Systems Network Architecture entered the market in 1974 as a hierarchical, single-host network structure. Since then, SNA has developed steadily in two directions. The first direction involved tying together mainframes and unintelligent terminals in a master-to-slave relationship. The second direction transformed the SNA architecture to support a cooperative- processing environment, whereby remote terminals link up with mainframes as well as each other in a peer-to-peer relationship (termed Low Entry Networking (LEN) by IBM). LEN depends on the implementation of two protocols: Logical Unit 6.2, also known as APPC, and Physical Unit 2.1 which affords point-to-point connectivity between peer nodes without requiring host computer control. The SNA model is concerned with both logical and physical units. Logical units (LUs) serve as points of access by which users can use the network. LUs can be viewed as terminals that provide users access to application programmes and other services on the network.

Physical units (PUs) like LUs are not defined within SNA architecture, but instead, are representations of the devices and communication links of the network.