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IBM SNA NAU: Network Accessible Units (PU, LU and CP)
Network Accessible Units (NAUs) are the IBM Systems Network Architecture (SNA) components to facilitate the communication between a Transaction Program (TP) and the SNA network, formerly called "network addressable units". NAUs are unique network resources that can be accessed through unique local addresses by other network resources. SNA provides the following types of NAUs:
- Physical units (PU)
- Logical units (LU)
- Control points (CP)
Protocol Structure - PU, LU and CP Details
U - Physical units
Each SNA node contains a physical unit (PU). The PU manages resources (such as link resources) and supports communication with a host.
LU - Logical Units
Each SNA node contains one or more logical units (LUs). An LU provides a set of functions that are used by TPs and end users to provide access to the network. LUs communicate directly with local TPs and devices.
SNA defines several types of LUs, each optimized for a specific class of applications. LUs of different types cannot communicate with each other, but LUs of the same type can communicate even though they reside on different kinds of systems.
For example, a TP running on a workstation that uses the AIX operating system can communicate with a TP on an AS/400 computer as easily as it can with a TP on another AIX workstation, as long as both TPs use the same LU type. IBM Communication Server (CS/AIX) supports the following LU types:
LU 6.2 supports program-to-program communication (APPC) in a distributed data processing environment. The LU 6.2 data stream is either an SNA general data stream (GDS), which is a structured-field data stream, or a user-defined data stream. LU 6.2 can be used for communication between two type 5 nodes, a type 5 node and a type 2.0 or 2.1 node, or two type 2.1 nodes. (Type 2.1 nodes can serve as APPN nodes.) This LU type provides more functions and greater flexibility than any other LU type. Unless you are constrained by existing hardware or software, LU 6.2 is the logical choice when developing new applications.
LU 3 supports application programs and printers using the SNA 3270 data stream. For example, LU 3 can support an application program running under Customer Information Control System (CICS) and sending data to an IBM 3262 printer attached to an IBM 3174 Establishment Controller.
LU 2 supports application programs and display workstations communicating in an interactive environment using the SNA 3270 data stream. Type 2 LUs also use the SNA 3270 data stream for file transfer. For example, the LU 2 protocol can support 3270 emulation programs, which enable workstations to perform the functions of IBM 3270-family terminals. In addition, LU 2 is used by other programs to communicate with host applications that normally provide output to 3270 display devices. Such TPs enable the workstation to achieve a form of cooperative processing with the host.
LU 1 supports application programs and single- or multiple-device data processing workstations communicating in an interactive, batch-data transfer, or distributed data processing environment. The data streams used by LU type 1 conform to the SNA character string or Document Content Architecture (DCA). For example, LU type 1 can support an application program running under Information Management System/Virtual Storage (IMS/VS) and communicating with an IBM 8100 Information System. This enables a workstation operator to correct a database that the application program maintains. Applications that use LU 1 are often described as remote job entry (RJE) applications.
LU 0, an early LU definition, supports primitive program-to-program communication. Certain host database systems, such as IMS/VS (Information Management System/Virtual Storage) and some point-of-sale systems for the retail and banking industries (such as the IBM 4680 Store System Operating System) use LU 0. Current releases of these products also support LU 6.2 communication, which is the preferred protocol for new applications.
CP - Control Points
A control point (CP) is an NAU that manages network resources within its domain, controlling resource activation, deactivation, and status monitoring. The CP manages both physical resources such as links, and logical information such as network addresses. SNA defines the following types of network control points:
System services control point
On a type 5 node, the CP is called a system services control point (SSCP). It manages and controls the network resources in a subarea network. For example, an SSCP can use a directory of network resources to locate a specific LU under its control, and can establish communication between two LUs in its domain. An SSCP can also cooperate with other SSCPs to establish connectivity between LUs in different subarea domains.
The SSCP also provides an interface to network operators at the host system, who can inspect and control resources in the network.
Physical unit control point
On type 4 nodes and type 2.0 nodes in a subarea network, the control point is called a physical unit control point (PUCP).
On type 2.1 nodes, the control point provides both PU and LU functions, such as activating local link stations, interacting with a local operator, and managing local resources. It can also provide network services, such as partner LU location and route selection for local LUs.
In a subarea network, the CP on a CS/AIX node acts as a type 2.0 PU. It communicates with an SSCP on a host and does not communicate with other CPs in the subarea network.
When participating in an APPN network, the CP exchanges network control information with the CPs in adjacent nodes. The CP can also function as an independent LU of type 6.2. The CP acts as the default LU for TPs on the local node.
SNA , APPN , APPC , LU0, LU1, LU2, LU3, LU6.2, PU, SSCP, NAU
NAUs are IBM SNA components.