Summary

42 IT Essentials: PC Harware and Software Companion GuideDefine the OSI ModelThe OSI model is an industry-standard framework that is used to divide network communicationsinto seven distinct layers. Although other models exist, most network vendors todaybuild their products using this framework.A system that implements protocol behavior consisting of a series of these layers is knownas a protocol stack. Protocol stacks can be implemented in either hardware or software, or acombination of both. Typically, only the lower layers are implemented in hardware, and thehigher layers are implemented in software.Each layer is responsible for part of the processing to prepare data for transmission on thenetwork. Table 8-7 describes each layer of the OSI model.Table 8-7OSI ModelOSI Model Layer DescriptionApplication 7 Responsible for network services to applicationsPresentation 6 Transforms data formats to provide a standard interface forthe application layerSession 5 Establishes, manages, and terminates the connectionsbetween the local and remote applicationsTransport 4 Provides reliable transport and flow control across a networkNetwork 3 Responsible for logical addressing and the domain of routingData link 2 Provides physical addressing and media access proceduresPhysical 1 Defines all the electrical and physical specifications fordevicesIn the OSI model, when data is transferred, it is said to virtually travel down the OSI modellayers of the sending computer and up the OSI model layers of the receiving computer.When a user wants to send data, such as an e-mail, the encapsulation process starts at theapplication layer. The application layer provides network access to applications.Information flows through the top three layers and is considered to be data when it getsdown to the transport layer.At the transport layer, the data is broken into more manageable segments, or transport layerprotocol data units (PDU), for orderly transport across the network. A PDU describes dataas it moves from one layer of the OSI model to another. The transport layer PDU also containsinformation such as port numbers, sequence numbers, and acknowledgment numbers,which is used for reliable data transport.