanation, it will be assumed temporarily that the system is a single priority system; that is, that there is a jetwork bit in the reservation field of the access control field.
A metropolitan area network may include DQDB subnetworks 12 for carrying voice and data signals originating with different users throughout the area.
According to one proposed standard, each data cell is fifty-three bytes long. The two busses, considered together, provide effective full duplex communications between any two nodes.
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. Each access control set also includes a DAR register 58 which is used to store a count of downstream access requests.
The new or all zero pattern notifies nodes 76A, 76B and 76C that there are no outstanding access requests for Bus A. 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.
There are three salient points about the method. Node 76B won’t know which two nodes or how much data each of those nodes has to send.
A downstream node is one which receives data after the local node. 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 first step in the process begins when a cell is received on one of the busses, arbitrarily designated as Bus B. If the payload segment is idle, the node receiving the cell may write data into the segment if certain conditions are satisfied.
To accomplish this, a check 93 is made to see whether the current request string count exceeds the DAR register value. 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:.
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.
A node with data to send on one bus, can request access to that bus, by sending a bus request signal to all nodes upstream on the 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, at time T2, node 76B requests access to Bus A.
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Each bit position in the reservation field is assigned to one of four priority levels. The figure does not show the cell delimiters and any data integrity checking characters that might be used as these are well known. The bus request signal is sent to the nodes as part of the traffic flowing on the second bus. DQDB networks can also be linked through a multiport bridge networm A check 88 is then made to determine whether the current DAR count exceeds a DAR register value established during a prior operation.
Dqbd of providing reserved bandwidth in a dual bus system. Method of providing effective priority access in a dual bus system. Given this structure, it is implicit that each cell can contain only one access request per priority level netowrk may contain as many as four access requests–one for each of the four possible priority levels.
The DQDB may be thought of as two token rings, one carrying data in each direction around the ring.
When a node initiates a request for access to Bus A, the newtork RQ count is transferred to a countdown CD counter in the node. 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. The following description uses the terms upstream and downstream to define netdork location of one node relative to another local node.
A single set is described below. A SumoBrain Solutions Company.
The three remaining bits in the access control field are reserved or perform defined functions that are not relevant to the present invention. The local node monitors the busy bit of each cell it receives on Bus A in an operation The CD counter is decremented by one for each idle slot passing the node on Bus A while the RQ counter begins to count newly arrived reservation bits.
All data on a bus flows from its head towards its tail.