Communications networks typically include a plurality of data terminal equipment (DTE), also referred to as terminals or stations. A computer connected to a network is considered to be a terminal. Terminals communicate with each other through a transmission medium which may, for example, be a coaxial cable in the case of Ethernet networks (which will be understood to include Cheapernet networks in the discussion below).
The various terminals of a network send information messages and receive those sent by other terminals. An information message is composed of a set of elementary information blocks, called frames, that contain a certain quantity of binary information. Each frame is structured and contains data which define its beginning and its end, the address of the terminal for which the message is intended, the address of the send terminal, the data length, and other useful data.
A local area network (LAN) is a network limited to a geographic location within a restricted area, the distances between the various stations being on the order of a few meters to tens of meters, even reaching a few kilometers. LANs of the CSMA/CD type are frequently used in current practice. They are standardized by the Institute of Electrical and Electronic Engineers (IEEE) in accordance with a standard known as 802.3 (adopted by the International Standards Organization (ISO), as ISO Standard 8 802.3).
This standard defines a number of provisions that establish the communication mode between the various terminals as well as the frame format and the protocol governing dialogs between the various stations. A protocol defines the access rules to the various stations.
Ethernet networks have a data transmission speed of 10 Mbits/s and their standardized transmission medium is a coaxial cable with a characteristic impedance of 50 ohms.
It is known that a computer is composed of one or more central processing units, input/output processors, random access memories, and read only memories associated with all these processors, input/output controllers, as well as several peripherals such as disk memories or input/output peripherals allowing exchange of data with users (screen terminals, printers, etc.), these peripherals being associated with respective peripheral controllers.
All the component parts listed above (aside from the peripherals) are disposed on a set of boards whose dimensions are standardized. These boards are generally connected to a parallel bus which provides communications between the various processors and data transport between the various boards, as well as providing electrical power.
One bus commonly used in present-day practice is called MULTIBUS II, a registered trademark of the INTEL COMPANY. The architecture of such a bus is structured around a principal bus of the parallel type standardized according to IEEE Standard 1296.
The rapid technological evolution of networks and the increasing numbers of terminals in use are leading to the development within the computers themselves of programmed communications servers whose role is to reduce the load on the computer's central processing unit by performing some of the management of the messages sent by the various component elements of the computer to the telecommunications network to which it is connected, as well as the messages coming from other terminals on the network. In current practice, such a communications server is built around a microprocessor connected to one or more memories, which microprocessor works with a basic program (simpler than that of the central processing unit) containing specialized modules allowing the bus common to the various component elements of the computer and the network transmission line to be managed. An example of such a program, called communications software, is the program called CNS which is used in the Bull S. A. Company's DN-7XXX series, and also in the CNS-A.sub.0 and CNS-A.sub.1 products in the DPS-7000 computer series.
Extremely rapid development in the power and processing capacity of central processing units makes it possible for an increasingly large number of input/output peripherals to communicate with central processing units. These input/output peripherals are synchronous or asynchronous peripherals that communicate with their environment via transmission lines having speeds that range from 300 bits/s to 64 kbits/s. These input/output computer terminals communicate with the outside via modems whose function is to adapt the electrical signal delivered by the input/output terminal to the transmission medium connecting the terminal to its outside environment. These modems are, for example, defined by the V-24, V-28, V-11, V-35, and V-36 standards of the Consultative Committee in International Telegraphy and Telephony (CCITT). These various standards also define the transmission modes and protocols for the corresponding transmission links. These are supported physically by transmission media which are in fact cables or sets of individual wires.
The large computer systems of today, composed of a computer, a communications server, and the various input/output terminals connected thereto, can have as many as several hundred terminals which are thus connected by the same number of cables or sets of wires to the computer. In practice, it is convenient to connect each computer system to other systems and to the input/output terminals via an Ethernet network. When the terminals are distributed in a geographic location within a restricted area, the existence of a large number of cables and wires poses mechanical and physical problems, such as crosstalk, also occupying too much space.