Abstract: Systems, methods, and computer program products generate a DIL signal in which the length of the sign pattern used for symbols in the DIL signal is a positive integer that wholly divisible by four and is not wholly divisible by three. The sign pattern may be viewed as comprising both an even and an odd subsequence with the number of positive signs balancing the number of negative signs in each subsequence. The DIL signal may be distorted by analog impairments, such as transformers, that may introduce nulls at direct current (DC) and severe non-linear distortion below 100 Hz. Moreover, low-pass filters in the PSTN typically have a sharp roll-off at the Nyquist frequency of 4 kHz. A DIL signal that includes one or more DIL segments with each DIL segment incorporating the aforementioned sign pattern may be characterized by spectral nulls at both direct current (DC) and the Nyquist frequency, which is 4 kHz for a V.90 modem application.

Abstract: Methods, systems and computer program products are provided for monitoring performance of a modem which obtain diagnostic data directly from memory associated with the modem's digital signal processor (DSP). A secondary path to the DSP memory is utilized for the monitoring operations so that real time data can be obtained during connection startup procedures and during an active connection. First-in first-out (FIFO) buffers are incorporated in the DSP memory to track state transitions of one or more of the state machines within the modem and various performance data measurements may be obtained directly from the DSP memory responsive to different state transition events. The real time collected data may be stored in a file and provided to a remote location for use in diagnosing customer problems with specific customer line connections. Accordingly, real time monitoring of digital and analog line conditions and modem performance may be utilized to diagnose problems with modems and line connections.

Abstract: Glitch filters, methods, and computer program products that utilize the generally monotonically increasing characteristics of the expected levels of code points to detect and remove noise spikes by replacing values in the code point sequence with new values based on the code points around a suspect value are provided. Measured values associated with two code points in the sequence of code points which are immediately higher in the sequence of code points than a code point of interest are evaluated so as to select a larger value of the two code points in the sequence as a first reference value. The first reference value is compared with a measured value associated with a code point in the sequence of code points immediately lower than the code point of interest to determine if the first reference value is smaller than the measured value associated with the code point in the sequence of code points immediately lower than the code point of interest.

Abstract: Modems, methods, and computer program products provide a dual power mode capability in which a maximum power limit is determined for a received signal and, based on that determination, at least one modem operational parameter is adjusted and/or digital impairment learning (DIL) sequence is selected. Adjustments made to the operational parameters may allow a modem to operate in a high power mode to achieve a higher downstream data rate. The higher downstream data rate may be attributed to an improved SNR resulting from an increase in data transmission power. Moreover, the modem may also operate in a normal power mode where the operational parameters may be set to maximize the client modem receiver's dynamic range. By selecting the DIL sequence based on the power limit, a DIL signal containing symbols having power levels within the power limit may be selected.

Abstract: Methods, modems and computer program products for detecting whether a remote modem is of a particular design type and for adjusting the communication configuration for a communication session based on a type of the remote modem. The modem type is recognized based on a knowledge that the modem design of certain manufacturers has a unique associated data pattern contained within the startup communication sequence which is not specified by the protocol but which can be demodulated and detected to thereby recognize that the remote modem is a particular manufacturer's design type. Based on knowledge of the characteristics of the remote modem design, various steps are taken which may improve performance of the communication connection in light of the particular modem design's characteristics. For example, a different Total Harmonic Distortion (THD) threshold may be used for falling back to V.34 communications when the remote modem design only supports up to 2 look ahead for spectrum shaping.

Abstract: Receivers, methods, and computer program products can be used to demodulate a data signal transmitted from a digital source, which has a network sampling rate that is synchronized with a network clock. In an illustrative embodiment, a receiver includes a two-stage interpolator that receives digital samples of the data signal as an input and produces an interpolated digital sample stream to be filtered by an adaptive fractionally spaced decision feedback equalizer. The digital samples received in the interpolator are synchronized with a local clock; however, the interpolated sample stream is synchronized with the network clock. A slicer generates symbols for the samples output from the decision feedback equalizer by comparing the samples with a reference signaling alphabet. The receiver can be used in a V.90 client modem to demodulate pulse code modulated (PCM) data transmitted as pulse amplitude modulated (PAM) signals from a digital network.

Abstract: Modems, methods, and computer program products select a data rate based on error signals in a modem. In an illustrative embodiment, error signals representing the difference between an output of an equalizer and an output of a detector are accumulated and an average error value is computed therefrom. A signal to noise ratio is determined using the average error value. The signal to noise ratio is then used to select a data rate. In another illustrative embodiment, the mean squared error at the output of the equalizer is determined and then used, along with the probability of error in correctly detecting a symbol, to select a data rate. Incorrect decisions in detecting received data symbols can cause a catastrophic failure in a decision feedback equalizer used in a modem receiver as errors are repeatedly fed back causing the tap coefficients for the equalizer filters to be shifted from their normal operating values.

