IBM SNA Interoperability Concepts
IBM SNA Interoperability Concepts |
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LUs are SNA protocols that provide a standardized format for delivery of data for applications. APPN networks typically use type 6.2 LUs, also known as Advanced Program-to-Program Communications (APPC) LUs. LU 6.2 is the most recently developed and the most advanced LU type. Unlike dependent LUs that are used in hierarchical SNA networks, LU 6.2 does not depend on centralized mainframe communications software. Rather, LU 6.2 provides a foundation for distributed computing in which programs on separate computers can communicate directly with each other across the network.
Although LU 6.2 provides the foundation for APPN communications, APPN can use additional LU types for AS/400 printers and display terminals as described in Table A.8. However, LU 6.2 is the only widely implemented APPN LU type.
Table A.8 APPN LU Types
| APPN LU Type | Description |
|---|---|
| LU 6.2 | Supports APPC for a broad range of SNA nodes and includes functions to support any type of SNA-based application. The most recent and advanced LU type. |
| LU 4 | Supports printers that use the IBM 5250 data stream. Not widely implemented because it does not provide the broad functionality of LU 6.2. |
| LU 7 | Supports display terminals that use IBM 5250 data streams, such as AS/400 display stations. Not widely implemented because it does not provide the broad functionality of LU 6.2. |
LU 6.2 forms the basis for IBM's Advanced Program-to-Program Communications (APPC), the network communications protocol most commonly associated with APPN. APPC is a general purpose network access method that supports applications such as:
The programs that use APPC LU 6.2 to communicate are called transaction programs (TPs). Figure A.8 illustrates TPs communicating through APPC sessions.
Figure A.8 Transaction Programs Communicating Through APPC Sessions
APPC LU 6.2 serves as a translator between TPs and the network. When a TP on one computer passes information to the APPC software, APPC establishes a session and sends it to the destination node. On the receiving end, APPC translates the information back into its original format and passes it to the corresponding partner TP. APPC can be used across any of the standard types of connections supported by SNA.
APPC generally uses a local APPC LU and one or more remote APPC LUs. Local APPC LUs can be dependent or independent.
Dependent LUs are used to provide backward compatibility with hierarchical networks. As such, dependent LUs require a host VTAM SSCP to establish and manage LU to LU sessions. Dependent LUs can support only a single session between a given pair of LUs.
A dependent LU must be used when an APPC TP needs to communicate with a mainframe that uses a version of VTAM earlier than V3R2. Independent LUs are not supported by earlier versions of VTAM. Also, dependent LUs cannot be used to communicate with AS/400 systems.
In full peer-oriented APPN networks, typically implemented in AS/400 environments, independent APPC LUs are used. Independent APPC LUs can establish and manage LU to LU sessions without the aid of a mainframe VTAM SSCP. Independent LUs provide the ability to run multiple, concurrent, parallel sessions between a single pair of LUs. Support for independent LUs is one of the main advantages of APPN networks.
