IBM Advanced Program-to-Program Communication
From Wikipedia, the free encyclopedia
In computing, Advanced Program to Program Communication or APPC is a protocol which computer programs can use to communicate over a network. APPC is at the application layer in the OSI model.
APPC was developed as a component of IBMs Systems Network Architecture or SNA. Several APIs were developed for programming languages such as COBOL or REXX.
APPC is linked with the term LU 6.2 ( Logical unit type 6.2 )
APPC is to a large extent limited to the IBM operating systems such AS/400, OS/2 and AIX. Microsoft also include SNA support in Microsoft Windows server editions. Major IBM software products have included support for APPC, including CICS, DB2, CIM and MVS.
Contrary to TCP/IP, in which both communication partners always possess a clear role (one is always server, and others always the client), the communication partners in APPC are equal, i.e. everyone can be both servers and clients equally. The role, and the number of the parallel sessions between the partners, is negotiated over so-called 'CNOS' sessions (Change Number Of Session) with a special log mode (e.g. at IBM, 'snasvcmg'). Communication of the data is made then by 'data sessions', their log modes can be determined in detail from the VTAM administrator (e.g. length of the data blocks, coding etc..).
With the wide success of TCP/IP, APPC has declined.
