DTJ Number 3 September 1987 - Digital Technical Journals

New ProductsTable 1Data Communication LayersISO LayerApplicationPresentationSessionTransportNetworkData linkPhysical linkDNA LayerUser/network managementNetwork applicationSession controlEnd-to-end communicationRoutingData linkPhysical linkDECnet-DOS QomponentsIIProgramming libraryJob spawner INetwork control program (NCP)Network test utility (NTU)Network file transfer (NFT)Virtual terminal service (SETHOST)Virtual disk/printer service (NDU)Network mail (MAIL)File access server (FAL)SESSIONNSPROUTINGIAsynchronous :DATA LINKEthernet DATA LINKAsynchronous controllersEthernet controllersIIData Link ServicesAsynchronous Data Link LayerThe Digital Network Architecture standard speci- _fies a protocol providing a reliable data communicationspath between two processors oversynchronous and asynchronous serial communicationlines. This protocol is the Digital DataCommunications Message Protocol (DDCMP) .The asynchronous data link layer providesDDCMP protocol processing and device driversupport for the asynchronous controllers contained.inthe PCs.We found that no existing software could beborrowed for the DDCMP protocol modules.However, existing DDCMP software programsfrom other products were used as models to constructour own modules. We also had to designand code all device drivers for the various asynchronouscontrollers. At first we were not sureexactly how the asynchronous controller chipand the interrupt controller chip workedtogether. Reading the specifications from thechip manufacturers along with the documentationfrom the makers of the controller boardsresolved any questions we had. The code wethen developed worked properly at lowerspeeds; at 9600 baud, however, we found thatcharacters were being lost during reception.After calculating the bytes per second at9600 baud and the instrutions per second onthe lower-speed PCs, we raiized that very fewinstructions could be exetuted between eachreceived character. In this ase the advantages ofI .coding in a higher-level !language were outweighedby other considerations. After carefullyrecoding the interrupt handler for received charactersin assembler language, we reduced but didnot eliminate the character loss. Using debugtracing of the interrupt stack, we discovered thatthe PC BIOS code handling the clock interruptscould leave the interrupt; system disabled forlong periods. Changing this clock interrupt codesolved the character-loss problem.IEthernet Data Link LayerIThe Ethernet data link layer provides buffer managementservices, transmfts and receives messages,and dispatches the received messagesbased upon their protocol types. The goal forDECnet-DOS included support for a number ofEthernet controllers. Unfortunately, the code forthe device drivers is often the hardest to designand debug.Our search for existing code led us to two separateengineering groups lwithin Digital. Bothgroups had already written device drivers for PCbasedEthernet controllers.! We decided to use aDigital Techntcal]ournalNo. 3 September 1986Ill