session and presentation layer

8/3/2019 Session and Presentation Layer

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Layer 5 - The Session Layer


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Layer 5

After data packets provided by the four lowerlayers travel through the transport layer, they

are turned into sessions by the layer 5protocol or OSI session layer.

This is accomplished by implementing

various control mechanisms: Conversation control.

Session parameter negotiation.

The session layer establishes, manages, and

terminates sessions between applications.


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Layer 5

Layer 5 the Session Layer is responsiblethe interhost communication.

It establishes, manages, and terminatessessions between applications.

This includes starting, stopping, and re-synchronizing two computers that are havinga "session".


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Layer 5 Protocols

Layer 5 has a number of important protocols.

Examples of Layer 5 protocols are:

Network File System (NFS) Structured Query Language (SQL)

Remote Procedure Call (RPC)

X-Window System

AppleTalk Session Protocol (ASP)

Digital Network Architecture Session ControlProtocol (DNA SCP)


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The Session Layer

Data communications travel on packet-switched networks.

The session layer coordinates applications

as they interact on two communicatinghosts.

The session layer addresses two datacommunication problems:

Collision of data.

Re-synchronizing communication after an

interruption.


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Dialog Control

When the session layer decides whether touse 2-way simultaneous or 2-way alternatecommunication it is called dialogue control.

The session layer uses two major forms of

dialog control:

1. 2-way simultaneous communication

allows 2 messages to cross during a

conversation2. 2-way alternate communication

It Avoids interrupts by taking turns.


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Two-Way Simultaneous

Devices transmit to each other, regardless ofwho is transmitting, and you would assumethat more information is always on the way.

If two-way simultaneous communication isallowed, then the session layer does little inthe way of managing the conversation, andallows the other layers of the communicating

computers to manage the conversation. If these session layer collisions are intolerable,

then the session layer will use two-way alternatecommunication.


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Two-Way Alternate

The session layer establish a set of rules(protocols) dictating how each device willtake turns transmitting.


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Synchronizing After a Collision

Dialogue separation is the orderly initiation,termination, and managing ofcommunication.

To solve synchronization problems, devicessend a checkpoint to each other.

Two very important checkpoints are how the

conversation starts (orderly initiation) and how itends (termination).


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Synchronizing After a Collision

When hosts perform checkpoints, both hostsperform the following routine:

back up the particular files

save the network settings

save the clock settings

make note of the end point in the conversation


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Layer 6 The Presentation Layer


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Layer 6 Presentation Layer

The presentation layer is responsible forpresenting data in a form that the receivingdevice can understand.

The presentation layer has 3 main functions:

data formatting

data compression

data encryption


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Data Formatting

To understand how data formatting works,imagine two dissimilar systems. The firstsystem uses Extended Binary CodedDecimal Interchange Code (EBCDIC) to

format text. The second system usesAmerican Standard Code for InformationInterchange (ASCII) to format text. Layer 6provides the translation between these two

different types of codes.


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Graphic File Formats

The Internet uses two binary file formats todisplay images:

Graphic Interchange Format (GIF)

Joint Photographic Experts Group (JPEG).

Any computer with a reader for the GIF andJPEG file formats can read these file types,regardless of the type of computer.


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Markup Language

Another type of file format is markuplanguage. This format acts as a set ofdirections that tell a Web browser how todisplay and manage documents. Hypertext

Markup Language (HTML) is the language ofthe Internet.


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Encryption & Compression

Layer 6 is responsible for data encryption.Data encryption protects information duringits transmission.

The presentation layer is also responsible for

the compression of files.

session and presentation layer

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