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Osi Model

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Michael Rauseo IT310

OSI Model

In the early years of computer and network research and development many systems were designed by a number of companies. Although each system had its rights and were sold across the world, it became apparent as network usage grew, that it was difficult, to enable all of these systems to communicate with each other.

In the early 1980s, the International Organization for Standardization (ISO) recognized the need for a network model that would help companies create common network implementations. The OSI reference model, released in 1984, addresses this need. The OSI reference model became the primary architectural model for communications. Although other architectural models have been created, most network vendors relate their network products to the OSI reference model.

The OSI model describes the processes necessary for effective communication in terms of a seven layered model. The seven layers are :

Physical Layer

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors, are defined by physical layer specifications.

Data Link Layer

The data-link layer provides error-free transfer of data frames from one computer to another over the physical layer. The layers above this layer can assume virtually error-free transmission over the network. The data-link layer provides the following functions.

- Establishing and terminating a logical link between two computers identified by their unique network interface card.

- Controlling frame flow by instructing the transmitting computer not to transmit frame buffers

- Sequentially transmitting and receiving frames

- Providing and expecting frame-acknowledgment, and detecting and recovering from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipts

- Managing media access to determine when the computer is permitted to use the physical medium

- Eliminating frames to create and recognize frame boundaries

- Error-checking frames to confirm the integrity of the received frame

- Inspecting the destination address of each received frame and determining if the frame should be directed to the layer above

Network Layer

The network layer controls the operation of the subnet. It determines which physical path the data takes, based on the network conditions, the priority of service, and other factors.

The network layer provides the following functions.

- Transferring the frame to a router if the network address of the destination does not indicate the network to which the station is attached

- Controlling subnet traffic to allow an intermediate system to instruct a sending station not to transmit its frame when the router's buffer fills up. If the router is busy, the network layer can instruct the sending station to use an alternate destination station.

- Resolving the logical computer address with the physical network interface card address.

- Keeping an accounting record of frames forwarded to produce billing information

Transport Layer

The transport layer makes sure that messages are delivered in the order in which they were sent and that there is no loss or duplication. It removes the concern from the higher layer protocols about data transfer between the higher layer and its peers. The size and complexity of a transport protocol depends on the type of service it can get from the network layer or data link layer. For a reliable network layer a minimal transport layer is required. Functions of the transport layer include the following.

- Accepting messages from the layer above and, if necessary, splitting them into frames

- Providing reliable, end-to-end message delivery with acknowledgments

- Instructing the transmitting computer not to transmit when no receive buffers are available

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