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APPC (Advanced Program-to-Program Communications)

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APPC, APPN (Advanced Peer-to-Peer Networking), and CPI-C (Common Programming Interface for Communications) are networking technologies that are available on many different IBM and non-IBM computing platforms. APPC, also known as LU 6.2, is software that enables high-speed communications between programs on different computers, from portables and workstations to midrange and host computers over SNA (Systems Network Architecture), Ethernet, X.25, token ring, and other network topologies. APPC software is available for many different systems, either as part of the operating system or as a separate software package. It is an open and published communications protocol.

APPC represented a major strategy change for IBM when it was introduced. It demonstrated a shift in network control away from the centralized host systems to the systems that were attached to the network. Systems running LU 6.2 sessions do not need the services of a host system when establishing sessions.

LU 6.2 was developed to allow computers on the network with their own processing power to set up their own sessions. In the older hierarchical approach, terminals attached to host computers relied completely on the host to set up and maintain sessions. LU 6.2 provides peer-to-peer communications between systems other than hosts and allows those systems to run distributed applications like file sharing and remote access. The entire range of IBM platforms is supported by LU 6.2, including LANs, desktop systems, and mainframes.

LU 6.2 relies on SNA Type 2.1 nodes. Type 2.1 nodes are different than other SNA nodes in that they run CP (Control Point) software that allows them to engage in peer-to-peer connections with other Type 2.1 nodes. This arrangement became increasingly important as LANs were installed in IBM SNA environments. While the LAN provided a connection from a network node to a connected host, those LAN nodes could also use LU 6.2 to communicate directly with other LAN nodes, rather than go through the host.

Applications using the LU 6.2 protocols are called TPs (transaction programs). Examples of TPs are IBM DDM (Distributed Data Management), which provides file sharing and database sharing among systems that implement DDM, and DIA (Document Interchange Architecture), which is a document exchange standard that defines searching, browsing, printing, and the distribution of documents.

A TP opens a session, performs data transfers, and closes. A TP performs a "unit of work" on a channel that interconnects IBM systems. The sessions are designed to be short-lived because some systems cannot perform other tasks until they complete the transactions. A transaction is like a conversation, and a TP can hold multiple conversations with multiple systems. Each conversation has a name and buffers for sending and receiving data, along with a code that is returned to indicate success or failure of the transaction. The parameters are simple, so code can be portable among systems.

Programs use LU 6.2 services through an interface called the LU 6.2 Protocol Boundary or through the CPI-C (Common Programming Interface for Communications). CPI-C is the currently preferred method. CPI provides a common environment for the execution of programs on different IBM platforms, and the C version provides the LU 6.2 communication interface. Recently, IBM has implemented CPI-C in its Open Blueprint, which supports TCP/IP (Transmission Control Protocol/Internet Protocol).

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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