Abstract: Modems, methods, and computer program products select a data rate based on error signals in a modem. In an illustrative embodiment, error signals representing the difference between an output of an equalizer and an output of a detector are accumulated and an average error value is computed therefrom. A signal to noise ratio is determined using the average error value. The signal to noise ratio is then used to select a data rate. In another illustrative embodiment, the mean squared error at the output of the equalizer is determined and then used, along with the probability of error in correctly detecting a symbol, to select a data rate. Incorrect decisions in detecting received data symbols can cause a catastrophic failure in a decision feedback equalizer used in a modem receiver as errors are repeatedly fed back causing the tap coefficients for the equalizer filters to be shifted from their normal operating values.

Abstract: Methods, systems and computer program products are provided for constellation generation which determine initial ucode boundaries and an initial minimum signal level spacing. A work constellation set is generated based on the initial ucode boundaries and the initial minimum signal level spacing. The work constellation set is then iteratively adjusted so as to provide an adjusted work constellation set which achieves an error probability target. The adjusted work constellation set is then iteratively pruned so as to provide a pruned work constellation set which achieves a power limit. The pruned work constellation set is then iteratively fine-tuned so as to provide a final constellation set which is within a specified tolerance of the error probability target and/or the power limit.

Abstract: Error rate control systems, methods, and computer program products request a data rate slow-down based on the status of the error rate. A linear penalty is generated if a data rate slow-down is requested, which is then used to determine a minimum distance between constellation points in a signaling constellation or alphabet. Because the minimum distance between constellation points may be directly related to the error probability, the error rate may be controlled through the linear penalty.

Abstract: Robbed Bit Signaling (RBS) and PAD digital impairments in sets of Digital Impairment Learning (DIL) signals that are repeatedly transmitted from a server modem to a client modem during DIL intervals are identified by identifying a PAD level for the sets of DIL signals in the DIL intervals and then identifying an RBS type for individual sets of DIL signals based upon the PAD levels so identified. The PAD level may be identified using model DIL signals that correspond to PAD levels that are not subject to RBS. The RBS type may be identified using model DIL signals that correspond to multiple RBS types that are subject to the identified PAD level. More specifically, one of the DIL intervals that contains DIL signals that are not subject to RBS is identified. A PAD level for the DIL signals in the one of the DIL intervals so identified then is determined.

Abstract: Modems, methods, and computer program products select a data rate based on error signals in a modem. In an illustrative embodiment, error signals representing the difference between an output of an equalizer and an output of a detector are accumulated and an average error value is computed therefrom. A signal to noise ratio is determined using the average error value. The signal to noise ratio is then used to select a data rate. In another illustrative embodiment, the mean squared error at the output of the equalizer is determined and then used, along with the probability of error in correctly detecting a symbol, to select a data rate. Incorrect decisions in detecting received data symbols can cause a catastrophic failure in a decision feedback equalizer used in a modem receiver as errors are repeatedly fed back causing the tap coefficients for the equalizer filters to be shifted from their normal operating values.

Abstract: Modems, methods, and computer program products identify a signaling alphabet that is in variance with an ideal alphabet due to digital impairments and is used to decode received data symbols. In particular, a set of alphabet thresholds are established that correspond to the data symbols. The data symbols are received in an equalizer, which generates output symbols in response thereto. A collection of data symbols corresponding to a particular alphabet threshold are accumulated and an average value is computed, which is then used to replace the current value for the particular alphabet threshold to which the average value corresponds. Equalizer performance is enhanced as the improved accuracy of the alphabet thresholds results in better error information being fed back for adapting the equalizer filter coefficients.

Abstract: Modems, methods, and computer program products for falling back to a low speed mode of operation upon detection of abnormal line conditions during startup perform a first phase of a multiphase startup procedure in accordance with a high speed mode of operation. A determination is then made whether the line conditions will support high speed communication. If a high speed connection cannot be established, then subsequent phases of the startup procedure will be performed in accordance with the low speed mode of operation. Otherwise, subsequent phases of the startup procedure will be performed in accordance with the high speed mode of operation.

Abstract: Decision feedback equalizers having a stabilization capability, and methods and computer program products for stabilizing a decision feedback equalizer under severe error conditions use output samples from an equalizer to determine whether a severe error event has occurred in accordance with predefined criteria. If a severe error occurs, then a determination is made to evaluate whether the number of severe errors that have occurred has exceeded a threshold. If the threshold has been exceeded, then the coefficients for the filter(s) in the decision feedback equalizer are preserved in their current state. Severe errors can cause the equalizer filter coefficients to be pulled away from their normal operating values, which can result in several modulation cycles passing before the coefficients are restored.

Abstract: In a receiver (3 ) for a DSSS signal (DSSS=direct sequence spread spectrum), an inverse filter (12) is utilized in place of a matched filter for detecting the symbols (B.sub.m). The inverse filter (12) is distinguished in that it responds to the predetermined pulse sequence as such with a pure Kronecker delta sequence. Preferably, the DSSS signal is generated with a pulse sequence realizing a maximum process gain.