Distributed Queue Dual Bus

Distributed Queue Dual Bus, or just DQDB is a network protocol for high-performance transmission. DQDB adapted for transmission rates of 34 Mbit / s to 155 Mbit / s and has the advantage that it is particularly efficient at high load.

Overview and History

DQDB has been developed in the mid 80s in coordination parallel to ATM. Thus both standards have a similar definition of the cell size. The aim was to meet the demands for new services such as video telephony and video conferencing needs.

DQDB has been standardized as IEEE 802.6.

Topology

DQDB consists of:

• 2 contra-rotating, unidirectional buses
• Head stations at each beginning of the bus
• Coupling of the stations on both buses

Often, these two buses are routed ring, which coincide both headends. Each station is connected to both read and write buses. The connection to the MAN is handled via Bridge and Router.

Transmission

The head sends a steady stream of 53 -byte "empty" cells that are "swallowed " at each end of bus from a terminating resistor. Each station may describe an empty cell when it is justified. Only stations are reached, which are downstream, respectively. The decision also implicitly which bus is used. This in turn also means that the stations have to have knowledge of the topology, that is, which stations behind and what lies ahead of them.

Access method

To make the access to the media persons, 2 method in the standard were provided:

• Queue (or Queued - Arbitrated )
• Isochronous access (or even pre- arbitrated )

Control over queue

There is a queue which is processed according to the FIFO principle for each bus. A bus regulates the control of the queue for the others. The queues are implemented distributed, ie in the stations. Thus, each station knows the current state of the network. Send Willige stations lined up in the queue.

In each node are 2 counters (per bus) implemented:

• Request Counter INT, the node wishing to transmit one
• Countdown Counter CD, indicating how many cells must be transmitted

In every cell there is a busy bit and a request bit which indicates whether a cell is busy, or whether a ( following ) station wishes to transmit. If a node has compiled a cell for bus A, it sends a request over the bus as all nodes of B thus obtained this request bit. If an empty cell, the RC is decremented by one, because this cell has been requested by someone else, the one no longer has to wait. Immerse a request on which RC is increased by one. In a cell with occupied busy bit is ignored.

After sending the request bit, the transmitter transmits the meter reading by the RC on the CD and sets the RC back to 0

Isochronous access

Here are accessed using predetermined ( PA ) cells, ie, that each station can only describe it for certain cells. The procedure is also connection-oriented, ie an acceptance of a connection between two stations must have preceded.

Note: Both types of access can be mixed