It then adds a header hi to each packet and passes

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Unformatted text preview: ransmission. The presentation layer software in the receiving computer transforms the message data from the network representations to the formats used in its own computer. 3. For applications dealing with confidential or secret data, the presentation layer software in the sending computer encrypts message data, before passing it to the session layer. On the receiver side, the encrypted message data is decrypted by the presentation layer before being passed on to the application layer. 4. In a similar manner, when message data is large in volume (such as multimedia data) or with networks that are slow or heavily loaded, message data may be compressed and decompressed by the presentation layer software in the sending and receiving computers, respectively. Application Layer The application layer provides services that directly support the end users of the network. Obviously, the functionality implemented at this layer of the architecture is application-specific. Since each application has different communication needs, no fixed or standard set of application layer protocols can meet the needs of all applications. Therefore, the application layer is basically a collection of miscellaneous protocols for various commonly used applications such as electronic mail, file transfer, remote login, remote job entry, and schemas for distributed databases. Some popular application layer protocols are X.400 (Electronic Mail Protocol), X.500 (Directory Server Protocol), FTP (File Transfer Protocol), and rlogin (Remote Login Protocol). Example of Message Transfer in the OSI Model The example below has been reprinted, with permission, from the book titled Distributed Operating Systems: Concepts and Design by Pradeep K. Sinha. © 1997 IEEE. To illustrate the functions of the various layers of the OSI model, let us consider a simple example of message transmission. With reference to Figure 17.25, let us assume that a process at the sending node wants to send a message M to a process at the receiving node. The sending node's process builds the message M and passes it to the application layer (layer 7) on its machine. The application layer software adds a header (H7) to M and passes the resulting message to the presentation layer (6) via the interface between la...
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This document was uploaded on 04/07/2014.

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