SNA - Computer Definition
A five-level design architecture developed by IBM in 1974, SNA comprises software and hardware interfaces that permit various IBM systems and software to communicate. SNA that has grown into a seven-layer model that more closely corresponds to the OSI Reference Model, an internationally recognized open model, although the two remain incompatible. SNA includes network nodes, physical units, and logical units , defined as follows:
(Systems Network Architecture) IBM's mainframe network standards introduced in 1974. Originally a centralized architecture with a host computer controlling many terminals, enhancements, such as APPN and APPC (LU 6.2), adapted SNA to modern peer-to-peer communications and distributed computing environments. SNA has mostly been replaced with TCP/IP as the world has migrated to the IP protocol. Following are some of SNA's basic concepts. Nodes and Data Links In SNA, nodes are end points or junctions, and data links are the pathways between them. Data links include high-speed local channels, the SDLC data link protocol, Token Ring and Ethernet. Nodes are defined as Type 5 (hosts), Type 4 (communications controllers) and Type 2 (peripheral; terminals, PCs and midrange computers). Type 2.0 nodes communicate only with the host, and Type 2.1 nodes communicate with other 2.1 nodes (peer-to-peer) without going to the host. SSCPs, PUs and LUs The heart of a mainframe-based SNA network is the SSCP (System Services Control Point) software that resides in the host. It manages all resources in its domain. Within all nodes of an SNA network, except for Type 2.1, there is PU (Physical Unit) software that manages node resources, such as data links, and controls the transmission of network management information. In Node Type 2.1, Control Point software performs these functions. In order to communicate user data, a session path is created between two end points, or LUs (Logical Units). When a session takes place, an LU-LU session is established between an LU in the host (CICS, TSO, user application, etc.) and an LU in the terminal controller or PC. An LU 6.2 session provides peer-to-peer communication and lets either side initiate the session. VTAM and NCP VTAM (Virtual Telecommunications Access Method) resides in the host and contains the SSCP, the PU for the host, and establishes the LU sessions within the host. NCP (Network Control Program) resides in the communications controller (front end processor) and manages the routing and data link protocols.