Explain the functions of the User Support Layers of the OSI Model.

Explain the functions of the User Support Layers of the OSI Model.

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Explain the functions of the User Support Layers of the OSI Model.

CBSGS: OCT – 2017 | OCT – 2015

IDOL: DEC – 2017

SOLUTION:

User Support Layers of the OSI Model:

The Session, Presentation, and Application Layers are the User Support Layers.

Application Layer: [CBSGS: OCT – 2015]

The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.

Specific Services provided by the Application Layer include the following:

• Network Virtual Terminal: A network virtual terminal is a software version of a physical terminal and allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host. The user’s computer talks to the software terminal, which, in turn, talks to the host, and vice versa. The remote host believes it is communicating with one of its own terminals and allows you to log on.
• File Transfer, Access, and Management (FTAM): This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.
• E-mail Services: This application provides the basis for e-mail forwarding and storage.
• Directory Services: This application provides distributed database sources and access for global information about various objects and services.

Presentation Layer: [IDOL: DEC – 2017]

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

Specific responsibilities of the presentation layer include the following:

• Translation: The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information should be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format.
• Encryption: To carry sensitive information a system must be able to assure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.
• Compression: Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.

Session Layer:

The services provided by the first four layers (physical, data link, network and transport) are not sufficient for some processes. The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction between communicating systems.

Specific responsibilities of the session layer include the following:

• Dialog Control: The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either halfduplex (one way at a time) or full-duplex (two ways at a time) mode.
• Synchronization: The session layer allows a process to add checkpoints (synchronization points) into a stream of data. For example, if a system is sending a file of 2,000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. Pages previous to 501 need not be resent.