PU Type 5 is the top of the SNA hierarchy and is expressed in the mainframe by a controlling software component called VTAM ( Virtual Telecommunications Access Method ). VTAM typically communicates directly to a PU Type 4 which is expressed in a hardware device that is called a Front End Processor, or FEP. Standard IBM nomenclature for these devices is 3745 or 3746 and they generally employ a management program called ACF/NCP, or Advanced Communication Function/Network Control Program. The NCP within a PU Type 4 controls and manages the resources of the next level closer to the end user, a PU Type 2. PU Type 2 is expressed in devices like an IBM 3174 Cluster Controller. An IBM 3174 is generally the last point of resource control prior to the actual end user or printer. PU Type 2.1 is the exception to the hierarchical model. PU 2.1 is the basis for Advanced Peer-to-Peer Networking (APPN) and is used in AS/400 networking. PU 2.1 and APPN grew out of a requirement for midrange systems to create their own SNA network environment without the involvement of a mainframe, or PU 5. There is substantial variation in the LU and PU combination theme. This description has been to provide general information about "traditional" SNA and its components. Figure 1 illustrates the components for mainframe application access in a traditional SNA environment.

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