The Physical Unit (PU) is a node in the SNA architecture and has the tasks of monitoring and managing the node resources( LU) from the connected links and neighboring stations. The PU type characterizes the hardware in the SNA network.
PU 1.0:Terminal node with severely limited features for connecting end devices.
PU2.1: Cluster controller with the functionalities of PU 1 and PU 2.0; additional support of Advanced Peer to Peer Networking( APPN). Networking capability of PU 2.1 among each other. Examples of PU 2.1 are the AS/400 and the RS/6000. PU 2.1 is mostly realized on personalcomputers( PC) with control components System Node Control Point (SNCP).
PUT :Front End Processors( FEP) which act as a control unit to control a network. A PU 4 usually has an SDLC connection to the remote cluster controllers (PU1/PU2) and optional SDLC connections to other PU4 controllers. Polling and addressing of the cluster controllers could be performed directly by the Network Control Program( NCP).
PU 5: A host that controls the FEPs and whose resources can be used.