Method of restoring the correct cell sequence, particularly in an ATM exchange, and output unit therefor
First Claim
1. Method of restoring the correct sequence of data packets or cells in a packet switching system, wherein, during transmission from an input unit to an output unit, related cells belonging to a single data stream from a specified input unit to a specified output unit of the switching system may overtake each other, said method comprising the steps of:
- prior to transmission form the input unit, adding a respective sequence label to each of the cells to identify the original transmission order of the cells;
upon reception at the output unit, buffering each cell in an output buffer;
waiting for at least a predetermined minimum delay time to elapse following the reception of a candidate cell still contained in the buffer;
comparing the sequence label of said candidate cell with the corresponding sequence labels of other cells in said output buffer;
outputting said candidate cell if the comparison indicated that the candidate cell was originally transmitted before the other cells in the output buffer; and
instead of the candidate cell, outputting another cell if the comparison indicated that the other cell was originally transmitted before the candidate cell.
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Abstract
To be able to load an ATM switching network as efficiently as possible, the successive cells of one and the same connection are routed to the output by as many different paths as possible; however, mutual overtaking of successive cells must be avoided or corrected. Each cell is held at the output until it is certain that no older cell can be buffered in the switching network. Before the cell is passed on, a check is made to determine whether the cells received from the switching network later include an older cell which must be passed on before that cell. At the input end, consecutive numbers ("Sequence Numbers") are allocated to the cells. A buffer (1) at the output end is operated, at least in part (1 . . . d), in the manner of a shift register, thereby introducing a predetermined delay. Prior to the output of a cell, at least the shift-register-like portion of the buffer is examined as to whether a cell written in later is to be put out earlier: if so, the two cells are interchanged.
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Citations
12 Claims
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1. Method of restoring the correct sequence of data packets or cells in a packet switching system, wherein, during transmission from an input unit to an output unit, related cells belonging to a single data stream from a specified input unit to a specified output unit of the switching system may overtake each other, said method comprising the steps of:
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prior to transmission form the input unit, adding a respective sequence label to each of the cells to identify the original transmission order of the cells; upon reception at the output unit, buffering each cell in an output buffer; waiting for at least a predetermined minimum delay time to elapse following the reception of a candidate cell still contained in the buffer; comparing the sequence label of said candidate cell with the corresponding sequence labels of other cells in said output buffer; outputting said candidate cell if the comparison indicated that the candidate cell was originally transmitted before the other cells in the output buffer; and instead of the candidate cell, outputting another cell if the comparison indicated that the other cell was originally transmitted before the candidate cell. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 11)
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9. Output unit for restoring the correct sequence of cells at the output of a packet switching system by means of respective sequence labels associated with each of said cells, said output unit comprising:
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a buffer for buffering cells to be put out; an access means for obtaining access to any cell contained in the buffer; timing means, associated with the access means, for determining the residence time of at least one cell in the buffer; and comparing means for comparing, following a predetermined minimum said residence time of said at least one cell, the respective sequence labels of said at least one cell and any other cell accessed by the access device to thereby determine the order the two cells entered said system. - View Dependent Claims (10, 12)
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Specification