Open System Interconnection (OSI)

In the network system, protocols are a set of instructions to exchange the information among two devices. A protocol specifies the message format & the rules for interpreting & reacting to messages.

The open system interconnection (OSI) reference model proposed by the International Standards Organization (ISO) incorporates a framework for modelling communication protocols. This particular model encompasses interoperability among dissimilar systems.  The entire communication process is divided into various layers by ISO/OSI as illustrated in Figure; the physical layer, the data layer, the network layer, the transport layer, the session layer, the presentation layer and the application layer.

The lower four layers are responsible for the transfer of information among applications. The upper three layers refer to support the applications. Standards are set for each of layer to handle communication from one device to another. A short discussion of the functions of these layers is as follows.

The Physical Layer

It has hardware that drives the network & circuits and specifies the type of cable that is utilized. It deals with, electrical, functional, mechanical & procedural characteristics to access the physical bits and is passed from one device to another.

The Data Link Layer

This performs the task of transferring information across the physical link through sending blocks of data (frames) with necessary synchronization, error control, & flow control functions. It is utilized to improve the error frequency for messages moved among adjacent nodes. The data integrity transmitted among two devices in the similar network is ensured on this layer. The ISO HDLC (high-level data-link control) standard is utilized in this layer. The HDLC standard starts & terminates along an 8-bit flag frame.

The Network Layer

 It uses to choose the outgoing line to send the message to a node. As it knows the physical connections and paths among the transport entities in a session, this relieves the transport layer of the have to know anything regarding the underlying network technologies utilized to connect end systems.

The Transport Layer

This ensures the transparent transfer of packets (data), to and from the session layer without disruption.  This layer contains three major functions; to establish an appropriate connection and quality (speed), to start the data transfer & manage the data to be sent, and to release the connection.

The Session Layer

It controls the communication among applications by managing, establishing and terminating virtual connections among cooperating applications.

 The Presentation Layer

It decodes the data to match the device & requirements. It makes the application procedure independent of differences in data representation.

The Application Layer

It provides the user interfaces to the networking system. The services provided by this layer include file transfer, terminal emulation, electronic mail, and distributed database managers.

Depend on this OSI reference model, a number of protocols have been built up. Examples include MAP/TOP, SNA from IBM, TCP/IP, DECNET from DEC and various others. A brief discussion of the above mentioned protocols, MAP/TOP & TCP/IP has been given in following two sections.