I want to learn these patterns for BERT, thing is i'm told by a site that they are pseudorandom sequences based on an 11 and 15 bit shift register, respectively. That doesn't explain the actual algorithm though :S could someone clear it up for me as well as explain what sequence of bits is used and why? thanks alot in advance :) oh and is the same algorithm also valid for 2E20-1, 2E23-1 and so on for 20 and 23 bits?
Check out the ITU I.150 series.
Bill Matern wtm at ncomm dot com
==== NComm provides complete software solutions, including the device driver, for the physical layer requirements of T1, E1, T3, E3 and SONET/SDH including APS.
There is a bit of an explanation of a number of test code algorithms in ATIS Technical Report # 25, although it's a little tough to get your hands around, too.
The basic "Linear Feedback Shift Register" (LFSR) technique is used to both generate AND detect all of these "PRBS" type patterns, which differ from each other primarily in the number of stages used and the feedback points.
Here's a simplified overview of LFSR operation:
To get an idea of the complexity that can be obtained with LFSRs, ATIS TR #25 states that the commonly-used QRS pattern consists of a PRBS with a 20-stage shift register with a constraint that forces the "put to a one" when the next 14 bits in the sequence would be zeros. The QRS generates every combination of 20-bit words, repeats every 1,048,575 bits, and contains high density sequences, low density sequences, and sequences that change from low density to high density and vice versa.
- Steve Pinkston Kentrox
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