Abstract: An apparatus for providing passive bus communication in an ISDN without use of services of a central office uses an alternative error checking (CRC) technique for communication between devices on the passive bus. This alternate CRC is interpreted as an error at the central office so that local frames are discarded by the central office.

Abstract: A method of providing an inband secondary channel for a data modem includes encoding the main channel data using a convolutional encoder so that main channel data symbols are selected from a first constellation. The secondary channel data are encoded using a differential encoder and secondary channel data symbols are selected from a second constellation. The transmission of the main channel symbols takes place during 48 frames of a 49 frame master frame. The differentially encoded secondary channel data symbols are transmitted during the remaining frame. The symbols of the secondary channel constellation are selected to have a higher average energy than those of the main channel data so that the secondary channel is easily detected and may be used to obtain frame synchronization for the modem after brief dropouts without having to retrain the modem.

Abstract: A method and apparatus for assigning time slots in a frame of time slots used for time division multiplexing, the time slots being representable as a position within the frame, in which it is determined what quantity of the time slots are required to provide a needed amount of bandwidth. A random number generator (40) generates (92) a uniformly distributed random number which is scaled (94) to represent a position in the first portion of the frame. A first of the positions within the frame is assigned based upon the value of the randon number. The remaining positions are assigned by fitting (64) the remaining required positions to the available positions in the frame spacing them at approximately equally spaced intervals as available. This prevents bunching of the used time solts which would otherwise prevent dynamic allocation of large blocks of bandwidth in the frame without complete recalculation of the frame.

Abstract: A modular housing for electronic apparatus includes an approximately C-shaped enclosure member having an inner portion and first and second sides. The enclosure member includes a male stacking enclosure connector on the first side; a female stacking enclosure connector on the second side; and a first cassette connector situated at the inner portion. A cassette for carrying an electronic apparatus engages the inner portion of the C-shaped enclosure member. The cassette includes a second cassette connector situated on the cassette suitable for engagement with the first cassette connector. A fan pulls air through the cassette through vents which mate with vents on the enclosure member.

Abstract: An apparatus for providing passive bus communication in an ISDN without use of services of a central office includes a passive bus suitable for ISDN D-channel frame communication, the passive bus having an echo channel and having a plurality of terminal devices coupled to the passive bus. A receiving circuit is coupled to the passive bus for receiving D-channel frames containing a SAPI address transmitted from the terminal devices over the passive bus. A decoding circuit is coupled to the receiving circuit for decoding D-channel addresses transmitted from the terminal devices over the passive bus. The decoding circuit includes a circuit for determining if one of the D-channel frames from the terminal devices include a predetermined SAPI address. A switching circuit is coupled to and responsive to the decoding circuit, and implements a logical communication channel between two or more of the plurality of terminal devices when the D-channel frame includes the predetermined SAPI address.

Abstract: An echo canceller according to the present invention includes means for storing an impulse response in a memory, the impulse response being indicative of the characteristics of a transmission line. It also includes a detector for detecting ranges of samples within the impulse response having relatively large distribution of echo energy, these ranges of samples of relatively large distribution of echo energy being indicative of echoes on the transmission line. An echo canceller includes a digital filter having a plurality of weighted taps, each of the weighted taps having an associated tap weight value. A tap allocation circuit establishes the tap weight values in response to said detecting means so that only taps within the regions of relatively large distributions of echo energy are turned on. The ranges of echo energy are detected coarsely by detecting ranges of samples having relatively high estimated average echo power.

Abstract: An architecture for a switching node of a communication network includes a dedicated hardware Layer 1 processing portion and a Layer 2 and 3 processing portion based on multiple programmed general purpose processors. An array of such processors are used with an arbitration scheme for selecting which of the processors is to be used for any given Layer 2 or Layer 3 process. The architecture allows the node's capacity to be expanded by simply adding more processors to handle increased traffic.

Abstract: A ventilation arrangement for a modular enclosure for electronic circuits includes a ventilation fan (8) including a motor (10) for providing movement of cooling air in the modular enclosure, the motor (10) being operable at at least two speeds including a lower speed and a higher speed wherein the higher speed induces greater air movement than the lower speed. A plurality of modular compartments (1, 2, 3, 4, 5) house a corresponding plurality of electronic circuits, with each of the modular compartments including an air path for the cooling air moved by the ventilation fan (8). A plurality of temperature sensors (32, 34, 36, 38, 40), one situated in each of the modular compartments (1, 2, 3, 4, 5), individually sense the temperature in each of the compartments.

Abstract: In a method for compensating for differences in a first clock signal's rate and a second clock signal's rate, transmitting voice samples from a first location synchronized to the first clock signal are received at a second location having a second clock for generating the second clock signal. The first clock signal and the second clock signal are compared to determine when a slip of one cycle has occured and it is determined which of the first and second clock rates is greatest. At the second location, the process includes adding a received voice sample for each predetermined number of slips which occur if the second clock is running too fast relative to the first clock, and discard an additional voice sample for each predetermined number of slips which occur if the second clock is running too slow relative to the first clock.

Abstract: An echo canceller stores echo signals in a plurality of sample and hold circuits. These echo signals are selectively switched to the input of a summing amplifier for subtraction from a data plus echo signal. A training sequence detector is utilized to detect the statistical occurrence of a predetermined training signal embedded within a stream of data. The echo canceller requires no analog to digital conversion or digital processer and operates at very high speeds. The circuit may be implemented in integrated circuit form at very low cost.

