Network and communication concepts

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This information is intended for system administrators, who are unfamiliar with general networking principles. If you’re familiar with UNIX networking, you can skip this information.

A network is the combination of two or more computers and their connecting links. A physical network is the hardware (equipment such as adapters, cables, and telephone lines) that makes up the network. The software and the conceptual model make up the logical network. Different types of networks and emulators provide different functions.

The complexity of modern computer networks results several conceptual models for explaining how networks work. One of the most common of these models is the International Standards Organization's Open Systems Interconnection (OSI) Reference Model, also referred to as the OSI seven-layer model.

The seven layers of the OSI model are described as follows:

Note: While the OSI Reference Model is useful for discussing networking concepts, many networking protocols don’t closely follow the OSI model. For example, in Transmission Control Protocol/Internet Protocol (TCP/IP), the Application and Presentation layer functions are combined, as are the Session and Transport layers and the Data Link and Physical layers.

Networks allow several user and application communication functions, for example:

Send electronic mail (email): You can send a message to another user. The two users can be on the same system, different systems in different buildings, or even in different countries. The underlying layers of software, hardware, and the physical network, allow a user to generate, send, receive, and process messages, letters, memos, invitations, and data files. These communications can be to or from any other user who resides on the physical network.
Emulate another terminal or log in to another computer: Through a communications network, a computer can emulate, or mimic, another and access information as if it were a different type of computer or terminal. Remote login capabilities provide users with an interactive command-line interface to log in to a remote system and access the same programs and files as if they were using the machine locally.
Transfer data: You can transfer data from one system to another. Files, directories, and entire file systems can be migrated from one machine to another across a network, enabling remote backup of data, as well as assuring redundancy if the machine fails. Password protection is usually provided as part of the protocol. Often a file transfer protocol includes functions for display and control so that users with read/write access can display, define, or delete files and directories.
Run programs that reside on a remote node: Several different protocols exist that allow users and applications on one system to invoke procedures and applications on other systems. This can be useful for a number of environments, including the offloading of many computer-intensive routines in engineering and scientific applications.
Data entry: Data entry consists of entering data directly into either local or remote data files. Increased accuracy and efficiency are natural by-products of a one-step data transfer.
Data queries: Data queries entail searching data files for specified information. Data updating involves altering, adding, or deleting data stored in local or remote files.
Remote batch entry: Remote batch entry consists of entering batches of data from a remote location, an activity often performed at night or during periods of low system usage. Because of such diverse capabilities, communications and networks are not only desirable but necessary.
Resource sharing: Sharing resources is another function of networks. Users can share data as well as programs, file-storage space, and peripheral devices like printers, modems, terminals, and hard disks.
Data sharing: Sharing of system resources is cost effective because it eliminates the problems of keeping multiple copies of programs and it keeps data consistent (in the case of program and file sharing).
Communication with other operating systems: Different types of computers can be connected on a network. The computers can be from different manufacturers or be different models from the same manufacturer. Communication programs bridge the differences in operating systems of two or more types of computers. Sometimes these programs require that another program is installed on the network. Other programs might require that such communications connectivity protocols as TCP/IP or Systems Network Architecture (SNA) exist on the network.