OSI (Open Systems Interconnection) Model

The ISO (International Organization for Standardization) is a worldwide federation that promotes international standards. In the early 1980s, it began work on a set of protocols that would promote open networking environments that would let multivendor computer systems communicate with one another using internationally accepted communication protocols.  It eventually developed the OSI reference model. The OSI model defines a layered architecture and the protocols defined in each layer are responsible for:  a. Communicating with the same peer protocol layer running in the opposite computer. b. Providing services to the layer above it (except for the top-level application layer). The techniques of Layered Protocols were developed to meet the following goals as well:  To provide a logical decomposition of a complex network into smaller, more understandable parts (layers) To provide standard interfaces between network functions, for example, standard interfaces between software program modules. To provide for symmetry in functions performed at each node in the network. Each layer performs the same functions as its counterpart in other nodes of the network. To provide means to predict and control any changes made to network logic. To provide a standard language to clarify communications between and among network designers, managers, vendors, and users when discussing network functions.

Network Introduction Network Cabling Wireless Network Network Interface Card OSI Model Protocols Ethernet - Network architecture Token Ring - Network architecture AppleTalk and ArcNet - Network architectures Repeaters, Bridges, Routers, and Gateways E-mail Standards Modem & Async Comm. T1, T3, and Switched 56 Digital Connectivities Packet-Switching Network,Frame Relay and ATM ISDN & FDDI

Layers included in OSI model are:
 

The lowest layer in the OSI model is called the Physical Layer. The functions within this layer are responsible for activating, maintaining, and deactivating a physical circuit between a DTE and a DCE. There are many standards published for the physical. The most notable ones are RS-232-C and V.24.

The Data-link Layer is responsible for the transfer of data over the channel. It provides for the synchornization of data delimit the flow of bits from the physical layer. It also provides for the identify of the bits. It ensures that data arrives safely at the receiving DTE. It provides for flow control to ensure that the DTE does not become overburdened with too much data at any one time. One of its most important functions is to provide for the detection of transmission errors and provide mechanism to recover from lost, duplicated, or errorneous data.

The Network Layer specifies the interface of the user DTE into a packet-switched network, as well as the interface of two DTEs with each other through a packet network. It also specifies network routing and the communications between networks (internetworking). The X.25 specification is included in this layer.

The Transport Layer provides the interface between the data communications net-work and the upper three layers (generally located at the user premises). It also gives the user options in obtaining certain levels of quality (and cost) from the network itself (i.e. the network layer). This layer is designed to keep the user isolated form some of the physical and functional aspects of the packet network. It also provides for the end-to-end accountability.

The Session Layer serves as a user interface into the transport service layer. The layer provides for an organized means to exchange data between users, and users can select the type of synchronization and control needed from the layer, such as:

a. alternative two-way or a simultaneous two-way dialogue.

b. synchronization points for intermediate checks/recovery for file transfer.

c. aborts or restarts.

d. normal and expedited data flow.

The Presentation Layer provides the syntax of data in the model, that is, the repre-sentation of data. The layer consists of many tables of syntax, such as teletype, ASCII, Videotext etc. The layer also provides for virtual terminal display and for services such as resolving the receipt of an electronic message from the application layer and negotiating with its peer layer a specific page image layout.

The Application Layer is concerned with the support of an end-user application process. Unlike the presentation layer, this layer is concerned with the semantics data. The layer contains service elements to support application processes such as job management, programming language send/receive, and business data exchange. The layer also supports the virtual terminal and virtual file concept.

Note:

OSI is a reference model, meaning that it defines a general description of services that should be provided at each layer, but it does not define any standard protocols. However, although the ISO has defined a set of protocols that follow the model, they are not part of the OSI definition. In addition, because OSI is a refernce model, it is often used when describing other protocols such as TCP/IP. For example, IP (Internet Protocol) is said to be a network layer protocol because it performs the functions defined in the network layer of the OSI model.

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