In IBM's Systems Network Architecture ( SNA ), a physical unit (PU) identifies a network node that supports communication sessions between logical unit s (LU). Logical units represent end users. Two logical units that communicate depend on physical connections being established through associated physical units. The network point at each end of a communication that sets up the communication session between logical units is called the system services control point (SSCP) . The SSCP sets up its own session with the PU on behalf of the LU-LU session. Typically, a logical unit is a unique connection to an application program. A physical unit is the hardware and routing aspect of a network node . IBM's Virtual Telecommunication Access Method ( VTAM ) is used by an application program to manage session and communication requests on behalf of logical units.
In SNA, there are different node types (formerly called physical unit types) that characterize routing, pooling, and other network capabilities.