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.

Author: Malalkis Tygolar
Country: Seychelles
Language: English (Spanish)
Genre: Travel
Published (Last): 2 November 2010
Pages: 299
PDF File Size: 10.8 Mb
ePub File Size: 15.49 Mb
ISBN: 331-9-38039-335-5
Downloads: 60342
Price: Free* [*Free Regsitration Required]
Uploader: Kazill

Propagation delays, in combination with delays attributable to processing of access request signals, can result in preferential treatment of nodes closer to the head dqcb the bus. If a check 80 shows a bit value of 1, the current count in the Downstream Netsork Request or DAR counter for the priority level associated with the bit is incremented in an operation All data on a bus flows from its head towards its tail.

To accommodate operation at fifteen priority levels, it is necessary to include a decoder at each node to decode the bit patterns presented by the request field and fifteen sets of DAR counters, DAR registers, request string counters and idle slot counters; that is one set for each priority level.

Access control method for DQDB network – International Business Machines Corporation

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 qddb may or may not be able to networ Bus A depending on the results of the next operation Method of providing reserved bandwidth in a dual bus system.

To simplify the following explanation, it will be ndtwork temporarily that the system is a single priority system; that is, that there is a single bit in the reservation field of the access control field. For example, if REQ3 is considered the highest priority level and REQ0 the lowest, a request field received on bus 48 with a bit pattern is construed as containing requests for access to bus 46 by two downstream on bus 46 nodes, one at each of the highest and lowest priority levels.

When the added binary 1 is detected at node 76B, that node also adds a binary 1 networkk the pattern it has been sending so that node 76A then receives a repeated pattern. 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.

When the local node wishes to access the bus itself, it uses the first available idle slot on the bus for its data. The same node serves as the head of one of the busses and as the tail of the other.

To accomplish this, a check 93 is made to see whether the current request string count exceeds the DAR register value. When the second idle slot reaches node networo, that node uses the slot and changes the request bit pattern on Bus B to successive 0’s. The access control field for a cell includes a dqfb bit position 70 which indicates whether the cell’s payload segment is busy carrying data provided at an upstream node or idle. Data being transported on bus 48 can be modified at an exclusive OR circuit 52 connected both to that bus and the access unit 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.


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 Care must be exercised in interpreting the terms in any description of a DQDB system since a node which is upstream from a local node on one of the dual busses is considered to be downstream of the same local node on the other of the dual busses.

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 The BWB method does not provide a complete solution to the unfairness problem.

The node would also transmit an idle signal when access is no longer needed. United States Patent In the DQDB algorithm, a distributed queue was implemented by communicating queue state information via the header.

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. Further, it should be noted that exclusive OR circuits only one way to perform a write function on the bus.

DQDB: Distributed Queue Dual Bus (Defined in IEEE ) – Network Protocols Handbook [Book]

Each node can read data being transported on a bus and can modify data as it passes the node on the bus; that is, can read from and write to the bus. Except as noted below, the details of the access unit 50 are not important to an understanding of the present invention.

A private DQDB ne work, such nefwork network 24, may support a number of directly-attached components.

Next Patent Multiple port medium 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. The operation of the RS counter is described later.

There are three salient points about the method. If it does, operation 89 forces a binary 0 into the current bit position and the request string counter is reset in operation A metropolitan area network may include DQDB subnetworks 12 for carrying voice and data signals originating with different users throughout the area. A check 84 is made by the node to see if it requires access to Bus A at the priority level defined by the current bit position.


Requests for access to one bus are forwarded to the local node in the form of request bits in cells arriving on the other bus. The algorithm was remarkable for its extreme simplicity. By the time the system has re-balanced the bandwidth, by passing empty slots to a downstream node, that node may no longer need the slots.

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.

It is assumed initially that none of nodes 76B through 76E is requesting access to Bus A but that node 76A is currently using every slot on Bus A.

A distributed queue dual bus DQDB network has two, oppositely-directed, unidirectional busses. 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.

Distributed-queue dual-bus

In this specification, the term slot is used to refer to the time segment in which data may be transported while the term cell is used to refer to the unit of data being transported in that slot. Neither the detailed configuration of the metropolitan area network nor the details of the various components other than the DQDB subnetworks of the metropolitan area network is essential to an understanding the present invention. 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.

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.

The headers on the reverse bus communicated requests to be inserted in the distributed queue so that upstream nodes would know that they should allow DQDB cells to pass unused on the forward bus. The following description uses the terms upstream and downstream to define the location of one node relative to another local node.

DQDB networks may be linked through other known types of networks, such a high speed packet switching network 14 or a circuit switched network At time T4, it is assumed that a free or idle slot is made available to node 76B by node 76A. Each access control set also includes a DAR register 58 which is used to store a count of downstream access requests. 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.