Private telephone installations

Private telephone systems developed at the same time as the large public telephone networks, extending and developing their functions of connecting people within the same organization.

In the days of manual switching, the private telephone system was both a tool for setting up internal communications and a means of multiplying the use of external lines, linking the group to the public network. An operator was in charge of connecting the inside lines to each other or to the outside lines, depending on user demand. This was done manually, using a connecting cord called a dicorde, which was plugged into jacks on both sides corresponding to the lines to be connected, thus establishing a communication link between users.

Automatic switching systems were then introduced. These used moving contacts and electromechanical activators to automate the functions of establishing and breaking communication links. Switching was controlled by means of electrical signals, known as "signaling", exchanged between the user terminals and the switches. Step selectors at first, then crossbar switches, enabled outgoing calls to the outside network, and internal calls to be handled automatically. However, the operator function was still required for incoming calls (from outside). Secondly, some large crossbar installations were equipped with the Direct Inward Dialing (DID) service, which automatically establishes incoming calls from the public network, provided that users benefiting from this service in the installation have been assigned a number in the national numbering plan.

Telecommunications techniques have undergone major changes following the development of computing and electronics. This has resulted in the following fundamental changes:

• the widespread use of electronics to replace electromechanics for switching and signalling devices;

• the switch to time switching as a replacement for space switching ;

• the widespread use of stored-program devices for plant control, replacing the so-called hard-wired logic of the time.

These developments were quickly applied to the field of PABXs (Private Automatic Branch Exchanges), completely changing the definition, form and even functions of private telephone systems. In particular, they enabled the PABX to become a tool offering multiple useful services to users, going far beyond the simple voice switching function for which it had originally been designed.

Today's PABX, as we'll describe it in the following, is a machine that uses the same basic technologies as computers, but implements application programs specific to the field of voice communication,...