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Systems Network Architecture is a proprietary networking protocol owned and marketed by International Business Machines (IBM) from 1974 to 2002. This protocol contains a group of interconnected applications, protocols and services that run on the IBM 3745/3746 communications controller. This controller is still common in thousands of different companies, most notably financial companies such as banks and brokerage houses. Even though production of the IBM 3745/3746 communications controller stopped, the system still receives updates from IBM, and a wide range of third-party systems use the controller as their base hardware.
The point of the Systems Network Architecture protocol was the connection of mainframe computers with other mainframes and communication terminals. This process was primarily achieved through hard connections and phone lines. When this technology came on the market in the mid-70s, the industry with the greatest need for fast and reliable interconnectivity was banking. As a result, Systems Network Architecture became a common method of exchanging information between financial systems.
This technology was designed to overcome two major technological disadvantages of the time. This first problem was the communication system itself. Terminals and mainframes of the time used hardwired communication ports to talk to one another. These ports were buggy on their own, but when ports of different makes or models attempted to communicate, the error rate often made connectivity impossible. Systems Network Architecture was a technological overlay that forced dissimilar ports to operate the same way, reducing error rates.
The other major disadvantage was built directly into the IBM systems. At the time, the phone network was so poor that transmissions were extremely slow. In order to overcome this technological limitation, large computers used line bundles to connect. Each of these bundles had hundreds of communication lines. Even though the connection was slow, so much information came in through the different lines that it allowed reasonable transmission speeds.
The IBM systems had a hard-coded limit of 256 peripheral connections per processor. While this was fine for most systems since they only had a handful of printers and keyboards connected, each line connection counted as its own peripheral. This severely limited the size of line bundles available to the computer. Systems Network Architecture allowed the system to read a group of lines as a single peripheral, increasing the number of available connections.
As computing changed, Systems Network Architecture changed with it, but not fast enough. Modern computing protocols and methods made certain aspects of Systems Network Architecture unwieldy or obsolete. As a result, when the contract for production of the IBM 3745/3746 communications controller ran out in 2002, it was not renewed. A third-party system continued producing the controller and upgrade kits until 2009.