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.
Read more about this topic: OSI Model
Famous quotes containing the words description of, description and/or layers:
“As they are not seen on their way down the streams, it is thought by fishermen that they never return, but waste away and die, clinging to rocks and stumps of trees for an indefinite period; a tragic feature in the scenery of the river bottoms worthy to be remembered with Shakespeare’s description of the sea-floor.”
—Henry David Thoreau (1817–1862)
“It is possible—indeed possible even according to the old conception of logic—to give in advance a description of all ‘true’ logical propositions. Hence there can never be surprises in logic.”
—Ludwig Wittgenstein (1889–1951)
“I think it’s the real world. The people we’re writing about in professional sports, they’re suffering and living and dying and loving and trying to make their way through life just as the brick layers and politicians are.”
—Walter Wellesley (Red)