OSI MODEL

THE APPLICATION LAYER - LAYER 7

The Application layer provides three basic services to applications:
It makes sure the resources needed to carry out a session are present.
It matches the application to the appropriate communication protocol or service.
It synchronizes the tgransmission of data between the application and its protocol.
The Application layer is used to support the following services:
File services - store, move control access to, and retrieve files
Print services - send data to local or network printers
Message services - transfer text, graphics, audio, and video over a network
Application services - process applications locally or through distributed processing
Database services - allow a local computer to access network services
In addition, the Application layer advertises any services that are being offered and determines whether requests made by the client should be processed locally or remotely (through another network resource).
The Application layer services and protocols you should know are:
· FTP (File Transfer Protocol)
· E-mail clients
· Web Browsers
· Telnet
· SNMP (Simple Network Management Protocol)
· BBS (bulletin board system) services
· EDI (Electronic Data Interchange) and other transaction services


THE PRESENTATION LAYER - LAYER 6

The main function of the presentation layer is to define the data formats used to provide a number of services to the Application layer.
Included in these services the ones you need to know are:
· Data encryption - coding data so that it is protected from unauthorizaed access
· Data compression - reducing the number of packets required for transport
· Data formatting
· Data conversion
Conversions standards defined on the Presentation Layer for data conversion and formatting:
Category
Standards


Data Conversion
ASCII, EBCDIC, encryption


Audio/video conversion
MIDI, MPEG, QuickTime, AVI


Graphics conversion
GIF, JPEG, PICT, TIFF


THE SESSION LAYER - LAYER 5

Layer 5 of the OSI model is the Session Layer, which establishes, manages, and terminates sessions between applications.
Following are some of the communication tasks performed at this layer:
Establishing connections
Maintaining connections
Synchronizing communications
Controlling dialogues
Terminating connections
When you create a connection, you authenticate the user%