# The algorithm for channel test patterns like 2047 and 2E15-1

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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?

Re: The algorithm for channel test patterns like 2047 and 2E15-1

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.

Re: The algorithm for channel test patterns like 2047 and 2E15-1

Perdition wrote:

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:

http://en.wikipedia.org/wiki/Linear_feedback_shift_register

The "feedback" aspect means that a taps are taken from selected
flip-flops in the shift register, and Exclusive-ORed back to the input
of the first flip-flop.

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