Data Communications Networking Devices - 4th Ed.pdf

246 __________________________________________________________ WIDE AREA NETWORKSFigure 2.46SNA routing based upon the headers in different message unitsAdvanced Peer-to-Peer Networking APPN)Although SNA represents one of the most successful networking strategiesdeveloped by a vendor, its centralized structure based upon mainframe-centeredcomputing became dated in an evoloving era of client-server distributive computing.Recognizing the requirements of organizations to obtain peer-to-peertransmission capability instead of routing data through mainframes, IBMdeveloped its Advanced Peer-to-Peer Networking APPN) architecture during1992 as a mechanism for computers ranging from PCs to mainframes to communicateas peers across local and wide area networks. The actual ability ofprograms on different computers to communicate with one another is obtainedfrom special software known as Advanced Program-to-Program CommunicationAPPC) which represents a more marketable name for LU6.2 software. SinceAPPC enables the operation of APPN, we will ®rst focus our attention upon APPCprior to examining the architecture associated with APPN.APPC conceptsAPPC represents a software interface between programs requiring communicationswith other programs and the network to which computers running those programsare connected. APPC represetns an open communications protocol that is availableon a range of platforms to include PCs, mainframes, Macintosh, and UNIX-basedsystems as well as IBM 3174 control units and its 6611 Nways router series.In its most basic structure, APPC can be considered to represent a stack existingabove a network adapter but below the application using the adaptor. Figure 2.47illustrates the general relationship of APPC to the software stack on two computerscommunicating with one another on a peer-to-peer basis. In this example, programA on computer 1 is shown communicating with program B on computer 2.