The Open System Interconnection (OSI) Architecture
This section is divided into two parts: an introduction and a slideshow.
- For an introduction to the OSI Architecture, continue reading this document.
- To go directly to the slideshow, click here.
Motivation
When communication is desired among computers from different vendors, the software development effort can be very difficult. Different vendors use
different data formats and data exchange protocols that do not allow computers
to communicate with one another. Recognizing the problem, the International
Organization for Standardization (ISO) develops a communications architecture
known as the Open System Interconnection (OSI) model that defines standards
for linking heterogeneous computers.
The Model
The OSI model consists of seven layers. The communication functions are
broken down into a hierarchical set of layers. Each layer performs a
related subset of the functions required to communicate with another system.
It relies on the next lower layer to perform more primitive functions and to
conceal the details of those functions. It provides services to the next
higher layer. The layers are defined in such a manner so that changes in one
layer do not require changes in the other layers. By partitioning the
communcation functions into layers, the problem at hand is much more
manageable.
The OSI Layers
- Physical
- Concerned with transmission of unstructured bit stream over the physical link. It invokes such parameters as signal voltage swing and bit duration. It deals with the mechanical, electrical, procedural characteristics to establish, maintain and deactivate the physical link.
- Data link
- Provides for the reliable transfer of data across the physical link. It sends blocks of data (frames) with the necessary synchronization, error control and flow control.
- Network
- Provides upper layers with independence from the data transmission and switching technologies used to connect systems. It is responsible for establishing, maintaining and terminating connections.
- Transport
- Provides reliable, transparent transfer of data between end points. It provides end-to-end error recovery and flow control.
- Session
- Provides the control structure for communication between applications. It establishes, manages and terminates connections (sessions) between cooperating applications.
- Presentation
- Performs generally useful transformations on data to provide a standardized application interface and to provide common communications services. It provides services such as encryption, text compression and reformatting.
- Application
- Provides services to the users of the OSI environment. It provides services for FTP, transaction server, network management, etc.
