ClockWeb's Computing Section 6

OSI, the layered architecture
By ClockWeb

A Brief Explanation of the OSI Layer

OSI Layer Definition

1. Physical

Concerned with transmission of unstructured bit stream over physical link; involves such parameters as signal voltage swing and bit duration; deals with the mechanical, electrical and procedural characteristics to establish, maintain and de-activate the physical link (RS-232-C, RS-449, X.21).

2. Data Link

Provides for the reliable transfer of data across physical links; sends blocks of data (frames) with the necessary synchronisation, error control and flow control (HDLC, SDLC, BiSync).

3. Network

Provides upper layers with independence from data transmission, and switching technologies used to connect systems; responsible for establishing, maintaining and terminating connections (X.25, layer 3).

4. Transport

Provides reliable, transport transfer of data between end points; provides end-to-end error recovery and flow control.

5. Session

Provides the control structure for communication between applications; establishes, manages and terminates connections (sessions) between cooperating application.

6. Presentation

Performs general useful transformations on data to provide a standardised application interface and to provide common communications services, example: encrytion, text compression and reformatting.

7. Application

Provides services such as transaction server, file transfer protocol (FTP), and network management to the users of the OSI environment.

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OSI Layer	Definition
1. Physical	Concerned with transmission of unstructured bit stream over physical link; involves such parameters as signal voltage swing and bit duration; deals with the mechanical, electrical and procedural characteristics to establish, maintain and de-activate the physical link (RS-232-C, RS-449, X.21).
2. Data Link	Provides for the reliable transfer of data across physical links; sends blocks of data (frames) with the necessary synchronisation, error control and flow control (HDLC, SDLC, BiSync).
3. Network	Provides upper layers with independence from data transmission, and switching technologies used to connect systems; responsible for establishing, maintaining and terminating connections (X.25, layer 3).
4. Transport	Provides reliable, transport transfer of data between end points; provides end-to-end error recovery and flow control.
5. Session	Provides the control structure for communication between applications; establishes, manages and terminates connections (sessions) between cooperating application.
6. Presentation	Performs general useful transformations on data to provide a standardised application interface and to provide common communications services, example: encrytion, text compression and reformatting.
7. Application	Provides services such as transaction server, file transfer protocol (FTP), and network management to the users of the OSI environment.