Page 210 - DCAP406_DCAP_207_Computer Networks

Unit 13: Session Layer and Presentation Layer

Self Assessment Notes

Fill in the blanks:
1. The ……………………… layer is located above the transport layer and intends to provide
value added services to the underlying transport layer services.

2. The ……………………… layer preserves and maintains the meaning of information
transmitted across a network.
3. The ……………………………. layer is thinnest layer with least numbers of protocols in the
OSI model.
4. Session layer enables the user to delimit data into logical units called …………………………
5. There are …………………………… types of synchronization points.

6. Synchronization is accomplished with the use of ……………………………….. numbers.

13.3 Presentation Layer

The presentation layer in a network model is concerned with the syntax and semantics of the
information exchanged between two systems.

The presentation layer determines how computers represent data format such as ASCII, GIF, etc
and implements coding and conversion functions for application layer data to ensure that data
sent from the application layer of source machine will be readable by the application layer of
destination machine. The presentation layer coding and conversion methods provide common
data representation formats, conversion of character representation formats, common data
compression techniques and common data encryption techniques. Common data representation
formats that may involve the use of standard image, sound and video formats allow to exchange
the application data between different types of computer systems from source machine to
destination machine. The machines interchanging data may use different text and data
representations like EBCDIC and ASCII. The data compression techniques allow a compressed
data at the source machine to be properly decompressed at the destination machine. Likewise,
data encryption techniques allow data encrypted at the source machine to be properly decryption
at the destination machine. Thus, the presentation layer makes the application layer free of the
responsibility of syntactical differences in data representation within the source and destination
machine or end user systems.

13.4 Presentation Layer – The Design Issues

The presentation layer deals with the translation, encryption/decryption, authentication and
compression which are explained as below:
Translation: It converts the complex data structures used by an application strings, integers,
structures, etc. into a byte stream that may be transmitted across the network. The message is
represented in such a way that communicating machines agree to the format of the data being
exchanged. For example, ASCII or EBCDIC character set.

The translation may be direct or indirect. In direct translation method, ASCII code is translated
as the EBCDIC at the destination machine. In the indirect method, the ASCII code is first translated
to a standard format at the source machine itself before transmission. The destination machine
converts it into EBCDIC code. Direct method is not desirable with obvious reasons as the
destination machine needs to deal with several computers in the network and therefore are
required to have a table of conversion for different data formats. The indirect method that

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