DTJ Number 3 September 1987 - Digital Technical Journals

The DECnet/SNA Gateway Product• How effectively can component fault isolationbe accoryplished?'• How will resource utilization be crosscharged?• How effectively can the combined networkmigrate to new technologies?The most important prerequisite for answeringthese questions is a clear understanding of enduserneeds at all times. Failure to understandthose needs can result in a significant expenditureof effort to solve the wrong problem, usuallyat the wrong time.Possible Solutions to TheseQuestionsTwo primary approaches to answering thesequestions are possible. First, an organizationcould take upon itself the effort of building customhardware, software, and procedures to effectthe desired solution. Unfortunately, thisapproach is usually an enormous task with drawbacksin terms of cost, time, maintainability, andSyStem migration, to name but a few. Some organizationshave done it, however, with varyingdegrees of success.The second approach is to use standard productsas the means to the desired end. Thisapproach that can take several forms:1. An organization could acquire computingequipment from only one vendor. While certainlylimiting the intercommunication risk,this approach has potential drawbacks interms of flexibility and cost-effectiveness.Furthermore, it creates the risky situation ofa business's having only a single supplier fora key organizational resource.2. An organization could limit purchases ofequipment to several vendors. While offeringbetter flexibility and cost control, thisapproach can complicate an interworkingstrategy unless the ability of the equipmentfrom different vendors to operate together iscarefully scrutinized.Neither approach, however, is satisfactory for theorganization that owns equipment from morethan three or four vendors and does not wish toincur the risk and expense of building customsolutions. This organization must depend onsome external communications standard that issupported by all the equipment that it intends toacquire.The Advent of Open SystemsFortunately for this organization (and many ofthe world's major corporations fit into this category), the international standards process isbeginning to provide a framework to solve thisproblem. Substantive definition is now underwayof the services and protocols at each layer ofthe Open Systems Interconnect (OSI) model,shown in Figure 1. Common services spanning amultitude of vendor equipment will begin to berealized by the end of the present decade. Insome cases, subsets of OSI services are being utilizedmuch now by major users to bring aboutstandards in particular application areas. Twoprominent examples are General Motors with itsManufacturing Automation Protocol (MAP) forfactory applications, and Boeing Computer Serviceswith its Technical Office Protocol (TOP)for the office environment.These efforts are significant and over time arecertain to create a much more open environment.However, what types of solutions areavailable now for organizations whose applicationsdo not fit these examples and who cannotwait for full OSI implementations? Such organizationsare generally faced with either buildinga custom solution or making use of vendor-suppliedsolutions. Two prominent examples ofthe second approach are the Systems NetworkArchitecture (SNA) from IBM Corporation andthe Digital Network Architecture (DNA) fromDigital Equipment Corporation. The next fewsections introduce the key properties of theseFigure 1APPLICATION (LAYER 7)PRESENTATION (LAYER 6)SESSION (LAYER 5)TRANSPORT (LAYER 4)NETWORK (LAYER 3)DATA LINK (LAYER 2)PHYSICAL (LAYER 1)Open Systems Interconnect Model36Digital TecbnlcalJournalNo. 3 September 1986