Distributed Queue Dual Bus (DQDB) is a Data-link layer communication protocol for Metropolitan Area Networks (MANs), specified in the IEEE 802.6 standard, designed for use in MANs. DQDB is designed for data as well as voice and video transmission based on cell switching technology (similar to ATM). DQDB, which permits multiple systems to interconnect using two unidirectional logical buses, is an open standard that is designed for compatibility with carrier transmission standards such as SMDS, which is based on the DQDB standards.

For a MAN to be effective it requires a system that can function across long, “city-wide” distances of several miles, have a low susceptibility to error, adapt to the number of nodes attached and have variable bandwidth distribution. Using DQDB, networks can be thirty miles long and function in the range of 34 Mbps to 155 Mbps. The data rate fluctuates due to many hosts sharing a dual bus as well as the location of a single host in relation to the frame generator, but there are schemes to compensate for this problem making DQDB function reliably and fairly for all hosts.

Below is a picture of the basic DQDB architecture:

DQDB: Distributed Queue Dual Bus in IEEE 802.6

The DQDB is composed of a two bus lines with stations attached to both and a frame generator at the end of each bus. The buses run in parallel in such a fashion as to allow the frames generated to travel across the stations in opposite directions.

Protocol Structure

DQDB cell has the similar format as the ATM:

DQDB cell has the similar format as the ATM

DQDB cell header:

DQDB cell header

Related protocols: IEEE 802.6, ATM, SMDS

Sponsor Source: DQDB is defined by IEEE (http://www.ieee.org) 802.6.

Reference: http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/smds.htm: Switched Multimegabit Data Service