DTJ Number 3 September 1987 - Digital Technical Journals

Terminal Servers on Ethernet Local Area NetworksFuture versions of Digital's LAT products maymake more extensive use of the LAT service capability.That would make it possible to installapplications that are accessible to the extendedLAN but not to the wide area network. A form ofnondiscretionary access control is implicit inthis design.LAT group codes can be used to partition anEthernet logically when the number of nodesgets large. By large, we mean more than I 00 services.Having more than 20 services or so meansthat a server display with one line per servicewill no longer fit on a terminal display withoutscrolling.Product Implications of theLA T ArchitectureAl though not originally conceived as a distributedterminal switch, an Ethernet can be usedeffectively in that role if combined with the terminalserver products. This fact remains trueeven when the Ethernet and host system are runningother protocols simultaneously, such asDECnet and VAXcluster systems based on Ethernet.Our experience has shown that a single dedicatedEthernet segment, without bridges, caneasily support several thousand concurrentusers.Functioning as a distributed terminal switchin the Digital computing environment, LAToffers significant advantages over dataswitchesand backplane multiplexers. The most prominentof these advantages is that any terminalserver user can connect to any host system ."Blocking" connections to host systems (moreaccurately called "port contention") is not anissue because host-system ports are logical, notphysical. A VAX(VMS system is limited by theLAT architecture to about 6 million simultaneousconnections, or 32,000 terminal servers, eachwith up to 255 sessions. This large number representsa. significant cost advantage, especiallyconsidering that Ethernet controllers are standardoptions on many of Digital's processors. Inthis case the host-processor terminal connectioncost then becomes negligible, making backplane-orientedterminal switches much lessattractive . This cost advantage improves as thesize of the system increases. Table I comparesthe requirements of LAT with those of a dataswitchfor different numbers of terminals andhosts.Some additional advantages afforded by usingLAT are as follows:• Multisession capability, not offered by dataswitches• Simplified installation and management(especially where users and computer systemsare often added or moved around)• Higher availability due to the lack of any singlepoint of system failure• Simplified, incremental expansion andmigration capabilities inherent in Digital 'sextended LAN architecture , utilizing bridgesLAT PerformanceLAT performance is measured in terms of CPUload per user, which decreases as the number ofusers performing terminal IjO increases. ThusLAT performance increases with increasing CPUloads. Under light loads, LAT uses a relativelylarge amount of CPU resources. This is understandableif the cost of processing an Ethernetpacket containing a single character is comparedwith the cost of servicing a single DZ- I I characterinterrupt. As more data is exchanged, however,the number of messages exchanged doesTable 1A Comparison of Host Connections for LAT and DataswitchNumber of Terminals,Number of HostsLAT RequirementsDataswitch Requirements8 terminals1 host64 terminals8 hosts512 terminals16 hosts8 server connections1 Ethernet adapter64 server connections8 Ethernet adapters5 ·1 2 server connections16 Ethernet adapters8 terminal connections8 host connections64 terminal connections512 host connections512 terminal connections4096 host connections82Digital Tecbnical]ournalNo. 3 September 1986