(Released in 1984) Open System Interconnection reference model. Network architectural model developed by ISO and ITUT-T. The model consists of seven layers, each of which specifies particular network functions such as addressing, flow control, error control, encapsulation, and reliable message transfer. The highest layer (the application layer) is closest to the user; the lowest layer (the physical layer) is closest to the media technology. The lowest layer is implemented in hardware. The next to lowest layer is implemented in hardware and software, while the upper five layers are implemented only in software. The OSI reference model is used universally as a method for teaching and understanding network functionality. Application Layer (Network services to apps) - Layer 7 of the OSI reference model. This layer provides services to application processes (such as electronic mail, file transfer, and terminal emulation) that are outside of the OSI model. The application layer identifies and establishes the availability of intended communication partners (and the resources required to connect with them), synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity. 6. Presentation Layer (Data representation) - Layer 6 of the OSI reference model. This layer ensures that information sent by the application layer of one system will be readable by the application layer of another. The presentation layer is also concerned with the data structures used by programs and therefore negotiates data transfer syntax for the application layer 5. Session Layer (Interhost communication) - Layer 5 of the OSI reference model. This layer establishes, manages, and terminates sessions between applications and manages data exchange between presentation layer entities. 4. Transport Layer (End-to-end connection reliability) - Layer 4 of the OSI reference model. This layer is responsible for reliable network communication between end nodes. The transport layer provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recover, and information flow control. ----- Media Layers: controls physical delivery of the message over the network 3. Network Layer (Addresses and best path) - Layer 3 of the OSI reference model. This layer provides connectivity and path selection between two end systems. The network layer is the layer at which routing occurs. 2. Data Link Layer (Assess to networking media) - Layer 2 of the OSI reference model. This layer provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow control. The IEEE has divided this layer into two sublayers: the MAC sublayer and the LLC sublayer. Sometimes simply called link layer. A switch operates at the data link layer. 1. Physical Layer (Binary transmission; wires, connections, voltages, and data rates) - Layer 1 of the OSI reference model. The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. The function of the physical layer is the transmission of data. Typically such transmission is accomplished through the use of such things as wires, connectors, and voltages. ----- The upper 5 layers are software only ----- Layer 2 is hardware and software ----- Layer 1 is hardware only The OSI reference model describes how information or data makes its way from application programs (such as spreadsheets) through a network medium (such as wires) to another application program located on another computer on a network
OSI ref model describes how information from a software application in one computer moves through a network medium to a software application in another computer. OSI model is a conceptual model composed of 7 layers each specifying a particular network function. OSI model was developed by ISO in 1984 All People Seem To Need Data Processing LAN protocols operate on data link and physical layers of OSI models and define communication over various LAN media. WAN protocols operate at lowest layers of OSI model and define communication over various wide area media. Routing protocols are network layer protocols that are responsible for exchanging info between routers. Network protocols are upper layer protocols. Many protocols rely on others for operation.
Provides reliable transit od data across physical network link. Physical addressing define how devices are addressed at data link layer as opposed to network addressing. Network topology consists of data link layer specs that define how devices are physically connected by ring or bus. Logical link control (LLC)sub layer manages communications between devices over a single link of a network. LLC is defined in IEEE 802.2. supports both connectionless and connection oriented services of upper layers. IEEE 802.2 defines how to enable multiple higher-layer protocols to share a single physical data link. Media access control sub layer manages protocol access to the physical network medium. IEEE MAC specifications define MAC addresses, which enable multiple devices to uniquely define one another at the data link layer.
![Design issues of communication ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]](https://image.slidesharecdn.com/3osiarchitecture-091201224545-phpapp01/85/3-Osi-Architecture-1-320.jpg)
![ISO Reference model ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)