OSI Model - Description of OSI Layers

Description of OSI Layers

According to recommendation X.200, there are seven layers, labeled 1 to 7, with layer 1 at the bottom. Each layer is generically known as an N layer. An "N+1 entity" (at layer N+1) requests services from an "N entity" (at layer N).

At each level, two entities (N-entity peers) interact by means of the N protocol by transmitting protocol data units (PDU).

A Service Data Unit (SDU) is a specific unit of data that has been passed down from an OSI layer to a lower layer, and which the lower layer has not yet encapsulated into a protocol data unit (PDU). An SDU is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user.

The PDU at a layer N is the SDU of layer N-1. In effect the SDU is the 'payload' of a given PDU. That is, the process of changing an SDU to a PDU, consists of an encapsulation process, performed by the lower layer. All the data contained in the SDU becomes encapsulated within the PDU. The layer N-1 adds headers or footers, or both, to the SDU, transforming it into a PDU of layer N. The added headers or footers are part of the process used to make it possible to get data from a source to a destination.

OSI Model
Data unit Layer Function
Host
layers
Data 7. Application
6. Presentation
5. Session
Segments 4. Transport
Media
layers
Packet/Datagram 3. Network
Frame 2. Data link
Bit 1. Physical

Some orthogonal aspects, such as management and security, involve every layer.

Security services are not related to a specific layer: they can be related by a number of layers, as defined by ITU-T X.800 Recommendation.

These services are aimed to improve the CIA triad (confidentiality, integrity, and availability) of transmitted data. Actually the availability of communication service is determined by network design and/or network management protocols. Appropriate choices for these are needed to protect against denial of service.

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