Modems themselves are, of course, widely known and used to facilitate the transmission of digital signals over analog telephone lines by modulating the digital signals onto an analog signal at the transmitter and demodulating the received signal and decoding the appropriate digital significance at the receiver. The name "modem" is thus an abbreviation for the modulation-demodulation process carried out by a pair of modems communicating over an analog transmission line.
As is also well known, modems operate in pairs and may operate with synchronous or asynchronous DTEs. Synchronous modems generally are reserved for higher speeds and operate with block or periodic error checking protocols for detecting erroneous reception of transmitted characters or digital data. Asynchronous modems, in contrast, are often operated in the "echo back" mode of operation in which the receiving modem's attached data terminal equipment will transmit back over the line a received character to indicate to the transmitter what character has been received and that it has been received. While this mode of operation is inefficient, it is simple and is widely employed or even required as a discipline in various data terminal equipment (DTE) manufacturer's devices.
As modem designs have progressed over the years, wide-spread use has occurred of microprocessor-based logic and filtering mechanisms that employ algorithms instead of physical analog circuits. Such modem designs have appeared in numerous patents. For example, U.S. Pat. No. 4,549,302 shows the typical design of one such modem utilizing a microprocessor, onboard RAM, addressable registers for interfacing to various circuits and functions and a full complement of filtering, tuning and conversion operations for handling the signals coming to or for transmitting signals from the modem to the communication line over the RS232 interface or port to the attached data terminal equipment (DTE).
In fact, the modems have progressed to the point that they may receive and act upon commands directed to them by the DTE. In order to obtain a response from the modem it has been generally required that the modem receive an inband signal in the form of a special header or escape sequence which alerts the modem to the fact that any following characters or commands are intended for it instead of for transmission on the communication line. Indeed, the aforementioned U.S. Pat. No. 4,549,302 is itself directed to a method of correctly recognizing an escape sequence of characters and distinguishing such a sequence from the random occurrence of a sequence of similar appearance that might occur in the data stream. In general, the modem monitors for the occurrence of escape characters in a sequence which includes some periods of time in which no characters are received, i.e. a pause, followed by the receipt of characters which may or may not be followed by another pause. The presence of the pauses distinguishes an intentional occurrence of an escape sequence of characters from an unintentional or randomly occurring one embedded in a stream of data.
However, in an asynchronous modem system, there does exist a significant problem where the sending and receiving DTE's echo received characters for error detection back to their transmitting partner in order to signal reception of a previous character and to indicate what character was understood to be received. This echoing of characters can cause a unique lockup problem to occur in asynchronous modems. Assume that an escape sequence is being transmitted by a first DTE through a modem attached to it in order for the DTE to gain control over the modem's functions. The modem will receive the first character, examine it, determine that it is potentially an escape sequence character and transmit it. It will be received at the remote modem, demodulated, passed over the RS232 interface to the attached DTE and echoed back through the receiving modem to be retransmitted. Of course, the remote modem receiving such a character from its attached DTE will begin to examine the characters that follow to see if an escape sequence is intended. The net result of this form of operation is that, in echo back mode, the remote asynchronous modem will see the same sequence of one or more escape characters and pauses seen by the attached local modem from the DTE which is actually trying to control only the local modem. The echo back feature from the remote DTE to its attached modem will cause the remote modem to enter into the "escaped" mode, awaiting receipt of commands from its DTE. However, the remote DTE does not intend to command or control its modem in this scenario, and the remote modem remains locked to incoming or outgoing data transmission while it patiently awaits commands over its RS232 interface.