Different types of networks and emulators provide different functions. To assist you in understanding the basic concepts of networking and emulation, this section consists of the following information:
A network is the combination of two or more computers and the connecting links between them. A physical network is the hardware (equipment such as computers, cables, and telephone lines) that makes up the network. The software and other controlling devices and conventions make up the logical network.
All network communications involve use of hardware and software. Hardware consists of the physical equipment connected to the physical network. Software consists of the programs, procedures, rules or protocols, and associated documentation pertaining to the operation of a particular system. The system hardware and software communications support is determined by the hardware being used and the software necessary to run that hardware and interface with the network.
The system hardware consists of adapter cards that provide a path or interface between the system software and the physical network. An adapter card requires an input/output (I/O) card slot in the system. The adapter card connects the data terminal equipment (DTE) to the data circuit-terminating equipment (DCE); that is, it provides physical local addressing to a DTE port.
An adapter card prepares all inbound and outbound data; performs address searches; provides drivers, receivers, and surge protection; supports different interfaces; and in general relieves the system processor of many communications tasks. Adapter cards support the standards required by the physical network (for example, EIA 232D, Smartmodem, V.25 bis, EIA 422A, X.21, or V.35) and at the same time support software protocols (for example, synchronous data link control or SDLC, high-level data link control or HDLC, and bisynchronous protocols).
Networks allow for several user and application communications functions. They enable a user to:
The communications network enables one user to communicate with another user through electronic mail. The underlying layers of software and hardware, as well as 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. Electronic mail has the capability for message annotation, message sequencing, message packing, date sorting, and mail folder management.
Emulating another computer permits users to access applications in other systems as if directly attached to that system. Remote login allows remote login between a system based on this operating system and other UNIX-based systems.
A file transfer protocol allows users to access files and directories on remote hosts. Files may be transferred to and from the remote host. Password protection is usually provided as part of the protocol. With a file transfer, there is a client/server relationship between the user initiating the request and the remote system the user is accessing. 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.
Several different protocols have been devised to allow users and applications on one system to invoke procedures and execute 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.
Different types of computers can be connected on a network. That is, the computers can be from different manufacturers or be different models from the same manufacturer. The differences in operating systems of two or more types of computers can be bridged with programs.
Sometimes these programs require that another program be previously installed on the network. Other programs may require that such communications connectivity protocols as Transmission Control Protocol/Internet Protocol (TCP/IP) or Systems Network Architecture (SNA) exist on the network.