EE535 - Project 4

DQDB Bus Contention Resolution Interactive Applet

Overview of DQDB

DQDB stands for Distributed Queue Dual Bus, and is a Medium Access Control (MAC) layer intended for use in MANs (metropolitan area networks). DQDB is described in the IEEE 802.6 specification, and was originally developed in Australia. This protocol can be implemented over any particular bit rate.

The applications of DQDB are centered mainly around the interconnection of MANs. Nonetheless, it can also be used for circuit switched and packet switched services and other applications like CAD/CAM and medical imaging. [2]. DQDB presents also attractive characteristics like high fault tolerance and high performance regardless of the number of hosts connected to the network.

Currently DQDB systems are being installed by many carriers in entire cities, with lengths that reach up to 160 Km (100 miles) with speeds of a DS3 line (44.736 Mbps) [5]. Other implementations use optical fiber for a length of up to 100 Km and speeds around 150 Mbps [3].

In a DQDB system, there are two data buses to which all stations or nodes connect. The two buses carry traffic in opposite directions. At the head of each bus there is a cell (or slot) generator, that constantly transmits DQDB cells. The cells are used by the stations along the bus to communicate. At the end of the bus the cells are simply dropped. These slot generators are expected to be synchronized to the 8 KHz ([3]

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