OSI Layers Now that the basic features of the OSI layered approach have been described, each individual OSI layer and its functions can be discussed. Each layer has a predetermined set of functions it must perform for communication to occur. Application Layer The application layer is the OSI layer closest to the user. It differs from the other layers in that it does not provide services to any other OSI layer, but rather to application processes lying outside the scope of the OSI model. Examples of such application processes include spreadsheet programs, word-processing programs, banking terminal programs, and so on. The application layer identifies and establishes the availability of intended communication partners, synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity. Also, the application layer determines whether sufficient resources for the intended communication exist. Presentation Layer The presentation layer ensures that information sent by the application layer of one system will be readable by the application layer of another system. If necessary, the presentation layer translates between multiple data representation formats by using a common data representation format. The presentation layer concerns itself not only with the format and representation of actual user data, but also with data structures used by programs. Therefore, in addition to actual data format transformation (if necessary), the presentation layer negotiates data transfer syntax for the application layer. Session Layer As its name implies, the session layer establishes, manages, and terminates sessions between applications. Sessions consist of dialogue between two or more presentation entities (recall that the session layer provides its services to the presentation layer). The session layer synchronizes dialogue between presentation layer entities and manages their data exchange. In addition to basic regulation of conversations (sessions), the session layer offers provisions for data expedition, class of service, and exception reporting of session-layer, presentation-layer, and application-layer problems. Transport Layer The boundary between the session layer and the transport layer can be thought of as the boundary between application-layer protocols and lower-layer protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower four layers are concerned with data transport issues. The transport layer attempts to provide a data transport service that shields the upper layers from transport implementation details. Specifically, issues such as how reliable transport over an internetwork is accomplished are the concern of the transport layer. In providing reliable service, the transport layer provides mechanisms for the establishment, maintenance, and orderly termination of virtual circuits, transport fault detection and recovery, and information flow control (to prevent one system from overrunning another with data). Network Layer The network layer is a complex layer that provides connectivity and path selection between two end systems that may be located on geographically diverse subnetworks. A subnetwork, in this instance, is essentially a single network cable (sometimes called a segment). Because a substantial geographic distance and many subnetworks can separate two end systems desiring communication, the network layer is the domain of routing. Routing protocols select optimal paths through the series of interconnected subnetworks. Traditional network-layer protocols then move information along these paths. Link Layer The link layer (formally referred to as the data link layer) provides reliable transit of data across a physical link. In so doing, the link layer is concerned with physical (as opposed to network, or logical) addressing, network topology, line discipline (how end systems will use the network link), error notification, ordered delivery of frames, and flow control. Physical Layer The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications. [INTERNETTS HISTORIE] - [INTERNETT I DAG] - [UT PÅ NETTET] - [INTERNETT SOM INFORMASJONSKILDE] -  [INTERNETT SOM SOSIALT NETTVERK] - [INTERNETT SOM UNDERHOLDNING] - [ANARKI ELLER REGULERT AV LOVER?] - [ORDFORKLARINGER] - [KONKLUSJON] - [STARTSIDEN] - [MAIL MEG] - [OSI-MODELLEN] - [FLYT AV INFO PÅ INTERNETT] - [HVORDAN VIRKER WEBSIDER] - [TILKNYTNING TIL INTERNETT]