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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.
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. ©
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.
- Spring '14