The OSI Model in Software

Create Code39 in Software The OSI Model

The OSI model describes how information in one computer moves through a network to another computer It defines networking in terms of seven layers, each of which performs a particular set of functions within the networking task, breaking the networking task into seven smaller tasks Each layer can communicate with the layers above and below it and with its opposite layer in another networked computer, as shown in Figure 5-3 The communication between computers goes down through the lower layers, across the physical connection, and back up the layers in the other computer In the OSI model, the lower two layers are implemented in a combination of hardware and software, while the upper five layers are all implemented in software The software in the lower layers is dedicated to networking and may be burned into silicon The software in upper layers is a part of the operating system or an application for which networking is only one of its tasks For example, an e-mail client application wishing to get its mail off the server passes its request to the Application layer of the network system in its computer This information is passed down through the layers until it reaches the Physical layer, where the information is placed on the physical network The Physical layer in the server takes the information off the physical network and passes it up through the layers until it reaches the Application layer, which passes the request to the e-mail server application Each of the OSI layers can add, if necessary, specific control information to information being transferred over the network The control information can be for other layers in its own stack or for its opposite layer in the other computer and can be either requests or instructions The control information can be added either as a header at the beginning of information being transferred or as a trailer at the end The header or trailer added at

one layer becomes part of the basic information being transferred at another layer The next layer may add another header or trailer or, if an existing header or trailer is intended for that layer, remove the header or trailer The following lists the type of control information and function defined for each layer: Physical layer Defines the physical specifications, such as voltages and timing, to make the network interface function as intended These specifications are implemented in networking hardware Data Link layer Defines the addressing of frames and the sending and receiving of frames between two linked computers After passing a frame to the

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Physical layer to be sent, the Data Link layer waits for acknowledgment that the frame has been received before it sends another frame; if the acknowledgment is not received, the Data Link layer resends the first frame The Data Link layer provides point-to-point linkage between itself and the receiving computer by using physical addresses The physical address at the Data Link layer is called the media access control (MAC) address The Data Link specifications are implemented in a combination of hardware and dedicated networking software Network layer Defines the packaging of packets into frames and the logical addressing (as opposed to the physical addressing used in the Data Link layer) necessary to provide internetwork routing through multiple, connected networks Packets, which may be larger or smaller than frames, are broken up or combined to create a frame in the sending computer, and are reassembled or disassembled in the receiving computer to reproduce the original packets The Network layer s internetworking commonly uses the Internet Protocol (IP) addressing to identify where frames should be sent The Network layer specifications are implemented in dedicated networking software Transport layer Defines the division of a message into packets, the identification of the packets, and the control of the packet transmission to know whether the packets are being sent and received correctly, and if not, to pause and resend a transmission The Transport layer creates, regulates, and terminates a flow of packets by using a virtual circuit between the sending and receiving computers (the flow is still down through the other layers, across the connection, and up the other side, but it occurs as if the two Transport layers were directly talking to each other) The Transport layer commonly uses the Transmission Control Protocol (TCP) to start, regulate, and terminate the flow of packets The Transport layer specifications are implemented in networking-related operating system (OS) software, such as Windows Server 2008, using networking protocols Session layer Defines the dialog between computers so that they know both when to start and stop transmission, creating a session, and when to repeat a session if it is not correctly received The Session layer also handles security-related issues and has its roots in the mainframe/terminal timesharing environment The Session layer specifications are implemented in network-related OS software Presentation layer Defines the encoding of information so that it is easily and securely transmitted and read by the receiving computer This includes the conversion of character, graphic, audio, and video information into common data representation, the encryption and compression of information, and the return of the information to its native form upon receipt The Presentation layer specifications are implemented in OS software Application layer Defines the mechanism by which applications access the network to send and receive information This includes the two-way handling of information, as well as the identifying, locating, and determining of the availability of the partner for an information exchange The Application layer specifications are implemented in OS and application software