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Link Error Rate Estimation for Fiber Distributed Data Interface without Floating Point Operations

IP.com Disclosure Number: IPCOM000105963D
Original Publication Date: 1993-Sep-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 4 page(s) / 81K

Publishing Venue

IBM

Related People

Mieczkowski, DJ: AUTHOR

Abstract

Disclosed is a method for calculating the logarithmic Link Error Rate (LER) for FDDI Station Management (SMT) without using floating point operations. This method uses a variation of a conditional look-up table.

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Link Error Rate Estimation for Fiber Distributed Data Interface without Floating Point Operations

      Disclosed is a method for calculating the logarithmic Link
Error Rate (LER) for FDDI Station Management (SMT) without using
floating point operations.  This method uses a variation of a
conditional look-up table.

      The Link Error Rate (LER) is defined as the equation  N / (T *
125 * 10e6), where N is the number of link errors and T is the time
in seconds.  The LER may range from 1*10e-4 to 1*10e-15.

      As per the Fiber Distributed Data Interface (FDDI) standard,
the LER value must be reported as the absolute value of the integer
portion of the base 10 logarithm of the LER.  For instance, one link
error in 35 seconds corresponds to the ratio 1/4,375,000,000.  (log
(1/4,375,000,000) = -9.64) the LER would be reported as 9.  The
determination of the logarithm is trivial if floating point
operations are available on the processor.

      The problem is to determine the base 10 logarithm of the ratio
without using floating point operations and within the confines of 32
bits.

      Since only the integer portion of the logarithm is used a good
technique would be to use a conditional look-up table.  This solution
uses a combination of two tables.

      The first table is based on the ratio of N/T where T is the
time period in seconds and N is the number of link errors.

      1,250             < (N/T)                       LER = 4
        125             < (N/T)  <= 1,250             LER = 5
         12.5           < (N/T)  <=   125             LER = 6
          1.25          < (N/T)  <=    12.5           LER = 7
          0.125         < (N/T)  <=     1.25          LER = 8
          0.0125        < (N/T)  <=     0.125         LER = 9
          0.00125       < (N/T)  <=     0.0125        LER = 10
          0.000125      < (N/T)  <=     0.00125       LER = 11
          0.0000125     < (N/T)  <=     0.000125      LER = 12
          0.00000125    < (N/T)  <=     0.0000125     LER = 13
          0.000000125   < (N/T)  <=     0.00000125    LER = 14
                          (N/T)  <=     0.000000125   LER = 15

      However, this table can not be used below LER=7 since the
fractional values of N/T (values less than 1) can not be represented
as an integer.

      The second table is based on the inverse ratio (T/N) where T is
the time period in seconds and N is the number of link errors.

              0.00008 <= (T/N)  <         0.0008    LER = 4
              0.0008  <= (T/N)  <         0.008     LER = 5
     ...