DQDB
DQDB Abbrev. for distributed queue dual bus. A form of network originally designed for use with optical fibers in metropolitan area networks. The system is based on two unidirectional buses, usually referred to as the A-bus and B-bus, that pass in opposite directions through a number of nodes. Traffic on these buses is processed in cells, each holding 53 bytes, with a 5 byte header and a 48 byte payload. This cell size is chosen to allow seamless interworking with ATM-based networks.
The head end of the A-bus and the tail end of the B-bus are collocated in a single node, and the tail end of the A-bus and the head end of the B-bus are again collocated in a node. Each head end transmits a stream of cells, which it either uses for its own transmissions and marks as being full, or marks as free; at the tail end the incoming cells are discarded. Any node can use one of the buses to pass packets to a node downstream of the sending node, and the receiving node can similarly use the other bus for the return traffic; thus each bus carries traffic in only one direction, but any two nodes have a full duplex connection. If there is a failure of either a node or a bus, the nodes immediately adjacent to the failure will reconfigure so as to take on the roles of the head and tail ends for the appropriate buses. At start-up, or after a failure and reconfiguration, nodes can identify which bus to use for which addresses by examining the source addresses of incoming packets on each bus.
As each cell passes through each node, the node has the opportunity to convert a single bit within the cell to a request for access to the bus. Each node also maintains counters of requests for access to the bus, and of free cells, and the nodes are thus able to cooperate to implement what is in effect a first come first served queue for access to the two buses.
The head end of the A-bus and the tail end of the B-bus are collocated in a single node, and the tail end of the A-bus and the head end of the B-bus are again collocated in a node. Each head end transmits a stream of cells, which it either uses for its own transmissions and marks as being full, or marks as free; at the tail end the incoming cells are discarded. Any node can use one of the buses to pass packets to a node downstream of the sending node, and the receiving node can similarly use the other bus for the return traffic; thus each bus carries traffic in only one direction, but any two nodes have a full duplex connection. If there is a failure of either a node or a bus, the nodes immediately adjacent to the failure will reconfigure so as to take on the roles of the head and tail ends for the appropriate buses. At start-up, or after a failure and reconfiguration, nodes can identify which bus to use for which addresses by examining the source addresses of incoming packets on each bus.
As each cell passes through each node, the node has the opportunity to convert a single bit within the cell to a request for access to the bus. Each node also maintains counters of requests for access to the bus, and of free cells, and the nodes are thus able to cooperate to implement what is in effect a first come first served queue for access to the two buses.
More From encyclopedia.com
Mitosis , Mitosis is the process of dividing chromosomes during cell division in eukaryotic cells . Mitosis is followed by cytokinesis, the splitting of the cy… Schwann Cell , Schwann cell (neurilemma cell) A cell that forms the myelin sheath of a nerve fibre. Each cell is responsible for a single length of the fibre (calle… Cell , Cell
The cell is the basic unit of a living organism. In multicellular organisms (organisms with more than one cell), a collection of cells that work… Cytoskeleton , The cytoskeleton is responsible for cell shape, motility (movement) of the cell as a whole, and motility of organelles within a cell. There are three… T Cell , T cell
T cell (T lymphocyte) Any of a population of lymphocytes that are the principal agents of cell-mediated immunity. T cells are derived from the… Adherens Junction , adherens junction A cell junction that is commonly observed in epithelial (see EPITHELIUM) cells (e.g. those lining the intestine and those in cardia…
You Might Also Like
NEARBY TERMS
DQDB