OSI Model


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OSI Model By: Jeremy Davis : 

OSI Model By: Jeremy Davis ICTN 2154


INTRODUCTION Operating Systems Interconnection (OSI) was first released in 1984. The OSI model describes how information makes its way from application programs through a network medium to another application program in another computer. It divides this one big problem into seven smaller problems. Each of these seven problems is reasonably self-contained and therefore more easily solved without excessive reliance on external information. Each problem is addressed by one of the seven layers of the OSI model.

Seven Layers of The OSI Model : 


Diagram of the Seven Layers : 

Diagram of the Seven Layers

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Acronyms used to help people remember the OSI Model is: All Serious Teenagers Never Do Physics Pretty All Seem To Need Data Processing People

Now let’s take a more in depth look at each of these layers…. : 

Now let’s take a more in depth look at each of these layers….


APPLIC ATION LAYER The Application Layer is Layer 7 of the seven-layer OSI model The Application Layer contains all protocols and methods that fall into the realm of process-to-process communications via an Internet Protocol (IP) network using the Transport Layer protocols to establish underlying host-to-host connections. In the OSI model, the definition of its Application Layer is narrower in scope, distinguishing explicitly additional functionality above the Transport Layer at two additional levels: Session Layer and Presentation Layer. OSI specifies strict modular separation of functionality at these layers and provides protocol implementations for each layer. The common application layer services provide semantic conversion between associated application processes.


PRESENTATION LAYER The Presentation Layer is Layer 6 of the seven-layer OSI model The Presentation Layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems The Presentation Layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in form of datagrams or packets between hosts Devices: -Encryption -EBCDIC and ASCII -GIF & JPEG


SESSION LAYER The Session Layer is Layer 5 of the seven-layer OSI model The Session Layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session Layer services are commonly used in application environments that make use of remote procedure calls (RPCs). Within the service layering semantics of the OSI network architecture, the Session Layer responds to service requests from the Presentation Layer and issues service requests to the Transport Layer.


TRANSPORT LAYER The Transport Layer is Layer 4 of the seven-layer OSI model The Transport Layer is responsible for delivering messages between networked hosts. As part of this, the Transport Layer is also responsible for fragmentation and reassembly. In addition, some Transport Layer protocols also provide services to manage flow control and end-to-end error recovery. The transport layer provides mechanisms for: -multiplexing upper layer applications -the establishment, maintenance, and orderly termination of virtual circuits -information flow control -transport fault detection and recovery


NETWORK LAYER The Network Layer is Layer 3 of the seven-layer OSI model The Network Layer is responsible for end-to-end (source to destination) packet delivery including routing through intermediate hosts, whereas the Data Link Layer is responsible for node-to-node (hop-to-hop) frame delivery on the same link. The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination host via one or more networks while maintaining the quality of service and error control functions. Functions of the Network Layer include: -Connection model - Host addressing - Message forwarding


DATA LINK LAYER The Data Link Layer is Layer 2 of the seven-layer OSI model It corresponds to or is part of the link layer of the TCP/IP reference model. The Data Link Layer is the protocol layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. The Data Link Layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the Physical Layer. Examples of data link protocols are Ethernet for local area networks (multi-node), the Point-to-Point Protocol (PPP), HDLC and ADCCP for point-to-point (dual-node) connections. The Data Link Layer is concerned with local delivery of frames between devices on the same LAN.


PHYSICAL LAYER The Physical Layer is the first and lowest layer in the seven-layer OSI model The Physical Layer consists of the basic hardware transmission technologies of a network. It is a fundamental layer underlying the logical data structures of the higher level functions in a network. The Physical Layer defines the means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes. Physical Layer provides an electrical, mechanical, and procedural interface to the transmission medium.

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