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Systems Network Architecture (SNA) is IBM’s proprietary networking 5-level design architecture developed in 1974 for mainframe computers. SNA consists of a variety of hardware and software interfaces permitting hardware and software system communication. The 5-level design has evolved into a 7-level model closely corresponding to the internationally recognized Open Systems Interconnection (OSI) model, and now supports peer-to-peer networks of workstations.
SNA is not a program but rather a complete protocol stack (suite) used for interconnecting computers and their associated resources.
In the mid 1970s IBM was principally a hardware vendor attempting to increase hardware sales. To do so they induced customers toward interactive terminal-based systems and away from batch systems that executed programs without manual intervention. The strategy was to increase sales of mainframe computers and peripherals, and SNA was intended to reduce the main non-computer costs and other problems operating large networks. These problems included:
Thus, SNAs were intended to increase consumer spending on terminal-based systems, at the expense of telecommunications companies. At that time each CPU could only handle 16 peripherals at once, and each communication line counted as a peripheral. So the number of terminals a powerful mainframe computer could handle was severely limited.
Technology improvements resulted in more powerful communications cards, resulting in “multi-layer communications protocols” being proposed; SNA and ITU-T's X.25 later became the dominant communication protocols.
Critical elements of SNA included:
SNA has since been mostly replaced with TCP/IP.
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