A layer serves the layer above it and is served by the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that comprise the contents of that path. Two instances at the same layer are visualized as connected by a horizontal connection in that layer.
The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO), maintained by the identification ISO/IEC 7498-1.
Contents
History
In the late 1970s, one project was administered by the International Organization for Standardization (ISO), while another was undertaken by the International Telegraph and Telephone Consultative Committee (CCITT, from French: Comité Consultatif International Téléphonique et Télégraphique). These two international standards bodies each developed a document that defined similar networking models.In 1983, these two documents were merged to form a standard called The Basic Reference Model for Open Systems Interconnection. The standard is usually referred to as the Open Systems Interconnection Reference Model, the OSI Reference Model, or simply the OSI model. It was published in 1984 by both the ISO, as standard ISO 7498, and the renamed CCITT (now called the Telecommunications Standardization Sector of the International Telecommunication Union or ITU-T) as standard X.200.
OSI had two major components, an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols.
The concept of a seven-layer model was provided by the work of Charles Bachman at Honeywell Information Services. Various aspects of OSI design evolved from experiences with the ARPANET, NPLNET, EIN, CYCLADES network and the work in IFIP WG6.1. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system was divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacted directly only with the layer immediately beneath it, and provided facilities for use by the layer above it.
Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly described the functionality provided to an (N)-layer by an (N-1) layer, where N was one of the seven layers of protocols operating in the local host.
The OSI standards documents are available from the ITU-T as the X.200-series of recommendations.[1] Some of the protocol specifications were also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model were available from ISO. Not all are free of charge.[2]
Description of OSI layers
The recommendation X.200 describes seven layers, labeled 1 to 7. Layer 1 is the lowest layer in this model.| OSI Model | ||||
|---|---|---|---|---|
| Layer | Protocol data unit (PDU) | Function[3] | ||
| Host layers |
7. Application | Data | High-level APIs, including resource sharing, remote file access | |
| 6. Presentation | Translation of data between a networking service and an application; including character encoding, data compression and encryption/decryption | |||
| 5. Session | Managing communication sessions, i.e. continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes | |||
| 4. Transport | Segment (TCP) / Datagram (UDP) | Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing | ||
| Media layers |
3. Network | Packet | Structuring and managing a multi-node network, including addressing, routing and traffic control | |
| 2. Data link | Frame | Reliable transmission of data frames between two nodes connected by a physical layer | ||
| 1. Physical | Bit | Transmission and reception of raw bit streams over a physical medium | ||
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