From an SNA perspective, the HSM appears as two Network Addressable Units (NAU): the Physical Unit (PU) and a Logical Unit (LU). The HSM contains only one LU; a standard 3274 Control Unit (CU) contains up to 32.
Communications within SNA occurs in sessions, or logical relationships between NAUs. The Primary Logical Unit (PLU) is always the Host program; the HSM contains the Secondary Logical Unit (SLU) in all LU-LU sessions. The System Service Control Point (SSCP) resides in the communications Front End Processor (FEP) for SDLC CUs; it manages the active sessions between the HSM and the Host.
The HSM supports three concurrent sessions:
· SSCP - PU (access method - HSM PU).
· SSCP - SLU (access method - HSM SLU).
· PLU - SLU (Host program - HSM SLU).