The Computer Revolution/Networks/Models
The Open Systems Interconnection(OSI) model is a theoretical model of networking that organizes network functions into seven layers (physical, data link, network, transport, session, presentation, and application) and specifies the communication interfaces between the OSI model layers and network endpoints utilizing various protocols. Although the model is theoretical the seven layers provide standardized functions that take place at each layer. The model is designed so that a layer serves the layer above it and is served by the layer below it.
Layer 1: physical layer
The physical layer defines electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or optical fiber cable|fiber optical cable. This includes the layout of Lead (electronics)|pins, voltages, line Characteristic impedance|impedance, cable specifications, signal timing, network hub|hubs, repeaters, network card|network adapters, host adapter|host bus adapters (HBA used in storage area networks) and more.
The data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multi-access media, was developed independently of the ISO work in IEEE 802|IEEE Project 802. IEEE work assumed sublayer-ing and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 Logical link control|LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layer by protocols such as Transmission Control Protocol|TCP, but is still used in niches where X.25 offers performance advantages.
Layer 3: network layer
The network layer provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network (in contrast to the data link layer which connects hosts within the same network), while maintaining the quality of service requested by the transport layer. The network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. router (computing)|Routers operate at this layer, sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is not hierarchical.
Layer 4: transport layer
The transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state- and connection-oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred.
Layer 5: session layer
The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for Duplex (telecommunications)|full-duplex, half-duplex, or Simplex communication|simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls. On this level, Inter-Process (computing)|Process communication happen (SIGHUP, SIGKILL, End Process, etc.).
Layer 6: presentation layer
The presentation layer establishes context between application-layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack.
Layer 7: application layer
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.
The developers of the TCP/IP protocol suite created their own architectural model to help describe its components and functions. This model is similar to the OSI model except it is a four layer standard compared to the seven layer of the OSI model. Because the four layers account for all 7 layers of functions in the OSI model, each layers function is not as strictly defined in this model.
Wikipedia OSI Model Meyers, Mike. Network+ Certification All-in-One Exam Guide, Fifth Edition.