Page 208 - DCAP406_DCAP_207_Computer Networks

Unit 13: Session Layer and Presentation Layer

13.1 Session Layer – Design Issues Notes

The session layer is thinnest layer with least numbers of protocols in the OSI model. The session
layer aims to establish, maintain and synchronize dialogs between communicating upper layers.
The communication may take place between either users or applications.
The functions of the session layers are as follows:

Session to transport communication: To coordinate connection and release of connection
of dialogs between the communicating applications.
Dialog Management: To coordinate who sends when.

Activity Management: To make sure that the data transfer is complete before the session
closes.
Synchronization: To provide synchronization points for data transfer.
Session to Transport Communication: The session layer helps to coordinate connection and
release of connection of dialogs between the communicating applications, it communicates
with the transport layer. The communication may be one to one, many to one and one to many.
In one to one, one session layer connection establishes for each transport layer connection. In
many to one, multiple session layer connections are shared with the services of one transport
layer connection. The one to many connection communication is set up when one session layer
connection calls for many transport layer connections to handle the service.
Dialog Management: Session layer aims to decide whose turn it is to talk. Some of the applications
operate in half-duplex mode. The half duplex provides two sides alternate communication
between sending and receiving messages and never send data simultaneously. The dialog
management is implemented through the use of a data token which is transmitted back and
forth to provide a user a right to transmit only when it possesses the token.
Activity Management: Session layer enables the user to delimit data into logical units called
activities. Each activity is treated as a separate activity and independent from the preceding and
following activities to that activity. Activities are used to delimit files of a multi-file transfer.
Activities are used for quarantining, collecting all the data of a multi-message exchange together
before processing them. The receiving application begins processing data only after all the data
arrived. This ensures that all or none of a set of operations is performed. For example, a bank
transaction may involve locking a record, updating a value, and then unlocking the record.
When an application processes the first operation, but could not receive the remaining operations
due to client or network failures. The record will remain locked forever. Quarantining solves
this problem.
Exception handling: It is a general purpose mechanism for reporting errors.

Tasks
1. Define session layer.
2. What are the functions of session layer?

13.2 Session Layer – Synchronization

Many a times during data transmission, some sort of error may creep in due to various reasons,
therefore the session layer aims to synchronise the data transmission so that the receiver receives
the data unit as desired rather than in any distorted form. This requires synchronization.

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