Transport layer makes sure that the entire message

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transport layer makes sure that the entire message arrives at the receivingtransport layer with out error. Error correction is achieved through retransmission.Session Layer: Session layer is the network dialog controller. It establishes, maintains,and synchronizes the interaction between communicating systems. Specific responsibilitiesof the layer areDialog Control:Session layer allows two systems to enter in to a dialog.Communication between two processes take place either in half-duplex or full-duplex. Example: the dialog between a terminal connected to a mainframe. Can behalf-duplex.Synchronization. The session layer allows a process to add checkpoints into astream of data. Example If a system is sending a file of 2000 pages , check pointsmay be inserted after every 100 pages to ensure that each 100 page unit isadvised and acknowledged independently. So if a crash happens during thetransmission of page 523,retransmission begins at page 501,pages 1 to 500 neednot be retransmitted.Presentation layer. It is concerned with the syntax and semantics of the informationexchanged between two systems. Responsibilities of the presentation layer areTranslation.The processes in two systems are usually exchanging information inthe form of character strings, numbers, and so on. The Since different computersuse different encoding systems ,the presentation layer is responsible forinteroperability between these different encoding methods. At the sender ,thepresentation layer changes the information from its sender-dependent format intoa common format. The presentation layer at the receiving machine changes thecommon format into its receiver dependent format.Encryption. The sender transforms the original information from to another formand sends the resulting message over the entire network. Decryption reverses theoriginal process to transform the message back to its original form.Compression. It reduces the number of bits to be transmitted. It is important inthe transmission of text, audio and video.Application Layer: It enables the user (human/software) to access the network. Itprovides user interfaces and support for services such as electronic mail, remote fileaccess and transfer, shared database management and other types of distributedinformation services. Services provided by the application layer are Network Virtual terminal. A network virtual terminal is a software version of aphysical terminal and allows a user to log on to a remote host.File transfer, access and management. This application allows a user to accessfiles in a remote computer, to retrieve files from a remote computer and to manageor control files in a remote computer.Mail services. This application provides the basis for e-mail forwarding and storage.
Directory services. It provides distributed database sources and access for global information about various objects3. IEEE 802.4 TOKEN BUS-EXPLAINIEEE 802.4 describes a token bus LAN standard.

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