OSI Model This memorandum will attempt to explain the Open Systems Interconnection Model, known more simply as the OSI Model. The OSI Model has seven levels, and these levels will be discussed in detail. Particular mention will be made to which level TCP/IP functions with the OSI Model. The seven levels of the OSI Model are as follows: 7) Application: Provides different services to the applications. 6) Presentation: Converts the information. 5) Session: Handles problems which are not communication issues.

4) Transport: Provides end to end communication control. 3) Network: Routes the information in the network. 2) Data Link: Provides error control between adjacent nodes. 1) Physical: Connects the entity to the transmission media. In simple terms, the OSI model defines a networking framework for implementing protocols.

Is defines seven levels to accomplish this. Control is passed from one level to the next, starting at the Application level (Level Seven) and working its way through the levels until it reaches and completes Level One, the Physical level. Once this cycle has been completed, control moves to the next station on the network and back up the hierarchy. Since the process begins at the seventh level, the Application level, it will be detailed first. As its name implies, this Application level supports applications, but it also supports other end-user processes.

User authentication and privacy on the network is considered at this level. This level also provides application services for file transfers (such as FTP and Telnet) and e-mail. Once the Application level has been completed, control is passed to the sixth level, or Presentation level. What occurs at this level involves transforming the data into a form that the application layer can accept.

Encrypted data is handled at this level. This layer formats and encrypts data to be sent across a network. Sometimes this level is known as the syntax level. Control is then passed to the Session level, or level five.

This level, as the name implies, deals with the different network sessions. This level establishes, manages and terminates connections between the different applications on the network. It also sets-up, coordinates and terminates exchanges and dialogue between the applications on either end. The Transport level assumes control at this point. This level provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. The Transport level ensures complete data transfer.

TCP, used on the Internet, is a Transport protocol. Next in line is the Network level. This level provides switching and routing technologies and creates what's known as virtual circuits. These virtual circuits are used for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internet working, error handling, congestion control and packet sequencing.

The Data Link level is next. At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub-layers: the Media Access Control (MAC) and the Logical Link Control (LLC) layer. The MAC sub-layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Finally, the bottom (first) level is reached. This first level, the Physical level, conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware a means of sending and receiving data, including defining cables, video / sound cards and other physical aspects. Ethernet is a protocol with Physical level components. The previously described OSI Model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI Model is a conceptual representation composed of seven levels, each specifying a particular network function and is now considered the primary architectural model..