Data Link Layer Nic Addresses example essay topic

Michael Rau seo IT 310 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 1980's, 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 sub net. 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 sub net 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 - Multiplexing several process-to-process message streams or sessions onto one logical link and keeping track of which messages belong to which sessions Session Layer The session layer establishes a communications session between processes running on different computers, and can support message-mode data transfer. Functions of the session layer include: - Allowing application processes to register unique process addresses.

It provides the means by which these process addresses can be resolved to the network-layer or data-link-layer NIC addresses, if necessary. - Establishing, monitoring, and terminating a virtual-circuit session between two processes identified by their unique process addresses. A virtual-circuit session is a direct link that seems to exist between the sender and receiver o Delimiting messages, to add header information that indicates where a message starts and ends. The receiving session layer can then refrain from indicating any message data to the overlying application until the entire message has been received.

- Informing the receiving application when buffer space is insufficient for the entire message and that the message is incomplete. The receiving session layer may also use a control frame to inform the sending session layer how many bytes of the message have been successfully received. The sending session layer can then resume sending data at the byte following the last byte acknowledged as received. When the application provides another buffer, the session layer can place the remainder of the message in that buffer and indicate to the application that the entire message has been received.

Presentation Layer The presentation layer ensures that information sent by the application layer of one system will be readable by the application layer of another system. If necessary, the presentation layer translates between multiple data representation formats by using a common data representation format. The presentation layer concerns itself not only with the format and representation of actual user data, but also with data structures used by programs. In addition to actual data format transformation, the presentation layer negotiates data transfer syntax for the application layer. Application Layer The application layer is the OSI layer closest to the user.

It differs from the other layers because it does not provide services to any other OSI layer, but rather to application processes lying outside the scope of the OSI model. Examples include spreadsheet programs, word-processing programs, banking terminal programs, etc. The application layer identifies and establishes the availability of intended communication partners, synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity. Also, the application layer determines whether sufficient resources for the intended communication exist. .