DTJ Number 3 September 1987 - Digital Technical Journals

cated from a common buffer pool, and each sideof the connection has a maximum allocationlimit. The buffers are allocated until theallocation exceeds the maximum allowed.This method provides the best allocation ofbuffer memory, but it does not guarantee afixed number of sessions since a single buffercan be allocated that exceeds the allocationlimit for a session. Therefore, the 32 sessionsthat the gateway documentation discussesonly occur if the allocation limits are notexceeded by the sessions. The server must performprotocol work only at the start and end ofthe session.Remote job Server (R]SRV)RJSRV is by far the most complicated program inthe gateway. RJSRV supports multiple SNAremote job entry workstations. Each workstationcontains a DECnet control link, multipleIBM sessions, and multiple network files.This handling of many different linkages hasalmost transformed the server from a simplemessage switcher to a "micro-operating"system, since it performs these activities inreal time. This micro-operating system providesa scheduler for events, common terminationroutines, and common buffer allocationmethods.As with GAS, a DECnet host program initiatesthe connection with RJSRV, thus establishingthe workstation connection. The number ofworkstations and the sessions per workstationare limited only by the available memory inthe gateway. Because of this limitation,each workstation is treated as an RSX programlogical address space (PLAS) region. Sessionscan then be allocated from the PLASregion.The messages from the DECnet control linkare parsed, and the actions taken vary dependingon the current workstation state. At the sametime, IBM SNA sessions may be active, receivingprinter or punch records, or transmitting readerrecords. The server provides all the SNA protocolsfor transmission control (TC) , data flowcontrol (DFC) , and function management headers(FMH) . In addition, RJSRV provides supportfor SNA character strings (SCS) and LU I compression.These facilities and the permutationsof different states make R)SRV rich in functionalityand fairly complex in terms of its internalstructure.Digital TecbnicalJournalNo. 3 September 1986Host Command Facillty Server(HCFSRV) !HCFSRV is a program lying1 midway in complex-1 .ity between GAS and RJSR:V. HCFSRV performssome SNA protocols fo the sessions thathave been established. It d,iffers, however, fromthe other servers in that the IBM applicationinitiates the connection. After the IBM applica-1tion session has been eStablished, HCFSRVreceives the VMS host name and establishes aIDECnet logical link with that node. HCFSRVthen continues to provide SNA protocol supportIafter the session to the VMS host has been established.This server can haddle multiple sessions. Ifrom the IBM network, but the number of sessionsis limited by the arriount of buffer spaceavailable.riIThe Gateway Hardw4reThe DECnetjSNA Gateway [software runs on twohardware configurations': a PDP-1 1 j24 withRX02 disks, DMRlls for (the DECnet connection,and DUPl ls for the sNA connection; and1the Digital Ethernet Communications Server(DECSA) . DECSA is the nerwork equivalent of acommunications controller, such as the DZ 11,I•DMF32, or DUPl l. A serv(lr is a shared resourcefor the hosts in an Ethernet andjor wide areaInetworks connected to an 1Ethernet. The seerperforms specific communications functions for, Ithese hosts. The hardware components arepackaged in a freestanding, table-top unit withself-contained power and cooling; it can operatein an office environment oi- in a computer room.IAt start-up, the unit performs a brief self-test.Then the appropriate senter software is downlineloaded from a Phase W DECnet host on the• Isame Ethernet, and the unit begins operations asa DECnetjSNA Gateway.SummaryWe ·have enumerated the! many diverse issuesthat need to be addressed as part of a networkIinterconnection process. This process is, to saythe least, a complex one. An effective networkinterconnection scheme Ican result only froman effective architectural! and implementationprocess. 1·Numerous aspects of cross-network interconnectmust be considered i'f the final result is tomeet the end-user's ne ,eds. The followingaspects should be considered.IIIIIIiI51New Products