Physical Units (PUs) and Logical Units (LUs)

Among the components that constitute a network, SNA distinguishes between those responsible for managing network resources such as lines, memory, terminals, controllers, etc., and those that are applications such as user computer programs. The resource managers are called Physical Units (usually referred to as Pus), the user programs Logical Units (or LUs). Specialised service program products such as Customer Information Control System (CICS) and Information Management System (IMS) are regarded as Logical Units.

Communication sessions may be defined as the facilities between two stations that enable them to exchange data. IBM permits users to establish sessions and then use and reuse the same session for successive "conversations." Session creation establishes and commits resources (an imaginary pipe, if you will); conversations are traffic through it.

By making a distinction between Physical Units and Logical Units, SNA may categorise communication sessions. The same software routines are used when the participating components have similar characteristics. Different communications software is needed for sessions that allocate resources from that which controls the exchange of data between two user programs. Further classification of Physical Units or Logical Units into type is done to highlight the significant distinctions of functionality found in each. The table below describes the various LU types -

Fig. 1 Logical Unit (LU) Types

Type	Logical Unit (LU) Description	Comments
0	Non-SNA	TCAM, IMS, CICS, JES/2 3270 BSC
1	Session with one or more workstations; interactive or batch with SNA character stream.	(Obsolete) card readers;. 3770 IMS/VS IBM 8100
2	Session with workstation/s; uses SNA 3270 data stream.	3270 display stations; IMS/VS 3179
3	Session to line printer; uses SNA 3270 data stream.	CICS/VS; 3287 printer via 3274
4	Session with one or more workstations in interactive, batch or DDP environment: using SNA character stream.	CICS/VS to 6670; 6670s to other 6670s
6.1	Application subsystem sessions with another subsystem in DDP.	CICS/VS; IMS/VS
6.2	Sessions between applications in DDP; uses SNA Generalised data stream or user defined datastream.	APPC VTAM S/36 S/38 AS/400 CICS/VS
7	Sessions between host and midrange processors, e.g. S/3x	3090 with S/38

Types Among the above LUs, LU2 is used for host communication sessions with workstations such as 3270s. LU 3 is defined for host sessions driving line printers. The NCP And Other Important Pus Referring to the table below (Fig. 2) that describes Physical Unit (PU) types, SSCP is denoted as a PU Type 5. Closely linked with SSCP and responsible jointly with the remaining Pus for implementing SNA's hierarchical functions is the Network Control Program (NCP) software. It operates within the communications front-end equipment - in IBM's Models 3745.3720. 3725 or 3705 - as a PU type 4. Together with the SSCP the PU Type 4 manages all path control, routing, data links and physical connections. In "sub-area" (i.e. hierarchical, non peer-to-peer) networks, users provide the detailed information about the network's configuration prior to it's use. Tables, generated by a "net gen" procedure, must be updated every time the network's configuration changes. These repetitive manual network definitions are chores often associated with traditional SNA. 

Fig. 2 Types of Physical Units

PU Type	Physical Unit (PU) Description
1	(Obsolete) more-or-less intelligent terminal
2.0 and 2.1	Cluster controller. Type 2.1 is a cluster controller containing its own control point, enabling session establishment without requiring host participation. Type 2.1 also defines a Low Entry Networking (LEN) station that was an introductory product of APPN. Type 2.1 is found on the following platforms: AS/400, System/36, System/38 & PS/2. Type 2.0 does not have its own control point. Both Types 2.0 & 2.1 provide end-user access to the network; both support LU2, LU3 and LU6.2.
3	(Not used) intelligent controller, e.g. 3174, 3274, 3276, also PC, 3770 and AS/400.
4	(Front-end) Communications Controller, e.g. 3720, 3745, 3725, 3705.
5	Host node that contains the Systems Services Control Point (SSCP) such as ACF VTAM in the 308x, 9370, 3090, 4300.

 

Of the remaining Pus that we have not yet described, PU Type 2 is the resource manager within a cluster controller, minicomputer or free-standing PC. It controls the resources of devices attached to it, such as dumb terminals. Two subtypes of PU 2 exist: Type 2.0 and Type 2.1. PU Type 2.o needs the services of PU Type 5, the SSCP, to jointly help manage the resources under its control. Type 2.1, on the other hand, may use the SSCP for certain applications, but for others it manages the attached devices from its own control point without involving or needing the SSCP. The AS/400, System/36, and System/38 as well as the PS/2 have this capability. Note that PU Types 1 and 3 are obsolete.