DQDB

DQDB, short for Distributed Queue Dual Bus, is a medium access control (MAC) mechanism used in the Fiber Distributed Data Interface (FDDI) network standard. It governs how stations on a dual-bus fiber ring gain access to transmit data and coordinates contention in a scalable, distributed manner.

Architecture and operation

DQDB networks use two unidirectional buses, often labeled A and B, that run in opposite directions around a dual-ring topology. Stations connect to both buses and can place data transmissions onto the network without a central controller. Each station maintains a local, distributed queue and participates in a shared arbitration process. When a station has data to send, it issues a request into the network, which propagates along the buses. The arbitration logic ensures that requests are served in a fair order, yielding transmission rights to stations in a rotating or orderly fashion. Once granted, a station transmits its data on the appropriate bus, with the frame circulating around the ring until it reaches its destination.

Key properties

DQDB is designed to be fully distributed, avoiding a single point of control. It supports multiple stations and large topologies by encoding and propagating requests and grants along the buses, aiming to provide fair access and predictable throughput. The dual-bus arrangement allows data frames to travel in both directions, enabling efficient utilization of the medium and resilience to single-point faults in the ring.

History and use

DQDB was developed as the core MAC technique for FDDI in the 1980s and became a defining feature of FDDI networks. Over time, FDDI and DQDB were largely supplanted by other technologies, such as Ethernet and switch-based optical networks, but DQDB remains a notable example of a distributed arbitration approach in early high-speed LAN designs.

See also

FDDI, MAC, distributed arbitration, dual-bus topology.