DTJ Number 3 September 1987 - Digital Technical Journals

New ProductsAdvantages of BridgesBridges used to connect IANs have several usefulproperties:• Traffic Filtering - Bridges isolate each IANfrom traffic that does not have to traverse thatIAN. For example, in Figure 2, traffic betweennodes A and B is not sent on the IANs to whichP and Q are connected. Because of this filtering,the load on a given IAN can be reduced,thus decreasing the delays experienced by allusers on the extended IAN.• Increased Physical Extent - IANs are limitedin physical extent (at least in a practicalsense) by either propagation delay or signalattenuation and distortion. Being a storeand-forwarddevice, a bridge forwards framesafter having gained access to the appropriateIAN via the normal access method. In thisway the extended LAN can cover a largerextent than an individual IAN. The penalty forthis coverage is a small store-and-forwarddelay.• Increased Maximum Number of Stations -Because of either physical layer limitations orstability and delay considerations, most IANarchitectures have a practical limit on thenumber of stations on a single IAN. Since thebridge contends for access to the IAN as a singlestation, one bridge may "represent" manynodes on another IAN or an extended IAN.• Use of Different Physical Layers - Some IANarchitectures support a variety of physicalmedia (baseband and broadband coaxialcables, and optical fiber cable) that cannot bedirectly connected at the physical layer.Bridges allow these media to coexist in thesame extended IAN.• Interconnection of Dissimilar IANs - IANs ofdifferent architectures are typically interconnectedwith routers or gateways. Often thesedevices are complex with only moderatethroughput, not an appropriate situation for aLAN environment. It is possible to build abridge connecting dissimilar LANs (withinconstraints discussed in the section ."PerformanceConsiderations"). For example, such abridge would allow stations on an IEEE 802.3(CSMAjCD) IAN to send frames to stations onIEEE 802.4 (token bus) or IEEE 802.5 (tokenring) LANs. 2• 5•6The Extended LAN ArchitectureGeneral GoalsAn ideal extended IAN should possess a nuinberof characteristics that translate into design goalsfor the architecture. These design goals are asfollows:• Minimize Traffic - The primary traffic on theindividual LANs should be generated by theuser stations. Traffic due to complex routingalgorithms should be eliminated or at leastminimized.• No Duplicates - The· bridges should notcause duplicate frames to be delivered to thedestinations during noral operation.• Sequentiality - The col:nbination of LANs andIbridges should not permute the frame orderingas transmitted by thb source station. ·I• High Performance --+ The extended LANshould preserve the cl!taracteristics of highthroughput and low dely that users expect inLAN environments. In !practice, this meansthat the bridges shoulp be able to processframes at the maximum rate they can bereceived. Since LANs operate in the multimegabit-per-secondrange, fulfilling this goalrequires a fast switching operation.• Frame Lifetime Limit - Frames should not beallowed to exist in the extended IAN for anunbounded time. Some higher-layer protocolsmay operate poorly if frames are undulydelayed. This fact is especially true for protocolsused for interactive applications. Theseprotocols depend on the low delay characteristicsof a LAN. An example is the Local AreaTransport (LAT) protocol.10• Low Error Rate - LANS typically have a loweffective bit error rate. igher-layer protocolsare often designed to take advantage of thisIlow rate, which allows hem to operate mol"eefficiently since they cn assume that errorsIare infrequent. Extendetl IANs should not significantlyincrease this rror rate.I• Low Congestion Loss -; Individual LANs minimizecongestion by employing access controlschemes that prevent excessive traffic fromentering the LAN. Extended LANs are morevulnerable to congestion loss since thebridges may be forced to drop frames whenDtgital Tecbnical]ournalNo. 3 September 198659