At&t Bell Laboratories

At+t Corp.

Bell Telephone Laboratories

Gary D Martin

Kai Y Eng

Ronald A Spanke

Hiromi Ueda

Vikram Punj

Mark A Pashan

Linda K Bechtel

A data stream is converted from a synchronous transfer mode (STM) to an asynchronous transfer mode (ATM) by extracting data of a number of payload signals from the data stream, writing data of the payload signal into a random access memory (RAM), separately recording address locations of the data of each of the payload signals in separate buffers, directing the RAM to separately read the data of the payload signals, and attaching a cell header to the data read from the RAM to form ATM cells.

STM-to-ATM converters

The requirement of a separate payload clock in an ATM system is eliminated by increasing the number of bytes in an ATM cell and controllably generating and inserting idle cells so that a clock rate equal to that of the incoming SDH, SONET or the like clock rate may be used as a read clock. In one embodiment, the number of cells in a predetermined interval, for example 125 .mu.sec., is adjusted to be a predetermined integer number by inserting idle cells. This, in turn, allows the use of a 'standard' available clock to delineate cell boundaries.

Asynchronous transfer mode (ATM) payload synchronizer

For forward error correction, the least significant bit of each multibit symbol is encoded before transmission according to a block code that has an over-all parity bit. Since even syndromes can be produced by only an even number of errors and odd syndromes by an odd number of errors, a Chase decoder at the receiver considers either all double errors or all single errors and all triple errors that include the least reliable symbol in making the corrections to arrive at the most likely transmitted sequence.

Error correction method and apparatus

A technique for improving efficiency of error-correction is disclosed for use in a Gray-coded multilevel transmission system and, specifically, in a quadrature amplitude modulation (QAM) transmission system. To this end, information bits in information bit symbols are converted, i.e., mapped, into error-correction bit symbols having fewer bits than the corresponding information bit symbols. This bit-converting technique permits use of encoders and decoders having a code rate lower than the overall transmission code rate.

Error-correction coding for multilevel transmission system

A technique for maintaining frame synchronization in a OAM system transmitting nondifferentially encoded data is disclosed. In accordance with the present invention, multibit words (F.sub.I, F.sub.Q) comprising framing bits are formed and inserted into the transmitter data channels. The value of the framing bit is insensitive to the rotational effects of phase ambiguity by having a predetermined value and by being inserted within a predetermined bit position in each data channel. In the disclosed embodiments, regeneration errors can also be corrected by the transmission of predetermined quadrant bits within the multibit words. In addition, the spectral tones generated by multibit word transmission can be reduced by the use of utility bits in the multibit words.

Frame synchronization and phase ambiguity resolution in QAM transmission

An adaptive equalizer is disclosed comprising an amplitude equalizer circuit (12) having an adjustable Q, and a plurality of all-pass circuits (14,15) having adjustable delay bumps. A controller (18) adjusts the tuning, magnitude of the bumps, and the number of all-pass networks in the equalizer in response to variations in the power within a plurality of spectrum samples and the bit error rate of the equalizer output signal.

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North Andover, MA, United States of America

Gary D Martin