network protocols handbook.pdf

4Network Communication Architecture and ProtocolsOSI Network Architecture 7 Layers ModelFor example, if computer A has data from a software applicationto send to computer B, the data is passed to the applicationlayer. The application layer in computer A then communicatesany control information required by the application layer incomputer B by prepending a header to the data. The resultingmessage unit, which includes a header, the data and maybea trailer, is passed to the presentation layer, which prependsits own header containing control information intended for thepresentation layer in computer B. The message unit grows insize as each layer prepends its own header and trailer containingcontrol information to be used by its peer layer in computerB. At the physical layer, the entire information unit is transmittedthrough the network medium.The physical layer in computer B receives the information unitand passes it to the data link layer. The data link layer in computerB then reads the control information contained in the headerprepended by the data link layer in computer A. The header andthe trailer are then removed, and the remainder of the informationunit is passed to the network layer. Each layer performs thesame actions: The layer reads the header and trailer from itspeer layer, strips it off, and passes the remaining informationunit to the next higher layer. After the application layer performsthese actions, the data is passed to the recipient software applicationin computer B, in exactly the form in which it was transmittedby the application in computer A.One OSI layer communicates with another layer to make use ofthe services provided by the second layer. The services providedby adjacent layers help a given OSI layer communicate withits peer layer in other computer systems. A given layer in theOSI model generally communicates with three other OSI layers:the layer directly above it, the layer directly below it and itspeer layer in other networked computer systems. The data linklayer in computer A, for example, communicates with the networklayer of computer A, the physical layer of computer A andthe data link layer in computer B. The following chart illustratesthis example.ApplicationPresentationSessionTransportNetwortData LinkPhysicalApplicationPresentationSessionTransportNetwortData LinkPhysicalFigure 1-3: Data communication between peer layers