System Network Architecture (SNA) is IBM's a proprietary networking architecture created in 1974. It is used to connect a LAN to an IBM mainframe or an IBM peer system.
It is still used intensively in banks and other financial transaction networks.
External links:
Cisco article on SNA (http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ibmsna.htm)