Concord Communications, Inc.

Darrell Furlong

A local area network modem which meets the stringent signalling requirements of the IEEE 802.4 standard. The modem supports loopback commands by insuring that the receiver and transmitter portions are in phase-synchronism, regardless of the physical distance between the modem and upper layer control circuitry. The transmitter also uses only two digital signal wires to specify which of a number of functions are to be performed by the transmitter analog components. An automatic gain control (AGC) circuit is included which eliminates a number of problems associated with prior designs. In particular, the modem's receiver monitors itself to automatically correct drift in phase and gain control adjustment, without intervention from the upper layer.

Local area network modem

A symbol detection and correction scheme for local area network modems, especially those which receive one of three symbols at any given time, such as those operating in accordance with the IEEE 802.4 standard. The invention significantly improves the bit error rate observed by the layer above the modem. The modem operates on a received signal with a pair of slicers. One slicer operating as a two-symbol detector, and a second slicer operates as a three-symbol detector. A receiver state machine is used to keep track of which portion of a frame is currently expected to be being received. Depending upon the particular context, one or both of the slicer outputs are used by the state machine to determine which of the three symbols was received. For example, when the receiver is in a state where only one of two data symbols are expected, and no control symbols are expected, only the two-symbol slicer output is used. The three-symbol slicer output, and logical combinations of the three-symbol slicer output and the two-symbol slicer output, are used as other times. The receiver state machine recognizes errors in pseduo-silence, start delimiter, and end-delimiter sequences without reporting such errors to the upper level, thereby greatly decreasing the error rate observed there.

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Uxbridge, MA, United States of America

Darrell Furlong