DTJ Number 3 September 1987 - Digital Technical Journals

New ProductsVAXcluster SYSTEMVAX/VMSHOSTDECserver 100ETHERNETTERMINALSERVERTERMINALSTERMINALSDIAL-INMODEMSFigure 1Initial LA T Producting other asynchronous devices, such as printersor links to hosts other than those directly connectedto an Ethernet.The goals of the LAT protocol are as follows:• To permit dumb terminals to be connected tomultiple hosts• To be a transparent character transport mechanism(implying that character echo must beperformed by the host and not by a server)• To support a high-bandwidth LAN technology(specifically the Ethernet)• To use a fixed maximum bandwidth that ismuch less than the total LAN bandwidth,which should be used in a fair and predictablemanner• To be an efficient data link protocol, relativeto the higher-layer DECnet protocols, such asCTERM operating in a LAN environment• To provide for low CPU loads and memory useon the host system at the expense of higherCPU and memory utilization on the terminalservers• To allow for simple terminal server implementations,which means low-cost and highperformancehardware implementations• To permit automatic configuration so that, forexample, servers can determine, without manualintervention, the names and addresses ofhosts on the EthernetThe LAT protocol maks certain simplifyingassumptions:• Communication is local to a single logical Ethernet(possibly connected by repeaters andbridges); thus no routing capability isrequired.• Communication is inherently asymmetric,which simplifies connection management andpermits straightforwa!rd host implementations..I. .• The bandwidth of the Ethernet ( 10 megabitsper second) is much greater than the bandwidthneeded for a given terminal (e.g., 9,600bits per second), so that a timer-based protocolis appropriate.The normal model of dumb terminal usage isone of low-speed data entry, say a: few charactersper second, and higher-speed display in bursts ofseveral hundred characters at a time, taking severalseconds to display. In addition, a user is usuallysitting at his terminal while a program operatesat the host. LAT takes advantage of thisasymmetrical relationship. Also, the terminalconnection normally take place at the explicitIrequest ofthe user rather than of the host system.LA T also takes advantag 1 e of this asymmetricaspect.The server does not cmmunicate charactersto a host system as they are entered by the user;rather, it collects characters and periodicallytransmits them to the host. The time interval ofthis period, the "circuit timer," is quite short -Digital TecbntcaljournalNo. 3 September 198677