Concomitant with the development and expansion of data/signal processing systems, including those in the distributed commercial data processing industry and the personal (home) computer market, has been the need for a communication medium that is readily accessible and which provides a communication capability that serves the needs of variety of equipments and users. By virtue of its presence throughout industrial, commercial and home environments and its capability of supporting digital data traffic, the standard telephone link is currently employed as the medium for meeting these multiuser/equipments communications requirements.
Interfacing digital data traffic between the telephone link and an end user's data processing equipment, which may be synchronous or asynchronous, is a modem which operates at some prescribed signalling rate (e.g. 300, 600, 1,200, 2,400 Baud). Because the digital processing/communication equipment to which the modem is to be coupled is often operated by a "non-technical" individual, the modem should ideally be simple to install and operate, perform consistently, and be flexible enough to meet the changing needs of the user, whether that user be a non-technical operator or a digital computer.
Because data communications are of one of two types (synchronous or asynchronous), modems have typically been configured to uniquely conform with the type of signalling format employed by the user's equipment. Asynchronous data communications have historically been carried out in a full duplex fashion over voice grade telephone lines at signalling rates of up to 1,200 Baud, by using an interface (e.g. RS 232C) which complied with the telephone company's communication standards (e.g. a Bell 103 (0-300 Baud) or Bell 212A (1,200 Baud) (interface). For manually controlled communications, such modems require the use of an attendant "dial-up" device in order for the communication (call) to be set up. Such an attendant device may include a standard telephone handset and control (talk/data) switches located on the modem, or a special purpose "data telephone" wherein the control switches are located on the telephone rather than on the modem. The placement of calls automatically has been accomplished via a suitable interface (e.g. an RS 366 interface (which is distinct from the above mentioned RS 232C interface)) to an automatic calling unit (e.g. a Bell 801 Autocall unit) which places the call for the modem without manual intervention.
In recent years modems that are compatible with the telephone company's equipment (e.g. a Bell 212A interface) have been introduced which allow an asynchronous device (e.g. user terminal) to control the modem and set up the communication (place the call) via an (RS 232C) interface between the intelligent equipment and the modem, using some prescribed asynchronous protocol (e.g. Hayes' "AT" protocol).
Synchronous communications have been historically accomplished in a half-duplex mode over voice grade two-wire telephone lines at signalling rates of 2,400 or 4,800 Baud using standard modems provided by the telephone company (e.g. Bell 208 units). For these units the call has been placed either manually (by always using an attendant handset) or automatically, by using, for example, a Bell 801-type autocall unit. Because of the specialized format/timing requirements of synchronous communications, there is at present no commonly accepted protocol that would permit a synchronous device to control a modem dialler via an asynchronous (RS 232C) interface.
Facing the diversification of users of data processing equipment and the wide variety of demands of data handling terminals, manufacturers have developed an assortment of modem types each of which is tailored to meet the needs of one or a few users, but none of which is presently adaptable to the broader spectrum of needs of both commercial and home users. For example, as mentioned above, modems employing Hayes' "AT" protocol have gained acceptance by the home/personal computer user for allowing an asynchronous terminal to control the modem, and the provision of external (front panel) control switches allow the user to control part of the operation of the modem (excluding auto-dial). Some modems employ manual dialing for setting up a call, while others offer a stored phone number directory with automatic dialing via a terminal formatted Hayes' "AT protocol" command. Some units offer a battery-powered back-up RAM for storing modem configuration parameters, or the use of an audio speaker to monitor the progress of a call. In order to effect a constant asynchronous terminal signalling rate regardless of telephone line speed there are also those modems which provide for the buffering of data within the modem. Finally, some units have adopted the use of a transparent industry-accepted (Micom Networking Protocol (MNP)) error detection protocol for asynchronous data. At present, however, there is no modem that offers to the user all of the above enumerated features or additional signalling capability that would substantially improve the flexibility of the modem and thereby enhance its application to effectively any digital data traffic communication environment.