DQDB, which stands for Distributed Queue Dual Bus, is a network protocol that was standardized by the IEEE under the IEEE 802.6 standard. It was designed primarily for metropolitan area networks (MANs) to cover large geographic areas like cities or larger campuses.
Here’s a closer look at DQDB:
- Dual Bus: As the name suggests, DQDB utilizes two unidirectional buses, where data flows in opposite directions. Each station or node in the network is connected to both buses. The use of dual buses helps in providing redundancy and increasing reliability.
- Distributed Queuing: Unlike centralized queuing systems where a single entity controls access, in DQDB, the queue is distributed among all nodes. Each node maintains a queue for its own data and has an algorithm to determine when it can transmit data.
- Slot Passing: The buses have predefined slots that circulate continuously. When a node has data to send, it waits for an empty slot to pass by and then places its data into that slot. If a node doesn’t have data to send, it lets the slot pass unchanged.
- Bandwidth Balancing: DQDB has mechanisms to ensure that all nodes get a fair share of the available bandwidth. It can reallocate unused bandwidth from one bus to nodes on the opposite bus.
- Reliability: The dual-bus configuration ensures that if one bus fails, the other can still operate, providing network reliability.
- Use Cases: DQDB was envisaged to provide high-speed data communication in MANs, especially for cities and large campuses. Its primary application was to offer integrated services, including voice, data, and video over the same network.
- Standard: DQDB was standardized under IEEE 802.6, which is a part of the IEEE 802 series of standards that address various networking scenarios.
However, with the evolution of networking technologies, other protocols and systems like Asynchronous Transfer Mode (ATM) and Ethernet have gained more widespread adoption for large-scale networks. DQDB is less common in modern network deployments but represents an important step in the evolution of MAN technologies.