DTJ Number 3 September 1987 - Digital Technical Journals

New ProductsToken Passing Bus5The characteristics of the token-bus accessmethod scale reasonably well with physicalextent (see Figure 1), but poorly with the numberof nodes.3 This situation is complementary tothat of CSMAjCD. The sensitivity of a token busto the number of nodes makes it unsuitable for asingle LAN with many nodes. The token bus, likeCSMAjCD, is well suited to implementation on aCA1V-like cabie plant.Token Ring6The performance characteristics of the IEEE802.5 token ring are somewhat similar to thoseof the token bus. However, an IEEE 802.5 tokenring station will not reissue a token until the previouslytransmitted frame has circulated completelyaround the ring. This characteristicmakes the ring more sensitive than a token bus toincreasing physical extent. Moreover, a tokenring cannot be applied directly to a branchingtreephysical topology, such as the one in aCA1V-like cable plant.Slotted RingThe design tradeoffs made in most slotted ringsresult in small slots, usually less than 20 bytes.Therefore, it is important to minimize the slotoverhead, such as source and destinationaddresses and error detection fields. Such operationsare usually associated with connectionorientedservices, such as voice transmission. Inslotted ring networks, mechanisms are oftenpresent to impose a measure of "fairness" in thenetwork. Those mechanisms make it difficult foran individual station to acquire a significant fractionof the instantaneous transmission rate. Suchnetworks are often inadequate for handling thebursty traffic expected in the environments ofinterest.Time Division Multiplexed (TDM) BusThe principal disadvantages of using a TDMstructure are that the number of time slots isfixed, and each time slot is assigned to only onestation. Thus, with a large number of stations,even with low network utilization, the meanwaiting time is large. Furthermore, since the busis allocated in tum to each station, the maximumthroughput of any station is limited to the datatransmitted in that station's slot. The TDM bus iswell suited to isochronous traffic, such as voiceor video.Frequency Division Multiplexed (FDM) BusThe characteristics of an FDM bus are somewhatsimilar to those of the TDM bus. The FDM bus hasan additional degree of freedom in that it couldhave slots of different bandwidths. The problemwith the FDM bus, however, is logical connectivity.To have full connectivity, each node mustmonitor each frequency band for messages destinedfor that node. In practice, this monitoringis prohibitively expensive.1As an alternative, onecould apply a reservation' system to either theTDM or FDM buses. The dharacteristics of suchan approach, however, arejmuch better suited toa connection-oriented ser\rice, such as voice orIvideo, rather than one with bursts of data..II.Hy brid of FDM and CSM1fCDA hybrid scheme utilizing multiple slow-speed(approximately 1 million bits per second, orMbps) CSMAjCD channels, each in its own6-MHz band, is another specific alternative thatwas considered. Without increasing the minimumpacket size used in an Ethernet, eachCSMA/CD channel can span an extent of approximately30 kilometers. Multiple CSMAjCD channelscould be used to increase the aggregatecapacity of the network. Unfortunately, logicalconnectivity cannot be achieved without somemechanism for switching packets between thesechannels. Furthermore, th bandwidth availableto any station is limited to a rate of 1 Mbps. Sincethere is no industry standard for a 1-Mbps, 6-MHzCSMAjCD LAN, selecting;this approach wouldmake necessary an attemp t to standardize it.These evaluations con}rinced the team thatnone of these access methods sufficed for buildinga single LAN capable b f successfully operatingin all dimensions of interest to the project.Not one of these alternatives was capable ofdirectly employing all the types of media that thecustomers wished to utilize. Furthermore, anychoice was constrained , by the desire for anaccess method with a defined standard havingthe appropriate parameters. The project teamwould have to find a way to interconnect at leasta subset of the standard LANs if the project wereto be successful.LAN Interconnection AlternativesThe team next investigated a variety of interconnectionmethods, each of which had certainadvantages and drawbacks.Digital TecbnlcalJournalNo. 3 September 198657