Advanced Peer-to-Peer Networking
Advanced Peer-to-Peer Networking ( APPN ) is an extension of the Systems Network Architecture (SNA) protocol suite from IBM from 1985 .
With the 'classic' IBM network architecture (SNA), communication between two peripheral systems (LUs, logical units) is only possible via a central instance ( mainframe with SNA software, for example VTAM ), i.e. 'dependent LUs '.
APPN now sees these peripheral systems as equal partners ( peers ) who can set up SNA connections with one another without a central control mechanism, as 'independent logical units'.
The APPN design was favored by the steadily increasing computing power on the terminal side since the introduction of SNA in 1974 . Saw the classical SNA design still 'stupid' screen and printer terminals before, so were in the meantime, more and more multi-functional systems in use on which the terminal - emulation , thus providing the user interface for a mainframe application, only one of many tasks was. This means that additional tasks such as the independent switching of LU-LU sessions and negotiating the optimal connection (routing) can be assigned to these systems.
APPN initially received little attention when it was first introduced, as it was based exclusively on the LU Type 6.2 ( APPC ) protocol. It was therefore unsuitable for replacing mainframe-supported dialog applications based on the IBM 3270 terminal type, historically grown 'dependent logical units'.
Only with the introduction of the Dependent LU Server (on the mainframe) and the Dependent LU Requester (DLUS / DLUR) could the terminals previously used with the SNA protocols LU Type 2 (3270 screen terminal), LU Type 1 (printer terminal) and LU type 0 (freely defined) can also be operated in full in the APPN network, the 'new SNA'.
APPN has been further developed since it was first introduced: The High Performance Routing (HPR) component supports the optimal use of fast line connections between APPN nodes.
However, the new flexibility also means getting used to the administration: If all definitions were carried out centrally on the mainframe in the classic SNA, they must be distributed in the APPN model.