Data Communications Networking Devices - 4th Ed.pdf

74 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSLayer 7Ðthe application layerFinally,the application layer level 7) acts as a window through which theapplication gains access to all of the services provided by the model. Examples offunctions performed at this level include ®le transfers,resource sharing and databaseaccess. While the ®rst four layers are fairly well de®ned,the top three layersmay vary considerably,depending upon the network used. Figure 1.35 illustratesthe OSI model in schematic format,showing the various levels of the model withrespect to a terminal accessing an application on a host computer system.Figure 1.35OSI model schematicData ¯owAs data ¯ows within an ISO network each layer appends appropriate headinginformation to frames of information ¯owing within the network while removingthe heading information added by a lower layer. In this manner,layer n) interactswith layer …n 1† as data ¯ows through an ISO network.Figure 1.36 illustrates the appending and removal of frame header informationas data ¯ows through a network constructed according to the ISO reference model.Since each higher level removes the header appended by a lower level,the frametraversing the network arrives in its original form at its destination.As the reader will surmise from the previous illustrations,the ISO referencemodel is designed to simplify the construction of data networks. This simpli®cationis due to the eventual standardization of methods and procedures to appendappropriate heading information to frames ¯owing through a network,permittingdata to be routed to its appropriate destination following a uniform procedure.