To avoid chaos in computer communications, rules must be established for the exchange of data from one site to another. These rules are known as line protocol. Communications software packages control the speed and mode of communications between computer systems.

Many different standard network protocols exist to perform addressing, routing, and packetizing. All provide formal definitions for how addressing and routing is to be executed, and specify packet structures to transfer this information between computers. OSI, TCP/IP, IPX/SPX, and X.25 are commonly used routing protocols.

Open Systems Interconnection (OSI)

A major problem of early networked computer systems was that a lack of consistency existed among the protocols of different types of computers. Consequently, various efforts have resulted in the establishment of standards for data transmission protocols. For example, the International Standards Organization (ISO) developed a set of standard protocols called the Open Systems Interconnection (OSI). The OSI model separates each network's functions into seven layers of protocols, or communication rules. This model identifies functions that should be offered by any network system.

It is important to note that the physical layer, data link layer, and network layer appear in the user and host computers as well as units such as the front-end processor and the cluster control unit. The remaining layers appear only in the user and host computers.

• Physical Layer: The physical layer sends data from the user computer to a host computer (and vice versa). This layer is concerned essentially with computer hardware, whereas the upper layers are interested in communications software. This layer, for example, manages voltage of electricity, timing factors, or connector standards.

• Data Link Layer: The data link layer formats the received data into a record called a frame. This layer also is in charge of error detection.

• Network Layer: The network layer provides the physical layer with the ability to transfer records from one computer to another. This layer provides for the functions of internal network operations such as addressing and routing.

• Transport Layer: The transport layer allows communication to take place between the user and host computers. That is, this layer takes care of end to end validity and integrity of the transmission. OSI transport services takes place through layer 1 to layer 4, which are collectibly responsible for acquiring data and information from the sender to the receiver.

• Session Layer: The session layer is in charge of starting, maintaining, and ending each logical session between the interacting end user computers.

• Presentation Layer: The presentation layer formats incoming data so that it can be presented on the receiving terminal for end users. In other words, it is in charge of displaying, formatting, and editing user inputs and outputs.

• Application Layer: The application layer controls the input from the user computer and allows the application program to be run on the host computer. This layer is the end user's access to the network.