network protocols handbook.pdf

222Protocols GuideMAN ProtocolsProtocol NameDQDB: Distributed QueueDual Bus (Defined in IEEE802.6)DQDB cell header:BIT8 7 6 5 4 3 2 1ACF1Protocol DescriptionDistributed Queue Dual Bus (DQDB) is a Data-link layer communicationprotocol for Metropolitan Area Networks (MANs),specified in the IEEE 802.6 standard and designed for use inMANs. DQDB is designed for data as well as voice and videotransmission and is based on cell switching technology (similarto ATM). DQDB, which permits multiple systems to interconnectusing two unidirectional logical buses, is an open standard thatis designed for compatibility with carrier transmission standardssuch as SMDS.VPIVCIHECVCIVCIVCI PT CLPHECVircual Path IndentifierPT Payload TypeVircual Channel IndentifierCLP Coll Loss PriorityHeader Error controlACF Access Control Field2345OCTETFor a MAN to be effective it requires a system that can functionacross long, “city-wide” distances of several miles, havea low susceptibility to error, adapt to the number of nodes attachedand have variable bandwidth distribution. Using DQDB,networks can be thirty miles long and function in the range of34 Mbps to 155 Mbps. The data rate fluctuates due to manyhosts sharing a dual bus, as well as to the location of a singlehost in relation to the frame generator, but there are schemesto compensate for this problem making DQDB function reliablyand fairly for all hosts.The DQDB is composed of two bus lines with stations attachedto both and a frame generator at the end of each bus. The busesrun in parallel in such a fashion as to allow the frames generatedto travel across the stations in opposite directions. Below is apicture of the basic DQDB architecture.Related protocolsIEEE 802.6, ATM, SMDSSponsor SourceDQDB is defined by IEEE (http://www.ieee.org) 802.6.Referencehttp://standards.ieee.org/getieee802/download/802.6-1994.pdfDistributed Queue Dual Bus (DQDB) access Method and Physicallayer specificationshttp://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/smds.htmSwitched Multimegabit Data ServiceBus AFrameGeneratorFrameGeneratorBus BFigure 2-15: DQDB ArchitectureProtocol StructureThe DQDB cell has a similar format as to the ATM:OCTETS 5 1 1 44 1 1HoadorSTMIDInformationLENCRCCRC coverageSTMIDSagment typeMessage IdentifierLENCRCData lengthCyclic redendancy check