OSI Protocol Hierarchy
OSI Protocol Hierarchy
Session Layer
The session layer is the fifth among the seven layers of the Open System Interconnections (OSI) model. It resides above the transport layer and below the presentation layer, and provides “value added" services to the underlying transport layer services. The session layer (along with the presentation layer) add services to the transport layer that are likely to be of use to applications, so that each application doesn 't have to provide its own implementation.
The session layer the thinnest layer in the OSI model. At the time the model was formulated, it was not clear that a session layer was needed.
The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes. The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.
The session layer is responsible for dialog control and synchronization.
Dialog control. Decide whose turn it is to talk. The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either simplex (one way only), half-duplex (one way at a time) or full-duplex (two ways at a time) mode.
Synchronization. Move the two session entities into a known state. The session layer allows a process to add checkpoints, or synchronization points, to a stream of data. For example, if a system is sending a file of 2000 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.
Basic Functions for the Session Layer Protocols
Creating a connection – session/communication session between an APPLICATION in one computer and
Session Layer
The session layer is the fifth among the seven layers of the Open System Interconnections (OSI) model. It resides above the transport layer and below the presentation layer, and provides “value added" services to the underlying transport layer services. The session layer (along with the presentation layer) add services to the transport layer that are likely to be of use to applications, so that each application doesn 't have to provide its own implementation.
The session layer the thinnest layer in the OSI model. At the time the model was formulated, it was not clear that a session layer was needed.
The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes. The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.
The session layer is responsible for dialog control and synchronization.
Dialog control. Decide whose turn it is to talk. The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either simplex (one way only), half-duplex (one way at a time) or full-duplex (two ways at a time) mode.
Synchronization. Move the two session entities into a known state. The session layer allows a process to add checkpoints, or synchronization points, to a stream of data. For example, if a system is sending a file of 2000 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.
Basic Functions for the Session Layer Protocols
Creating a connection – session/communication session between an APPLICATION in one computer and
References: [1] Ampoloquio, Jason M., Self-Sufficient Guide to Electronics Engineering, Manila, Philippines, 2005. [2] Blake, Roy, Electronic Communication Systems, 2nd ed., Delmar, Singapore, 2002. [3] Forouzan Behrouz A., Data Communications and Networking, 4th ed., Mc Graw Hill, New York, 2007. [4] Tomasi, Wanye, Electronic Communications System; Fundamentals through Advance, 4th ed., Prentice Hall, New Jersey, 2001