OSI (Open Systems Interconnection)

The OSI Reference Model
In 1983, the International Standards Organization (ISO) created the OSI, or X.200, model. It is a multilayered model for facilitating the transfer of information on a network. The OSI model is made up of seven layers, with each layer providing a distinct network service. By segmenting the tasks that each layer performs, it is possible to change one of the layers with little or no impact on the others. For example, you can now change your network configuration without having to change your application or your presentation layer.

The OSI model was specifically made for connecting open systems. These systems are designed to be open for communication with almost any other system. The model was made to break down each functional layer so that overall design complexity could be lessened. The model was constructed with several precepts in mind:

1) Each layer performs a separate function;
2) The model and its levels should be internationally portable; and
3) The number of layers should be architecturally needed, but not unwieldy.

Each layer of the model has a distinct function and purpose:

Application layer--Provides a means for the user to access information on the network through an application. This layer is the main interface for the user to interact with the application and therefore the network. Examples include file transfer (FTP), DNS, the virtual terminal (Telnet), and electronic mail (SMTP).

Presentation layer--Manages the presentation of the information in an ordered and meaningful manner. This layer's primary function is the syntax and semantics of the data transmission. It converts local host computer data representations into a standard network format for transmission on the network. On the receiving side, it changes the network format into the appropriate host computer's format so that data can be utilized independent of the host computer. ASCII and EBCDIC conversions, cryptography, and the like are handled here.

Session layer--Coordinates dialogue/session/connection between devices over the network. This layer manages communications between connected sessions. Examples of this layer are token management (the session layer manages who has the token) and network time synchronization.

Transport layer--Responsible for the reliable transmission of data and service specification between hosts. The major responsibility of this layer is data integrity--that data transmitted between hosts is reliable and timely. Upper layer datagrams are broken down into network-sized datagrams if needed and then implemented using the appropriate transmission control. The transport layer creates one or more than one network connection, depending on conditions. This layer also handles what type of connection will be created. Two major transport protocols are the TCP (Transmission Control Protocol) and the UDP (User Datagram Protocol

Network layer--Responsible for the routing of data (packets) to a system on the network; handles the addressing and delivery of data. This layer provides for congestion control, accounting information for the network, routing, addressing, and several other functions. ). IP (Internet Protocol) is a good example of a network layer interface.

Data link layer--Provides for the reliable delivery of data across a physical network. This layer guarantees that the information has been delivered, but not that it has been routed or accepted. This layer deals with issues such as flow regulation, error detection and control, and frames. This layer has the important task of creating and managing what frames are sent out on the network. The network data frame, or packet, is made up of checksum, source address, destination address, and the data itself. The largest packet size that can be sent defines the maximum transmission unit (MTU).

Physical layer--Handles the bit-level electrical/light communication across the network channel. The major concern at this level is what physical access method to use. The physical layer deals with four very important characteristics of the network: mechanical, electrical, functional, and procedural. It also defines the hardware characteristics needed to transmit the data (voltage/current levels, signal strength, connector, and media). Basically, this layer ensures that a bit sent on one side of the network is received correctly on the other side.

Data travels from the application layer of the sender, down through the levels, across the nodes of the network service, and up through the levels of the receiver. Not all of the levels for all types of data are needed--certain transmissions might not be valid at a certain level of the model.

To keep track of the transmission, each layer "wraps" the preceding layer's data and header with its own header. A small chunk of data will be transmitted with multiple layers attached to it. On the receiving end, each layer strips off the header that corresponds to its respective level.

The OSI model should be used as a guide for how data is transmitted over the network. It is an abstract representation of the data pathway and should be treated as such.