As organizations grow and develop, and their information management needs to expand accordingly, it is no uncommon to reach for the quickest solution. As a result, "islands of information" sprout up, with departments unable to communicate or share information with one another. Proprietary solutions are
in abundance --computers, terminals, workstations, databases, and application packages--- but outside of their own work groups, users can't share information, or communicate electronically.
The next frontier in the development of information technology is the ability to share information and applications across the organization. The Open System Interconnection mode, or OSI, has the potential of creating bridges between the islands of information through connectivity international standards In its purest form, OSI is basically a set of technical standards (pages, and pages of technical standards) that provide a basis for data communications, such that computers from a multitude of different manufactures can share data, and even applications with one another. The OSI model does no imply that all the hardware vendors get together, share their proprietary secrets, and come up with one master computer. The OSI model provides standards that simply permit communication between different architectures so that, for example, an electronic mail message can be sent from user to user on an IBM machine to a user on a DEC or NCR machine. OSI allows users in an organization to communicate and share data as necessary, while the organization's hardware and software investments are both protected and maximized.
One of the unique aspects of the OSI model is its organization. The standards are arranged into seven layers, each representing one of the major functions required to effectively send data through a network. The layers are organized as a stack, so that, for example, a message going through a network would pass through the stack of seven layers. There are specifications in each layer, thus, a vendor who is implementing OSI in the product development process must select specifications from each level (Though not all the specifications at each level may be necessary) if the product is to fully conform to OSI standards. The relationship between the seven layers is in sense hierarchical. In transferring a message, for example, certain actions are expected before, and after, the message passes a specific layer.
The focus of the physical layer is on the actual physical attachment to the network, for example, the means of connecting two nodes (or links) in the network. Cables, modems, and other connections are regulated at this level. This layer is concerned with transmitting row bits over a communications channel. Its purpose is to provide a physical connection for the transmission of data among network entities and a means by which to activate and deactivate a physical connection.
The data link layer focuses on the control of communication between the two nodes, checking for errors and "packaging" the message for the rest of the trip through the network. The primary purpose of the data link layer is to provide a reliable means of transmitting data across a physical link. It breaks the input data up into data frames, transmits the frames sequentially, and processes the acknowledge frames sent back by the receiver. The data link layer is divided into two sublayers in the IEEE802 LAN standards project, namely, the Media Access Control (MAC) sublayer and the Logical Link control
At the network layer, a path is set up for the trip through the network, and the communication is routed to its destination, for example, from one node in the network to another. The network layer routes information from one network computer to another. These computers may be physically located on the same network or on another network that is connected is some fashion. Like a street map, the network layer defines all the possible avenues that a message could travel to get from one network to another.
The first three layers thus serve to package the communication and get if off to wherever its destination is in the network. From here, it passes through the fourth layer, called the transport layer, which essentially acts as buffer between the beginning point of the trip and the destination. Transport layer handles the control of the flow of data in the network, making sure that the data is not lost and that the destination is not inundated with incoming data. The purpose of the transport layer is to provide a network independent transport service to the session layer. The basic function of the transport layer is to accept data from the session layer, split it up into smallest units, if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end.
The top three layers, the services layers, focus on the actual destination of the communication and operate in reverse of the bottom three layers. It is in these layers that the communication is recompiled as necessary and passed on for processing of storage. As the communication reaches the session layer, it is passed to its appropriate destination, and the communication session is ended. The session layer is the user's interface into the network. It is with this layer that the user must negotiate to establish a connection with a process on other machine. Once the connection has been established, the session layer can manage the dialogue in an orderly manner, if the user has requested that service.
At the presentation layer, the communication, now that it has arrived, is translated into the appropriate syntax, so that the recipient can understand it, if the communication being sent is in the form of data, for example, it might be translated into a machine-readable language, appropriate to the destination, or even into a foreign language, understandable to a human. The presentation layer provides for the negotiation and establishment of the transfer syntax, which represents the encoding of values for the purposes of transferring structured data types. A data type may have more than one possible transfer syntax, providing greater or lesser degrees of compression or security.
The application layer, as the mane implies, focuses on specific applications unique to the organization. At this layer, communications are matched with specific user-applications to which they are appropriated, such as electronic mail, or an accounting package. At the top of the OSI model, the application layers exhibits some differences for the other layers. Specifically, the application layer does not provide services to a higher layer. The application layer does not provide services, but these services are provided to application processes that lie outside the seven layer architecture. They can be manual computerized, or physical. For example:
1. A person operating a banking terminal is a manual application process.
2. A COBOL program executing in a computer center and accessing a remote data base is a computerized application process. The remote database management systems server is also an application process.
3. A process control program executing in a dedicated computer attached to some industrial equipment and linked into a plant control system is a physical process.
Application processes in different open systems that wish to exchange information do so by accessing the application layer. The application layer contains application entities that employ application protocols and presentation services to exchange information. It is these application entities that provide the means for application processes to access the OSI environment.