Abstract: In the public switched telephone network, an originating modem can identify an answering modem, e.g., as to the type of modem it is, by detection of a low-level identification signal sent from the answering modem. The low-level identification signal is hidden within an industry standard "answer tone," e.g., a CCITT V.25 answer tone of 2100 Hz. If the low-level identification signal is detected by the originating modem, the industry standard "handshaking" procedure is terminated and a non-standard handshaking procedure is implemented. If the identification signal is not detected by the originating modem, the industry standard handshaking procedure is simply completed.

Abstract: The effect of non-linear distortion on a received signal point is reduced by the use of a circular limiter. In particular, the circular limiter computes the distance of the received signal point from the center of the signal point constellation. If this distance exceeds a desired limit, then both dimensions of the received signal point are reduced by the same factor to maintain the original phase. This reduction in the dimensions of the received signal point decreases the corresponding error signal that is calculated by a Viterbi decoder. As a result, this further reduces the value of the accumulated path metrics of the Viterbi decoder which results in an improvement in system performance. In addition, the circular limiter also realizes a slight improvement in system performance against additive white Gaussian noise. The value of the limiting distance can either be predefined, or determined adaptively to match channel conditions of the communications system.

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: The decoding depth of a multidimensional trellis-coding scheme with X-fold time diversity is reduced, or minimized. As a result, an improvement in error rate performance is achieved for the multidimensional trellis code, which can then be favorably applied to a fading channel application.