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 evenimpressive working knowledge of PCs, operating systems, applications,and common problems and solutions. Many of these people are wizards withdesktop computers.
Nội dung trích xuất từ tài liệu:
Networking: A Beginner’s Guide Fifth Edition- P1132 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 receiving computer, this process is reversed, with each layer unwrapping its appropriate control information, performing whatever work is indicated by that control information and passing the data on to the next higher layer. It all sounds rather complex, but it works very well in practice. Learning About Network Hardware Components This chapter is really about understanding networks, with a “view from 30,000 feet.” An overview of the hardware that enables networks to operate completes this discussion. Understanding the general types of devices you typically encounter in a network is important, not only for planning a network, but also for troubleshooting and maintenance. Chapter 3: Understanding Networking 33Servers A server is any computer that performs network functions for other computers. These functions fall into several categories, including the following: File and print servers, which provide file sharing and services to share network-based printers. Application servers, which provide specific application services to an application. An example is a server that runs a database that a distributed application uses. E-mail servers, which provide e-mail storage and interconnection services to client computers. Networking servers, which can provide a host of different network services. Examples of these services include the automatic assignment of TCP/IP addresses (DHCP servers), routing of packets from one network to another (routing servers), encryption/decryption and other security services, and VPN access. Internet servers, which provide Web, Usenet News (NNTP), and Internet e-mail services. Remote access servers, which provide access to a local network for remote users. As noted earlier, servers typically run some sort of network operating system, such as Windows Server 2008, Linux, or UNIX. Depending on the operating system chosen, the functions previously listed might all be performed on one server or distributed to many servers. Also, not all networks need all the services previously listed. NOTE Server computers can be nearly any type of computer, but today they are usually high-end Intel-based PCs. You might also see certain types of servers that use a different platform. For instance, many dedicated web s ...