DTJ Number 3 September 1987 - Digital Technical Journals

The DECnetjSNA Gateway ProductFrom an architecture standpoint, both thePU_T5 and PU_T4 nodes have a fairly highdegree of intelligence and possibly some massstorage. Thus their associated SNA definitions arefairly rich in function and rather complex in definition.On the other hand, such devices as theIBM 3274 information-display control unit andthe 3 7 7 6 remote job entry workstation areassumed to be limited in both intelligence andstorage. These operations are detailed in thephysical unit type 2 definition. The SNA nodedefinition for this class of device is more limitedin that both node and end-user communicationsoperate in the slave mode of a masterjslave relationship.The final node type in SNA terminology istypically associated with single-unit, limitedfunctionterminals, such as the 3 767 communicationsterminal and the 3 2 71 model-1 1 displayunit. This type is known as physical unit type 1.It was much more prominent in the early days ofSNA when the architecture was more orientedtoward terminal-mainframe communication thanis the case today. Given current technologytrends, it is likely that this particular node typewill become increasingly de-emphasized, exceptperhaps as a migration mechanism for the interconnectionof pre-SNA devices or non-IBMequipment.Program-to-Program Communicationwithin an SNA NetworkInterprogram communications within an SNAnetwork are realized via an architectural componentknown as a logical unit (LU). The LU can beenvisioned as a port from which an applicationprogram can obtain the services of an SNA network.SNA communications through a logicalunit are managed via an entity known as a logicalunit services manager. This entity is responsiblefor interfacing end-user communicationsrequests into the SNA network.Logical units are further classified by the typeof layered function the application programschoose to realize. There are specific logical unittypes that are pr edefined by IBM Corporation tocorrespond to particular layered functions that"standardize" the use of SNA capabilities in realizingmainstream usages. Most logical unit typespredefine terminal- and printer-to-host programfunctions; these LUs are called types 1, 2, 3, 4and 7. The exception to this classification is inthe definitions of logical unit types 0 and 6.2.Logical unit type 0 is unique by virtue of its"nondefinition" (that is, it can be defined by auser to implement any form of desired programto-programfunction). Logical-unit type 6.2 issignificantly different from the other LU types.Its definition changes the semantics of an LUfrom that of a network port to that of a distributedoperating system. LU6.2 is used mainlyfor transaction processing; it is a primary indicationof the future direction of SNA. 3An Example of LU-to-LUCommunicationThis section discusses briefly the LU-to-LU communicationwithin an SNA network. 4 Start withthe simple SNA topology as shown in Figure 4.Assume that an end user attached to a 3270 displaystation cluster controller node B wishes toenter into an SNA session, or communication dialogue,with a CICS subsystem executing on hostnode A. For this session to begin, one side mustinitiate the request. Typically, that is done at thedisplay station via either an unformatted log-onor a formatted SNA initiate-self message. Thismessage is issued by the logical unit servicesmanager at the cluster controller in response to3270CLUSTERCONTROLLERDISPLAYSFigure 4HOST SYSTEMCICSSUBSYSTEMFRONT ENDPROCESSOR3270CLUSTERCONTROLLERDISPLAYSSample SNA TopologytSNASESSION38Digital TecbnicaiJournalNo. 3 September 1986