Abstract: A mobile phone includes a modem, hereinafter referred to as a cellular radio modem. This cellular radio modem supports two modes of operation--an analog mode, where the cellular radio modem provides an analog signal for transmission; and a digital mode, where the cellular radio modem provides a digital bit stream for transmission. During operation, i.e., the existence of a data connection to a far-end modem, the cellular radio modem is alerted by the cellular network to switch to the appropriate mode--analog or digital. The cellular radio modem then switches and continues transmission in the new mode. In this embodiment, the far-end modem is part of a modem-pool located within the Mobile Telecommunications Switching Office of the cellular network. The far-end modem automatically detects this switch by the cellular radio modem and itself switches to the appropriate mode.

Abstract: In a mobile data communications system, a near-end modem initiates a data connection to a far-end modem at the highest permissible data rate, e.g., at 9600 bits per second (bps). If the communications channel subsequently degrades, either the near-end modem, or the far-end modem, initiates an autorate, or fallback sequence, e.g., to 4800 bps. However, if channel performance is still poor at 4800 bps, then one, or both, of the modems initiate a fallback to 1200 bps--skipping over the data rate of 2400 bps, which is not included within the fallback sequence.

Abstract: The Electronic Industry Association (EIA) RS-232 interface signals are manipulated to additionally convey operating mode information from a piece of data communications equipment (DCE) to its respective data terminal equipment (DTE). In particular, in establishing a data connection, the data communications equipment may transmit information in either half-duplex mode or full-duplex mode. In order to convey which mode of transmission is being used, the data communications equipment manipulates the EIA RS-232 interface signals to provide a notification signal, which is representative of the particular mode, to the data terminal equipment. As a result of receiving this notification signal, the data terminal equipment can then switch to the same mode of operation.

Abstract: A modem's data reception rate is increased by decreasing the amplitude of the signals transmitted by the modem. Decreasing the amplitude of the transmitted signals reduces the amplitude of the resulting echoes, and thereby reduces the residual echoes that remain after echo cancellation. Reducing the residual echoes permits increasing the data reception rate. Decreasing the amplitude of the transmitted signals also involves decreasing the data transmission rate; however, asymmetrical data transfer rates are acceptable for many modem applications.

Abstract: A V.32 compatible modem provides a full-duplex mode of operation, while a 208B compatible modem provides a half-duplex mode of operation. The connect sequence of a V.32 compatible modem is modified so that it will automatically determine if the other modem, whether the originating modem or the answering modem, is a 208B type of modem or a V.32 type of modem. In one embodiment of the invention, the connect sequence of an originating V.32 modem is modified to detect either a 2100 Hz answer tone, which represents that the answering modem is a V.32 type of modem, or a 2021 Hz answer tone, which represents that the answering modem is a 208B type of modem. In another embodiment of the invention, the connect sequence of an answering V.32 modem is modified to transmit first a 2100 Hz answer tone and then, if no 1800 Hz V.32 signal "AA" is received from the originating modem, to transmit a 2021 Hz answer tone, thereby simulating a 208B modem has answered the data call.

Abstract: A re-authentication procedure between the modems of a public switched telephone network (PSTN) data connection, which is between a computer facility and a user, provides a secure method for protecting the computer facility against an active wire tap, or spoofing, by an intruder. In particular, the user's modem and the computer's modem perform a re-authentication procedure throughout the duration of the data connection. This re-authentication procedure is transparently performed on a side channel of the data connection. This side channel can either be an in-band channel or an out-of-band channel. The re-authentication procedure comprises an exchange of encrypted information between the two modems. If one of the modems detects the presence of an active wire tap, that modem simply interrupts the data connection.

Abstract: A modem receiver used in a QAM or PSK data communication system is provided with various circuits generating error or monitoring signals indicative of the status of the system. These signals and the in-phase and quadrature components of the received signals are fed to an interface circuit which generates display signals suitable for driving a display unit such as an oscilloscope. These display signals define an image composed of the signal constellation of the received signals and various error or monitoring points.