Distributed-queue dual-bus
Encyclopedia
In telecommunication
, a distributed-queue dual-bus network (DQDB) is a distributed multi-access network
that (a) supports integrated communications
using a dual bus and distributed queuing, (b) provides access to local
or metropolitan area networks, and (c) supports connectionless data
transfer
, connection-oriented data transfer, and isochronous
communications, such as voice communications.
IEEE 802.6
is an example of a network providing DQDB access methods.
) algorithm is generally credited to Robert Newman who developed this algorithm in his PhD thesis in the 1980s at the University of Western Australia
. To appreciate the innovative value of the DQDB MAC algorithm, it must be seen against the background of LAN
protocols at that time, which were based on broadcast (such as ethernet IEEE 802.3) or a ring (like token ring IEEE 802.5 and FDDI
). The DQDB may be thought of as two token rings, one carrying data in each direction around the ring. The rives reliability which is important in Metropolitan Area Networks
(MAN), where repairs may take longer than in a LAN and wifi because the damage may be inaccessible).
The DQDB standard IEEE 802.6 was developed while ATM
(Broadband ISDN) was still in early development, but there was strong interaction between the two standards. ATM cells and DQDB frames were harmonized. They both settled on essentially a 48-byte data frame with a 5-byte header. In the DQDB algorithm, a distributed queue was implemented by communicating queue state information via the header. Each node in a DQDB network maintains a pair of state variables which represent its position in the distributed queue and the size of the queue. The headers on the reverse bus communicated requests to be inserted in the distributed queue so that upstream nodes would know that they should allow DQDB cells to pass unused on the forward bus. The algorithm was remarkable for its extreme simplicity.
Currently DQDB systems are being installed by many carriers in entire cities, with lengths that reach up to 160 km (99.4 mi) with speeds of a DS3 line (44.736 Mbit/s) [5]. Other implementations use optical fiber for a length of up to 100 km and speeds around 150 Mbit/s
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
, a distributed-queue dual-bus network (DQDB) is a distributed multi-access network
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
that (a) supports integrated communications
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
using a dual bus and distributed queuing, (b) provides access to local
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
or metropolitan area networks, and (c) supports connectionless data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
transfer
Transfer
Transfer may refer to:* Transfer * Transfer * Transfer DNA, the transferred DNA of the tumor-inducing plasmid of some species of bacteria such as Agrobacterium tumefaciens* Transfer...
, connection-oriented data transfer, and isochronous
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...
communications, such as voice communications.
IEEE 802.6
IEEE 802.6
IEEE 802.6 is a standard governed by the ANSI for Metropolitan Area Networks . It is an improvement of an older standard which used the Fiber distributed data interface network structure. The FDDI-based standard failed due to its expensive implementation and lack of compatibility with current LAN...
is an example of a network providing DQDB access methods.
DQDB Concept of Operation
The DQDB Medium Access Control (MACMedia Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
) algorithm is generally credited to Robert Newman who developed this algorithm in his PhD thesis in the 1980s at the University of Western Australia
University of Western Australia
The University of Western Australia was established by an Act of the Western Australian Parliament in February 1911, and began teaching students for the first time in 1913. It is the oldest university in the state of Western Australia and the only university in the state to be a member of the...
. To appreciate the innovative value of the DQDB MAC algorithm, it must be seen against the background of LAN
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
protocols at that time, which were based on broadcast (such as ethernet IEEE 802.3) or a ring (like token ring IEEE 802.5 and FDDI
Fiber distributed data interface
Fiber Distributed Data Interface provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to . Although FDDI logical topology is a ring-based token network, it does not use the IEEE 802.5 token ring protocol as its basis; instead, its...
). The DQDB may be thought of as two token rings, one carrying data in each direction around the ring. The rives reliability which is important in Metropolitan Area Networks
Metropolitan area network
A metropolitan area network is a computer network that usually spans a city or a large campus. A MAN usually interconnects a number of local area networks using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks and the...
(MAN), where repairs may take longer than in a LAN and wifi because the damage may be inaccessible).
The DQDB standard IEEE 802.6 was developed while ATM
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(Broadband ISDN) was still in early development, but there was strong interaction between the two standards. ATM cells and DQDB frames were harmonized. They both settled on essentially a 48-byte data frame with a 5-byte header. In the DQDB algorithm, a distributed queue was implemented by communicating queue state information via the header. Each node in a DQDB network maintains a pair of state variables which represent its position in the distributed queue and the size of the queue. The headers on the reverse bus communicated requests to be inserted in the distributed queue so that upstream nodes would know that they should allow DQDB cells to pass unused on the forward bus. The algorithm was remarkable for its extreme simplicity.
Currently DQDB systems are being installed by many carriers in entire cities, with lengths that reach up to 160 km (99.4 mi) with speeds of a DS3 line (44.736 Mbit/s) [5]. Other implementations use optical fiber for a length of up to 100 km and speeds around 150 Mbit/s