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ATM cell detector

  • US 5,485,461 A
  • Filed: 06/23/1994
  • Issued: 01/16/1996
  • Est. Priority Date: 07/01/1993
  • Status: Expired due to Term
First Claim
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1. Method for detecting, from a bit stream, the beginning of data packets such as ATM data cells, which data packets originate from different sources and are combined into one stream of data packets, each data packet being preceded by a preamble which comprises at least a gap, formed by a number of 0-bits, and a unique word UW having a unique word reference value which is formed by a specific combination of 0-bits and 1-bits and which unique word reference value has a length of b(uw) bits, wherein the unique word UW is preceded by a bit alignment word AW having an alignment word reference value which is formed by, beginning with a 1-bit, a specific combination of 0-bits and 1-bits and which has a length of b(aw) bits,the method comprising the following steps being carried out successively for the purpose of detecting a new data packet after a previous data packet has ended:

  • detecting, from a bit stream, the first 1-bit to occur and calculating, for a first detected bit string B(dl), which begins with the first detected 1-bit and which has a length b(dl) equal to the number of bits b(aw) of the alignment word reference value plus the number of bits b(uw) of the unique word reference value, a Hamming distance hd(1) between the first detected bit string B(dl) and a first reference bit string B(aw+uw) being equal to the alignment word reference value plus the unique word reference value;

    calculating, for a second detected bit string B(d2), which begins with the detected first 1-bit, (b(aw)-1) bits after said detected first 1-bit occurs and which has a length b(d2) equal to the number of bits b(uw) of the unique word reference value, the Hamming distance hd(2) between the second detected bit string B(d2) and a second reference bit string B(uw) which is equal to the unique word reference value;

    detecting, from the bit stream, a second 1-bit to occur and calculating, for a third detected bit string B(d3), which begins with the second detected 1-bit and which has a length b(d3) equal to the number of bits b(aw) of the alignment word reference value plus the number of bits b(uw) of the unique word reference value, the Hamming distance hd(3) between the third detected bit string B(d3) and the third reference bit string B(aw+uw) which is equal to the alignment word reference value plus the unique word reference value;

    calculating the position of the first bit of a new data packet on the basis of at least one of the calculated Hamming distances hd(1), hd(2) and hd(3) according to the following rules, where p is the number of bits of possible further preamble words which follow the gap, the alignment word AW and the unique word UW;

    if hd(1)=0 or hd(1)=l, then the first bit of the data packet is situated at a distance of b(aw+uw)+p bits after the detected first 1 -bit;

    if hd(2)=0, then the first bit of the data packet is situated at a distance of b(aw+uw)-l+p bits after the detected first 1-bit; and

    if hd(3)=0, then the first bit of the data packet is situated at a distance of b(aw+uw)+p bits after the detected second 1-bit.

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