System Network Architecture (SNA) is IBM's a proprietary networking architecture created in 1974. It is a complete protocol stack for interconnecting computers and their resources.
It is still used intensively in banks and other financial transaction networks.
Within SNA there are two types of datastream to connect local terminals and printers; there is the 3270 datastream mainly used by mainframe (S/390 family) and the 5250 datastream maily used by S/36, S/38 and AS/400 (nowadays called the i-series).
Starting from version 5.2 of IBM OS400, SNA for client-access is no longer supported.