3.1 The OSI Reference Model

The Open Systems Interconnection reference model (OSI reference model or OSI model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection (OSI) initiative. It is also called the OSI seven layer model. The layers, described below and shown in Figure 2 are, from bottom to top: Physical, Data Link, Network, Transport, Session Presentation and Application layer. A layer is a collection of related functions that provides services to the layer above it and receives service from the layer below it. 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 is much more manageable.

OSI model layers
Figure 2: OSI model layers

Physical layer – 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 layer – Provides reliable transfer of data across the physical link. It sends blocks of data (frames) with the necessary synchronization, error control and flow control.

Network layer – 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 layer – Provides reliable, transparent transfer of data between end points. It provides end-to-end error recovery and flow control.

Session layer – Provides the control structure for communication between applications. It establishes, manages and terminates connections (sessions) between cooperating applications.

Presentation layer – Performs 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 layer – Provides services to the users, FTP, HTTP, TELNET, etc.

Reference Models Reference Models The TCP/IP Reference Model