US 3,955,141 AGranted

Synchronizing circuit for modems in a data communications network

AssigneeIntertel, Inc.

InventorsDavid L. Lyon, Jerry L. Holsinger, David E. Williams

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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. Certain decision regions defined by the receiver use the receipt of the modulated carrier signals corresponding to a framing point to initiate a synchronizing operation.

A modem for connection between a digital unit and a transmission link, said modem comprising:

A. a transmitter for converting received digital signals from the digital unit into a modulated signal including
i. means for periodically transmitting a framing signal, thereby to establish a transmitted time reference,
ii. means for converting the digital signals to coordinate signals having a first set of values and a second set of values,
iii. control means for causing said converting means normally to transmit coordinate signals with the first set of values and, in response to a framing signal, to transmit coordinate signals with the second set of values, and
iv. modulator means responsive to the coordinate signals from said converting means for transmitting modulated signals onto the transmission link, and
B. receiver means including
i. means responsive to received modulated signals on the transmission link for producing received coordinate signals corresponding to the transmitted coordinate signals,
ii. means for comparing the received coordinate signals and predetermined signal levels corresponding to boundaries of first and second sets of decision regions, each of the first regions corresponding to a digital value and having one of the first set of values and each of the second regions corresponding to the second set of coordinate values and having a digital value which duplicates the digital value in one of the first decision regions, said comparison means transmitting a digital value corresponding to the selected decision region, and, when a second of the decision regions is selected, a received framing signal,
iii. synchronization detector means responsive to each received framing signal for establishing a received time reference corresponding to the transmitted time reference, thereby to detect synchronism between the transmitter and receiver operations during a transfer of data, and
iv. means for transmitting to the digital unit digital signals in response to the digital value from said comparison means.

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Abstract

Claims10