Abstract: Disclosed is a modem including a transmitter having a convolutional encoder for transforming each group interval digital data into an expanded bit sequence having symbol-selecting bits and subset-selecting bits forming a plurality of bit groups, with each bit group designating a 2-dimensional subset and the symbol-selecting bits being used to select one 2-dimensional symbol from each of the selected subsets, the transmitter further providing modulation of a carrier signal, and a receiver wherein synchronization of received frames of subset-selecting bits is provided by applying a parity check equation to a plurality of possible received frames of subset-selecting bits.

Abstract: This invention relates to an echo canceller, for a transmission line, including a shift register having an input for receiving an input signal from a signal source and having a plurality of output taps for providing a plurality of equally spaced delayed signals. A tap coefficient storage memory stores a plurality of tap coefficients. A multiplier multiplies certain of the delayed signals by certain tap coefficients to produce multiplier outputs. A tap allocation circuit determines, on a tap by tap basis, how many and which of the taps are to be turned ON so that its corresponding tap coefficients are multiplied by the multiplier. A subtracter cancels echoes by subtracting the multiplier outputs from signals received on the transmission line. Taps which are turned OFF may be assigned a status of STANDBY and these taps may be activated to compensate for changes in the characteristics of the transmission line.

Abstract: A differential correlator for a receiver including a voltage controlled current source which is controlled by an input data signal which is based on a received signal. A switching circuit selectively couples the controlled current to one of two storage capacitors as a function of a reference signal. A comparison circuit compares the integrated voltages on the storage capacitors at the end of each integration period T to provide user data as a function of the comparison. A discharge circuit discharges the storage capacitors to a common initial voltage after the integrated voltages are compared.

Abstract: A data communication system, comprising a first and a second transceiver coupled together through a transmission channel, the transmission channel having transmission characteristics. A measurement circuit is coupled to the first transceiver for measuring the effects of the transmission characteristics upon a known signal transmitted from the second transceiver to the first transceiver a mapping of the transmission characteristics to a code is made and the code is transmitted from the first transceiver to the second transceiver. A plurality of predetermined filters for use in filtering signals to be transmitted by the second transceiver is selected upon receipt of the transmitted code.

Abstract: A method for changing the data transmission rates in a synchronous data communication system including a first modem and a second modem coupled together through a transmission channel, the transmission channel including a first path for transmissions from the first modem to the second modem and a second path for transmission from the second modem to the first modem, includes the steps of determining, at the first modem, that the second path of the transmission channel is adequately high in quality to support an increase in transmission rate. Next it is determined, at the second modem, that the first path of the transmission channel is adequately high in quality to support an increase in transmission rate. A training sequence is then transmitted at an increased transmission rate from the second modem to the first modem. Next, a training sequence is transmitted at the increased transmission rate from the first modem to the second modem.

Abstract: A modem is disclosed having a data transmission protocol involving lower-speed, full-duplex operation during the connect sequence with a remote modem and an automatic switch to higher-speed, half-duplex operation for data transfer. Further, the modem data transmission involves transparently changing between lower-speed, interactive operation and higher-speed operation based upon transmission demands. The operation is controlled by a processor monitoring the contents of a transmit data buffer and providing a mode control command to the modem transmitter. The modem also adapts its speed to the quality of the telephone line by fallback or fallforward to a different speed based upon predetermined data frame retransmission criteria.

Abstract: Disclosed is a modem including a transmitter having a convolutional encoder for transforming each group interval digital data into an expanded bit sequence having symbol-selecting bits and a frame of subset-selecting bits forming a plurality of bit groups, with each bit group designating a 2-dimensional subset and the symbol-selecting bits being used to select one 2-dimensional symbol from each of the selected subsets, the transmitter further providing modulation of a carrier signal, and a receiver wherein a commonality-seeking, branch cost calculator determines the candidate branch associated with the minimum cost N-dimensional symbol subset for each coset and a Viterbi decoder which determines the maximum likelihood path from a plurality of surviving paths formed from branches selected from the candidate branches.

Abstract: Disclosed is a scrambler/encryption system for randomizing an information-containing data signal for transmission and for reproducing the original information at a receiver. The information-containing data to be transmitted, encoded with a preselected base of a numeric system, is applied to a modulo adder appropriate for that base. An output of the modulo adder is applied as an input to a multistage shift register. An arbitrary logic networ, having a plurality of inputs, connected to a plurality of stages of the shift register, responds to the arbitrary logic's selected code and the contents in the register by delivering an output signal to the modulo adder. At the receiver the inverse operation is accomplished by a circuit which includes substantially the same components except the modulo adder receives the scrambled/encrypted input signal which is fed into the receiver's shift register.

Abstract: An improved method of providing transparent packet access to a D-channel of an ISDN, includes the step of providing a user packet including an Address field, a Control field and an Information field, for transporting to an ISDN; Extracting the Address, Control and Information fields from the the user packet; and inserting the extracted fields into an Information field of an ISDN format LAPD packet. Preferably X.25 packets are provided access to the network, and the X.25 LAPB Address, Control and Information fields are inserted into the LAPD information field. This method provides independence of protocols and reduces complexity.

Abstract: The present invention relates to a communication system which includes a plurality of N communication links coupled to a central office. A plurality of N-1 user devices are coupled to N-1 of the N communication links with each of the N-1 user devices including a memory for storing a translation table relating a device identification code for each of the N-1 user devices to a directory code associated with each of the communication links. An input is situated at each of the N-1 user devices, for allowing a user to supply a device identification code of another user device to be called. A translating circuit is situated at each of the N-1 user devices, for translating the device identification code to it's associated directory code responsive to the input of a device identification code by the user by reading the translation table.