Systems Network Architecture (SNA)

Techopedia explains Systems Network Architecture (SNA)

In the mid 1970’s 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:

1. communication lines not shared by terminals using different applications with different communication protocols
2. inefficient and time consuming data transmissions
3. poor quality telecommunications lines

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:

• IBM Network Control Program (NCP): a primitive switching protocol for forwarding data packets similar to a modern “switch” and reducing the limitations on communications lines per CPU.
• Synchronous Data Link Control (SDLC): a protocol which greatly improved data transmission efficiency over a single link – a precursor to “data packet” communication which evolved into today’s IP technology.
• Virtual Telecommunications Access Method (VTAM): a software package for log-in, session and routing services within a mainframe computer

Evolving technologies such as APPN (advanced peer-to-peer networking – an extension to SNA) and APPC (advanced program-to-program communication – a protocol at the application layer in the OSI model) allowed computers to control many terminals; and SNA was adapted to handle modern peer-to-peer communications and distributed computing. Today (2011) SNA has mostly been replaced with TCP/IP.