SNA is the protocol that 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 TokenRing in mind. However cisco routers do incorporate a 'local ack' feature to cheat the ack's and allow SNA to operate over a wan.
There is some work involved when connected to a Token Ring network from an Ethernet as the destination MAC addresses all have to be flipped from big endian to little endian.