Data Communications Networking Devices - 4th Ed.pdf

2.6 SNA AND APPN _______________________________________________________________ 241SNA layersSNA was developed as a connection-oriented communications service in 1974,predating the OSI Reference Model. A second version of SNA developed by IBMto include program-to-program communications capability differs from its originalincarnation at the higher layers and more closery resembles the OSI ReferenceModel. Figure 2.44 provides a comparison between the current version of SNA andthe ISO Reference Model which we will use for a discussion of SNA layers.Figure 2.44A comparison of SNA with the ISO Reference ModelPhysical and data link layersSimilar to the OSI physical layer, SNA's physical control layer is concerned withthe electrical, mechanical and procedural characteristics of the physical media andinterfaces to the physical media. SNA's data link control layer is also quite similarto OSI's data link layer. Protocols de®ned by SNA include SDLC, System/370channel, Token-Ring and X.25; however, only SDLC is used on a communicationslink in which master or primary stations communicate with secondary orslave stations. Some implementations of SNA using special software modules canalso support Bisynchronous BSC) communications, an IBM pre-SNA protocolstill widely used as well as asynchronous communications.Path control layerTwo of the major functions of the path control layer are routing and ¯ow control.Concerning routing, since there can be many data links connected to a nose, pathcontrol is responsible for insuring that data is correctly passed through intermediatenodes as it ¯ows from source to destination. At the beginning of an SNA session