IEEE 802.6 (DQDB)

Introduction

DQDB (Distributed Queue Dual Bus), i.e IEEE 802.6 standard, is a MAN (Metropolitan Area Network) protocol. It is a bus network access control protocol that operates at a high speed across a shared media.

DQDB utilizes two unidirectional buses for transmission, enabling the concurrent transport of voice, video, and data traffic. Bandwidth allocation is achieved through time division multiplexing, where each device is assigned specific time slots for transmission.

One advantage of DQDB is its fault tolerance, facilitated by the presence of two separate buses. In case of a failure in one bus, communication can continue uninterrupted through the other bus.

The protocol supports speeds ranging from 34 to 55 Mbps and can span distances of up to 30 kilometers, making it suitable for connecting devices across a metropolitan area.

Directional Traffic

Every bus opposes the other and supports traffic in a single direction. The bus is depicted as having a square at the beginning and a triangle at the finish (Fig.1). While bus B travels from left to right (i.e., from station 5 to 1), bus A travels from right to left (i.e., from station 1 to 5).

Upstream and Downstream

The buses' direction of travel determines how the stations in the DQDB network are related to one another.

Examining bus A in Figure 1, where stations 1 and 2 are positioned upstream relative to station 3, while stations 4 and 5 are positioned downstream relative to station 3. In this instance, station 1 is at the head of bus A since there isn't an upstream station, and station 5 is at the end of bus A because it doesn't have a downstream station.

Working

Fig. 1

Bus A's head, or station 1, creates an open slot for its usage. In a similar vein, bus B's head, or station 5, creates an open slot for bus B to use. Until the transmission station drops data into the empty slot and the intended destination reads the data, the empty slot moves along its bus.

As an illustration

Station 2 selects a slot on bus A since station 4 is downstream in bus A if it want to deliver data to station 4 (Fig. 1). There is a gap created by the head of bus A, or station 1. Station 2 inserts its address and data into the passing slot of the destination. After reading the address, Station 3 marks the slot as unread. It is recognized by station 4, which then receives the data, modifies the slot's status, and passes it along to station 5, where it is absorbed.

How do you reserve a slot?

A station at the end of the bus suffers because of the imbalance that might result in a lower quality of service. In order to send data downstream, a station must wait for the arrival of the unoccupied slot. Here is where the question of how to prevent an upstream station from manipulating the bus arises. Making a reservation at the station is the answer to this issue. Station 2 can reserve a seat on bus B for bus A here. In order to notify each station, it goes through that Station 2 is reserving a slot on Bus A, Station 2 places a reservation bit on a slot on Bus B. Every station is required to honor the downstream station's reservation and give the requested station the slot.

Advantages of IEEE 802.6 (DQDB)

Scalability: IEEE 802.6 (DQDB) is designed to scale efficiently, making it suitable for metropolitan area networks (MANs) covering large geographical areas. It can accommodate the expansion of network devices and users without significant performance degradation.

Dependability: The protocol offers high reliability and fault tolerance, crucial for maintaining uninterrupted communication within MANs. With redundant pathways and fault detection mechanisms, it ensures robust connectivity even in the event of failures.

Speed: IEEE 802.6 supports high-speed data transmission, ranging from 34 to 55 Mbps. This makes it ideal for multimedia applications such as video conferencing, online gaming, and high-speed data transfer in office or business networks.

Cost-effectiveness: DQDB can be implemented using existing LAN wiring infrastructure, reducing the need for additional investment in network infrastructure. This makes installation simpler and more cost-effective, especially for organizations looking to upgrade their networks without significant expenditure.

Disadvantages of IEEE 802.6 (DQDB)

IEEE 802.6 also has some disadvantages. The protocol is not suitable for long distance networks or large organizations because of its limited transmission reach and lack of data transfer capacity for high data transfer rates. Similarly, the convention does not preserve quality of services (QoS) for multimedia applications which may restrict its feasibility for some applications requiring high quality video or audio streaming.

Conclusion

IEEE 802.6 (DQDB) is a standardized communication system framework designed for metropolitan area networks (MAN's), offering fast, dependable, and efficient communication across a wide area. It is ideal for real-time applications, data transfer, and video conferencing due to its distributed queuing algorithm, dual-bus architecture, and token passing mechanism. Its dependability, scalability and cost-effectiveness make it an appealing choice for small and medium-sized organizations.

In spite of the fact that it has a few disadvantages, for example, restricted transmission range and data transmission bandwidth limitations for the exchange of data, it keeps on being a effective and flexible benchmark that affects the creation of modern communication systems. There will always be new alternatives as technology develops, but IEEE 802.6 (DQDB) remains a useful option for MANs.