What Is OSI?

During the past two decades there has been a tremendous increase in the numbers and sizes of networks. Many of the networks, however, were built using different implementations of hardware and software. As a result, many of the networks were incompatible and it became difficult for networks using different specifications to communicate with each other. To address this problem, the International Organization for Standardization (ISO) researched many network schemes. The ISO recognized that there was a need to create a network model that would help network builders implement networks that could communicate and work together (interoperability) and therefore, released the OSI reference model in 1984.

Enterprise networks are designed and built to support current and future applications. To accommodate increasing requirements for bandwidth, scalability, and reliability, vendors and standards bodies introduce new protocols and technologies at a rapid rate. Network designers are challenged to develop state-of-the-art networks even though what is considered state-of-the-art changes on a monthly, if not weekly basis.

By dividing and organizing the networking tasks into separate layers/functions, new applications can be handled without problems. The OSI reference model organizes network functions into seven categories, called layers. Data flows from upper-level user applications to lower-level bits that are then transmitted through network media. The task of most wide area network managers is to configure the three lowest layers. Peer-to-peer functions use encapsulation and de-encapsulation as the interface for the layers.

There are seven layers in the OSI reference model, each of which has separate distinct functions. The Transmission Control Protocol/Internet Protocol (TCP/IP) models' functions fit into five layers. This separation of networking functions is called layering.

Each layer uses its own layer protocol to communicate with its peer layer in another system. Each layer's protocol exchanges information, called protocol data units (PDUs), with its peer layers. A layer can use a more specific name for its PDU. For example, in TCP/IP the transport layer of TCP communicates with the peer TCP function by using segments. Each layer uses the services of the layer below it in order to communicate with its peer layer. The lower layer service uses upper layer information as part of the PDUs that it exchanges with its peer. 

The TCP segments become part of the network layer packets (datagrams) that are exchanged between IP peers. In turn, the IP packets become part of the data link frames that are exchanged between directly-connected devices. Ultimately, these frames become bits, as the data is finally transmitted by the hardware that is used by the physical layer protocol.

Each layer depends on the services of the OSI reference model layer that is below it. In order to provide this service, the lower layer uses encapsulation to put the protocol data unit (PDU) from the upper layer into its data field, then it can add whatever headers and trailers the layer wishes to use to perform its function.

As an example, the network layer provides a service to the transport layer, and the transport layer presents data to the internetwork subsystem. The network layer has the task of moving that data through the internetwork. It accomplishes this task by encapsulating the data within a packet.

This packet includes a header containing information that is necessary to complete the transfer, such as source and destination logical addresses.

The data link layer in turn provides a service to the network layer. It encapsulates the network layer packet in a frame. The frame header contains information that is necessary to complete the data link functions (e.g. physical addresses).

As networks perform services for users, the flow and packaging of the user's original information go through several changes.

A computer converts an e-mail message into alphanumeric characters that can be used by the internetworking system. 

The message data is then segmented for transport on the internetwork system by the transport layer. The transport layer ensures that the message hosts at both ends of the e-mail system can reliably communicate.

The data is then converted to a packet, or datagram, by the network layer. The packet also contains a network header that includes a source and destination logical address. The address helps network devices send the packet across the network along a chosen path.

Each data-link layer device puts the packet into a frame. The frame enables the device to connect to the next directly-connected network device on the link.

The frame is changed to a pattern of 1s and 0s for transmission on the medium (usually a wire). A clocking function enables the devices to distinguish bits as they travel across the medium.
The medium on the physical internetwork can vary along the path. For example, an e-mail message may originate on a LAN, cross a campus backbone, and continue through a WAN link until it reaches its destination on another remote LAN.

The OSI Model