Abstract: A signal structure for high speed data communication selects each signal point in the structure to attain a low probability of error in the presence of both additive noise and multiplicative noise. Transmission apparatus for practice of the invention employs an encoder which responds to digital data to produce a QAM signal having parameters selected according to the new signal structure, and receiving apparatus employs a decoder which converts a received QAM signal to digital values according to the same structure.

Abstract: A signal structure for high speed Trellis coded data communications selects each signal point in the structure to attain a low probability of error in the presence of both additive noise and multiplicative noise. Viterbi decoding is modified to take account of the multiplicative noise energy content of the received signal.

Abstract: A network control and test system for application to data communications networks. In a first mode of operation, the system learns the network configuration automatically and builds a table describing the network configuration. In a second mode of operation, the system automatically monitors the status of the network and each of its component units, periodically polling the network for changes of status and updating the information in the network configuration table to reflect such changes. In a third mode of operation, selective sites in the network may be tested under control of the system at previously selected and entered times of day, at which times previously chosen tests are performed without the need for operator intervention. In a fourth mode, an operator can manually command the system to perform certain tests and control functions.

Abstract: A network control and test system for application to data communications networks. In a first mode of operation, the system learns the network configuration automatically and builds a table describing the network configuration. In a second mode of operation, the system automatically monitors the status of the network and each of its component units, periodically polling the network for changes of status and updating the information in the network configuration table to reflect such changes. In a third mode of operation, selective sites in the network may be tested under control of the system at previously selected and entered times of day, at which times previously chosen tests are performed without the need for operator intervention. In a fourth mode, an operator can manually command the system to perform certain tests and control functions.

Abstract: An automatic switching unit for connection to a modem at a remote site in a data communications network. The switching unit normally connects the modem to a primary data transmission path. If the primary path malfunctions, a central site operator can telephone the remote site and establish a secondary data transmission path. The switching unit answers the incoming calls and automatically disconnects the modem from the primary data transmission path. Once the primary data transmission path has been repaired, the central site operator causes the switching unit to return communications to the primary data transmission path. In one embodiment, this transfer can be made while maintaining communications over the secondary data transmission path.

Abstract: A device for measuring the power of analog signals on telephone lines in a data communications network. A selection circuit couples a selected analog signal from one of the telephone lines to a measuring circuit. A full-wave rectifier and a low pass filter process and convey the selected signal to a plurality of open-loop comparators. Each comparator produces a binary signal that indicates whether the selected signal is greater or less than a reference power that corresponds to a predetermined power level. Each comparator energizes an inverter and driver that in turn energizes a light-emitting diode thereby to indicate the line signal power visually. However, the drivers are interconnected so only one light-emitting diode is energized at any given time. A blanking circuit disables the light-emitting diodes when rapid signal level changes occur. A self-testing circuit includes a damped oscillator for testing all the circuitry in the measuring device.

Abstract: A testing facility for a data communications network including modulators and demodulators, or modems, at central sites, at intermediate, or hubbing, sites, and at remote sites. Hubbing site testing units connect to respective host modems at the intermediate sites. The central testing unit transmits control words containing line address, drop address, and command characters. A configuration control circuit in each hubbing site testing unit generates signals that designate the particular location of the testing unit at the hubbing site. A validation circuit in each testing unit enables the testing unit to respond to the command character if two valid control words are received in succession. Thereafter the testing unit transmits back to the central testing unit signals in response to the test defined by the command character.

Abstract: A modem with transmitter and reciever sections. Incoming digital data signals produce successive coordinate signals to define successive transmission points representing a given digital value and to modulate quadrature carrier signals. The coordinate signals have unit values of 0, 1, 2.707, 3 and 5 and together control the amplitude and phase of the modulated carrier signals. The receiver section uses received coordinate signals, after they are properly conditioned, to select a decision region which corresponds to a transmission point to produce the digital data signals. The decision regions have rectilinear boundaries on a Cartesian coordinate system at unit values 0, 1.25, 2 and 4 and each region is either rectangular or non-rectangular. Digital synchronism between separate modems is maintained by transmitting coordinate signals corresponding to a framing point which is separate from the transmission points, but which has the same digital value as a transmission point.