Systems Network Architecture
It is used by IBM midrange AS/400's and mainframe 370/390's to communicate over a LAN to FEP's and other networked devices. A common machine found on SNA networks is Microsoft's SNA server which allows users to use a Windows client protocols to communicate to the mainframe/midrange.
One of the 'annoying' things about SNA is that every 2 seconds the host will send a ACK request, and if it is not responded to quickly enough all the sessions associated with that endpoint will be dropped. The SNA protocol was designed with local Token Ring in mind. However cisco routers do incorporate a 'local ack' feature to cheat the ack's and allow SNA to operate over a WAN.