tailieunhanh - Networking: A Beginner’s Guide Fifth Edition- P11

Networking: A Beginner’s Guide Fifth Edition- P11:I have run into many people over the years who have gained good even impressive working knowledge of PCs, operating systems, applications, and common problems and solutions. Many of these people are wizards with desktop computers. | 32 Networking A Beginner s Guide Application Layer The application layer layer 7 controls how the operating system and its applications interact with the network. The applications you use such as Microsoft Word or Lotus 1-2-3 are not a part of the application layer but they certainly benefit from the work that goes on there. An example of software at the application layer is the network client software you use such as the Windows Client for Microsoft Networks or the Windows Client for Novell Networks. It also controls how the operating system and applications interact with those clients. Understanding How Data Travels Through the OSI Layers As mentioned earlier in this section data flows from an application program or the operating system and then goes down through the protocols and devices that make up the seven layers of the OSI model one by one until the data arrives at the physical layer and is transmitted over the network connection. The computer at the receiving end reverses this process The data comes in at the physical layer travels up through all the layers until it emerges from the application layer and is made use of by the operating system and any application programs. At each stage of the OSI model the data is wrapped with new control information related to the work done at that particular layer leaving the previous layers information intact and wrapped within the new control information. This control information is different for each layer but it includes headers trailers preambles and postambles. For example when data goes into the networking software and components making up the OSI model it starts at the application layer and includes an application header and application data the actual data being sent . Next at the presentation layer a presentation header is wrapped around the data and it is passed to the component at the session layer where a session header is wrapped around all of the data and so on until it reaches the physical layer. At the .